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  • 2

  • 2.09  periodic quantity. Oscillating quantity whose values identically recur at equal increments of the independent variable.
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  • 6.01  absolute pitch. Ability of a person to identify the pitch of a pure tone without the aid of an external reference.
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  • 4.18  normal mode of vibration. Mode of free vibration of an undamped system. In general, any composite motion of the system can be analyzed in terms of a summation of normal modes. Annotation 1          The characteristic pattern of motion typically consists of a space distribution, one(...)
  • 4.21  modal numbers. Set of integers by which the normal modes of a system are ordered with respect to the number of anti-nodes along each respective coordinate for the mode shape. Annotation              For 1-dimensional structures with no curvature, the modal numbers are also ordered(...)
  • 9

  • 5.04  far field. The acoustic field sufficiently distant from a distributed source that the sound pressure decreases linearly with increasing distance (neglecting reflections, refraction, and absorption). Annotation 1          For a source of extent L, the "Fresnel range" can be used to(...)
  • 1

  • 5.50  backscattering cross section. Of an object, 4 π steradians multiplied by the differential scattering cross section of that object, evaluated back in the direction from which the sound originates. Unit, meter squared (m2).
  • 2

  • 5.07  detectability index; d′. The square root of the ratio of the difference between the mean signal plus noise power and the mean noise power to the variance of the noise at the point where the detection decision is made. The detector may be a subject or any other decision-making(...)
  • 10.63  critical frequency. Of a panel or partition, the lowest frequency at which the coincidence effect occurs. Unit, hertz (Hz).      
  • 7.11  cross spectrum. Product of the complex conjugate of the Fourier transform of the input signal to an acoustical device and the Fourier transform of the output signal. Annotation         The cross spectrum is typically used to determine the linear frequency response of a system using a(...)
  • a

  • 9.44  A-scan. In ultrasonic testing, method of data presentation utilizing a horizontal base line that indicates distance or time and a vertical deflection from the base line which indicates amplitude.
  • 11.30  abnormal growth of loudness; abnormal loudness growth function; recruitment. In certain cases of hearing impairment (e.g., cochlear) an increase in loudness with increasing stimulus magnitude at a rate greater than for a normal ear. Annotation              Although in wide use, the(...)
  • 11.30  abnormal growth of loudness; abnormal loudness growth function; recruitment. In certain cases of hearing impairment (e.g., cochlear) an increase in loudness with increasing stimulus magnitude at a rate greater than for a normal ear. Annotation              Although in wide use, the(...)
  • 5.01  absolute threshold; absolute limen; sensory threshold; stimulus threshold. Minimum stimulus that evokes a response in a specified fraction of the trials. Annotation         For hearing, the sound pressure level that establishes a threshold of audibility at a specified frequency is an(...)
  • 5.01  absolute threshold; absolute limen; sensory threshold; stimulus threshold. Minimum stimulus that evokes a response in a specified fraction of the trials. Annotation         For hearing, the sound pressure level that establishes a threshold of audibility at a specified frequency is an(...)
  • 5.37  absorption loss. That part of the transmission loss caused by dissipation or conversion of sound energy into other forms of energy (e.g., heat) either within the medium or attendant upon reflection. Unit, decibel (dB).
  • 2.53  acceleration. Vector that specifies the time rate of change of velocity. Unit, meter per second-squared (m/s2). Annotation 1          Various self-explanatory modifiers such as peak and time-mean-square are often used. The time interval should be indicated over which the mean (for(...)
  • 2.05  acoustic, acoustical. Qualifying adjectives meaning containing, producing, arising from, actuated by, related to, or associated with sound. Acoustic is used when the term being qualified designates something that has the properties, dimensions, or physical characteristics associated with(...)
  • 6.47  acoustic admittance. Reciprocal of acoustic impedance. Unit, (cubic meter per second) per pascal [(m3/s)/Pa].
  • 7.63  acoustic amplifier. Device in which energy is added to an acoustical wave by interaction with mechanical or electric energy.
  • 5.31  acoustic attenuation constant. Real part of the acoustic propagation constant. Unit, neper per section or per unit distance.
  • 7.64  acoustic bridge. Device for measuring the acoustic impedance by comparison with an adjustable known impedance.
  • 6.46  acoustic compliance. Reciprocal of acoustic stiffness. Unit, meter per pascal (m/Pa).
  • 7.47  acoustic coupler. Cavity of specified shape and volume used for the calibration of earphones or microphones in conjunction with a calibrated microphone adapted to measure the sound pressure developed within the cavity. Annotation   See ANSI/ASA S3.7, American National Standard Methods(...)
  • 5.42  acoustic dispersion. Change of speed of sound with frequency. Unit, meter per second per hertz, m/(s/Hz).
  • 7.02  acoustic equivalent volume. Alternate method of stating acoustic compliance. Acoustic equivalent volume Va may be expressed as:      Va =  γ po Ca =  ρo c2 Ca where γ is the ratio of specific heat at constant pressure for air to the specific heat at constant volume, 1.400 at sea(...)
  • 9.27  acoustic holography. Inspection method using the phase interference between sound waves from an object and a reference signal to obtain an image of reflectors in the object under test.
  • 7.97  horn; acoustic horn. Tube of varying cross section intended to achieve an acoustic impedance match and to produce a directional effect.
  • 6.41  acoustic impedance. At a specified surface, complex quotient of acoustic pressure by volume velocity through the surface. Unit, pascal per (cubic meter per second) [Pa/(m3/s)]. Annotation              Pascal per (cubic meter per second) is also called an acoustic ohm.
  • 6.44  acoustic mass; inertance. At a frequency for which inertial forces are dominant, quotient of sound pressure by the resulting in-phase volume acceleration during sinusoidal motion. Unit, pascal per (cubic meter per second squared) [Pa/(m3/s2)], or kilogram per (meter to the fourth power)(...)
  • 3.01  acoustic nerve; auditory nerve; eighth cranial nerve; vestibulocochlear nerve. Nerve, consisting of two sets of fibers: the anterior branch or cochlear nerve and the posterior branch or vestibular nerve. Afferent fibers of the nerve conduct neural signals from the inner ear to the(...)
  • 8.25  acoustic particle motion sensor. A vector sensor that measures underwater sound by sensing particle motion magnitude and direction.
  • 5.32  acoustic phase constant. Imaginary part of the acoustic propagation constant. Unit, radian per section or per unit distance.
  • 5.30  acoustic propagation constant. Natural logarithm of the complex ratio of the steady-state particle velocities, volume velocities, or pressures at two points separated by unit distance in a uniform system (assumed to be of infinite length), or at two successive corresponding points in a(...)
  • 7.83  acoustic radiating element. Vibrating surface of a transducer that produces sound waves.
  • 2.68  acoustic radiation pressure. Unidirectional pressure exerted upon a surface exposed to a sound wave. Unit, pascal (Pa).
  • 7.65  acoustic radiometer. Instrument for measuring acoustic radiation pressure.
  • 6.43  acoustic reactance. Imaginary part of acoustic impedance.
  • 3.02  acoustic reflex. Middle-ear muscle (stapedius and tensor-tympani) reflex elicited by an acoustic stimulus.
  • 3.03  acoustic-reflex activating stimulus. Acoustic signal that is used to elicit an acoustic reflex. See 3.02, 3.62.
  • 5.43  acoustic refraction. Process by which the direction of sound propagation is changed by spatial variation of the speed of sound in the medium.
  • 6.42  acoustic resistance. Real part of acoustic impedance.
  • 5.47  acoustic scattering. Irregular reflection, refraction, or diffraction of sound in many directions.
  • 6.45  acoustic stiffness. For a system in which friction and inertia are negligible, quotient of sound pressure by the resulting in-phase displacement during sinusoidal motion. Unit, pascal per meter (Pa/m).
  • 5.41  acoustic streaming. Name given to unidirectional flow in a fluid caused by the presence of sound waves.
  • 2.08  acoustic variable. Time varying or oscillatory acoustic quantity; examples are acoustic pressure and vibrational acceleration.
  • 2.05  acoustic, acoustical. Qualifying adjectives meaning containing, producing, arising from, actuated by, related to, or associated with sound. Acoustic is used when the term being qualified designates something that has the properties, dimensions, or physical characteristics associated with(...)
  • 7.81  acoustical baffle. Shielding device used with a loudspeaker unit to increase the effective acoustic path length between the front and back of the loudspeaker unit.
  • 7.01  acoustical coupler. Cavity of specified shape and volume used for the calibration of earphones or microphones in conjunction with a calibrated microphone adapted to measure the sound pressure developed in the cavity. Annotation         Acoustical couplers are specified in ANSI/ASA(...)
  • 7.82  acoustical enclosure. Assembly of an enclosure, one or more loudspeaker units, and other associated parts such as filters, transformers, or other passive elements.
  • 6.02  acoustical system. System capable of receiving, transmitting, recording or generating acoustic signals.
  • 5.01  acoustical transmission system. Assembly of elements adapted to the transmission of sound.
  • 11.51  acoustical trauma. Ear injury caused by a sudden and intense acoustic stimulus that causes a degree of permanent or temporary hearing loss, or both.
  • 2.04  acoustics. (a) Science of sound, including its production, transmission, and effects, including biological and psychological effects. (b) Those qualities of a room that, together, determine its character with respect to auditory effects.
  • 8.02  active sonar. Method or equipment by which information concerning a distant object is obtained by evaluating its effects on the sound generated by the equipment. Annotation              Monostatic: transmitter and receiver are collocated. Bistatic: transmitter and receiver are(...)
  • 6.07  active transducer. Transducer such that the energy of the output signal is derived at least in part from sources other than the input signal.
  • 5.02  adaptive psychophysical method. Procedure primarily used to determine thresholds in which the stimulus magnitude is dependent upon the subject’s response to a previously presented stimulus. The stimulus magnitude may be (a) under the direct control of the subject, or (b) adjusted by the(...)
  • 6.17  admittance. Reciprocal of impedance.
  • 4.53  presbycusis; age-associated hearing loss. Loss of hearing sensitivity that results from physiological changes that occur with advanced age.
  • 9.06  agglomeration. Union of small particles suspended in a fluid medium into larger aggregates by the action of sound waves.
  • 11.20  air-bone gap. For the ear of an individual at a specified frequency, the difference between the hearing threshold levels for air conduction and bone conduction. Unit, decibel (dB).
  • 11.18  air conduction. Transmission of sound through the outer and middle ear to the inner ear.
  • 7.03  air pressure in an ear (or calibration cavity). Difference between the pressure of air within the external auditory canal (or in a calibration cavity) and atmospheric pressure at the location of the measurement. Unit, pascal (Pa). Annotation 1      Air pressures observed in aural(...)
  • 7.02  all-pass filter. Filter designed to introduce a phase shift or delay over a band of frequencies without introducing appreciable attenuation or distortion at those frequencies.
  • 2.33  ambient noise. All-encompassing sound at a given place, usually a composite of sounds from many sources near and far. Annotation 1                  When the intent is to measure or record a specific source or signal, the ambient noise does not include sounds of interest. Annotation(...)
  • 8.09  ambient noise. All noise that would be present in the absence of the measuring equipment and its deployment platform, and (for an active sonar) in the absence of reverberation from the sonar transmitter. Annotation                      See also 2.33.
  • 2.20  amplitude. Magnitude of the largest departure from its equilibrium value of an acoustic variable.
  • 9.50  amplitude. In ultrasonic testing, the vertical pulse height of a signal, usually base to peak, when indicated by an A-scan presentation.
  • 10.25  anechoic room. Test room whose surfaces absorb essentially all of the incident sound energy over the frequency range of interest, thereby affording nearly free-field conditions over the measurement surface. Annotation              The word anechoic is derived from the Greek words(...)
  • 9.34  angle beam. Wave train traveling at an angle measured from the normal to the test surface to the centerline of the beam.
  • 6.68  angular deviation loss. Sensitivity level along the principal axis minus the sensitivity level of the transducer for a specified direction. Unit, decibel (dB).
  • 2.14  angular frequency. Frequency multiplied by 2π. Unit, radian per second; symbol, ω.
  • 6.02  annoyance. Any sound that is perceived as irritating or a nuisance. Unit, noy; symbol, n. See also C11.14, C11.15.
  • 4.06  anti-resonance. Phenomenon that occurs when the driving system produces zero response in the driven system at a frequency between two mode frequencies. Annotation              An anti-resonance frequency is a frequency at which anti-resonance occurs.
  • 5.25  antinode. Point, line, or surface in a standing wave where some characteristic of the wave field has maximum value. Annotation              An appropriate modifier should be used before the word "antinode" to signify the type that is intended; e.g., displacement antinode, velocity(...)
  • 5.03  antiphasic. Condition in which the phase or time difference of the signal presented at each ear differs by a fixed nonzero value from the phase or time difference of a second sound (often noise) presented at each ear. Annotation         An example of an antiphasic condition is one in(...)
  • 3.27  incus; anvil. Middle member of the three auditory ossicles that are located in the middle ear. The body of the incus is attached to the head of the malleus and the rounded projection at the lower end of the incus (lenticular process) is attached to the head of the stapes.
  • 6.31  apparent mass. Quotient of force by the resulting in-phase acceleration during sinusoidal motion. Unit, kilogram (kg).
  • 4.39  applied shock. Excitation that, if applied to a system, would produce mechanical shock. The excitation may be either a force applied to the system or a motion of its support.
  • 8.26  array gain. Signal-to-noise ratio in decibels at the output of a beamformer minus signal-to-noise ratio in decibels at the input of the beamformer. Unit, decibel (dB).
  • 11.45  articulation; intelligibility. Percentage of speech units correctly received out of those transmitted. Annotation 1          The word "articulation" is used when the units of speech material are syllables or other meaningless fragments of speech; the word "intelligibility" is used(...)
  • 6.03  articulation index. Numerical value indicating the proportion of an average speech signal that is understandable to an individual. See also C11.45. Annotation 1      See ANSI/ASA S3.5. Annotation 2      Although the term articulation sometimes has been used to mean intelligibility(...)
  • 7.53  artificial mastoid; mastoid simulator. Device simulating the mechanical impedance of an average human mastoid which is used to calibrate bone vibrators.
  • 7.55  artificial voice; voice simulator. Complex sound, usually emitted by an artificial mouth, whose spectrum corresponds to that of an average human voice.
  • 12.17  attack. Term used to designate the initial portion of a sound.
  • 2.15  audio frequency. Frequency of a sound wave nominally audible to humans. Unit, hertz (Hz). Annotation 1          Audio frequencies range roughly from 15 Hz to 20 kHz. Annotation 2          The word "audio" may be used as a modifier to indicate a device or system intended to operate(...)
  • 4.01  audiogram. Graph of hearing threshold level (4.42) as a function of frequency. Also see C11.27. Annotation            Standard audiograms for pure-tone air or bone-conduction thresholds are generally plotted for hearing levels from –10 to 120 dB HL for audiograms used in clinical(...)
  • 4.02  audiologist. Individual whose primary expertise is in the identification and measurement of hearing loss and the rehabilitation of those with hearing impairments.
  • 4.03  audiology. The study of hearing and hearing disorders, with primary emphasis on the conservation of hearing; the prevention, identification, and assessment of hearing impairment; and the rehabilitation of persons with hearing impairments.
  • 4.04  audiometer. Electronic instrument for measuring hearing sensitivity, specifically hearing levels, as a function of frequency.
  • 7.57  audiometer. Device used to measure hearing sensitivity, specifically hearing level, as a function of frequency. Annotation 1          A manual audiometer is a pure-tone audiometer in which the signal presentations, frequency, hearing level selection, and recording of results are(...)
  • 4.12  audiometric test room. Enclosed space used for testing hearing. Annotation 1      An audiometric test room may also be known as an audiometric test area, hearing test space, or hearing test room. An example would be a prefabricated room known as an audiometric test booth, suite, or(...)
  • 10.28  audiometric test room. Enclosed space used for testing hearing. Annotation              See ANSI/ASA S3.1 American National Standard Maximum Permissible Ambient Noise Levels for Audiometric Test Rooms.
  • 4.11  audiometrically normal listener. Individual having hearing threshold levels within a percentile range, typically 10th to 90th percentile values, for a population specified in reference to age, ethnic background, and gender at the audiometric frequency of interest. See(...)
  • 4.13  audiometry. Measurement of hearing, including aspects other than hearing sensitivity.
  • 4.14  audition. Sense of hearing.
  • 4.15  auditory adaptation; perstimulatory adaptation. Change (decrease) in loudness during presentation of an acoustic stimulus. Annotation 1      Auditory adaptation may occur following acoustic stimulation too weak to produce auditory fatigue and its effects on the threshold of audibility(...)
  • 6.04  auditory after-effect; auditory negative afterimage. Phenomenon in which a familiar sound appears to take on a modulated quality after listening to rapid high-intensity pulses for about one minute. Annotation         The after-effect, which varies with the sound pressure level and(...)
  • 4.16  auditory brainstem response; brainstem auditory evoked response. Electrical response emanating from the auditory nerve and brainstem, which can be obtained from electrodes on the scalp.
  • 11.32  auditory critical band. (a) Frequency band within which the loudness of a band of continuously distributed sound of constant sound pressure level is independent of its bandwidth. (b) Frequency band of sound, being a portion of a continuous-spectrum noise covering a wide band, that(...)
  • 4.17  auditory fatigue; fatigue; post-stimulatory fatigue. Temporary decrease in hearing sensitivity resulting from a previous auditory stimulus. Auditory fatigue produces a temporary threshold shift that may last for seconds, minutes, hours, or days. Annotation            Auditory fatigue(...)
  • 6.05  auditory flutter. Wavering auditory sensation produced by periodic interruption of a continuous signal.
  • 6.82  auditory gap. Frequency band of low auditory sensitivity above and below which the sensitivity is better. Unit, hertz (Hz).
  • 6.06  auditory lateralization; lateralization. Determination by a subject that the apparent direction of a sound is either left or right of the frontal-medial plane of the head. Annotation 1      Auditory lateralization is usually restricted to sounds presented by earphones or by bone(...)
  • 6.07  auditory localization; localization. Determination by a subject of the apparent direction and distance, direction alone, or distance alone, of a sound source. Annotation         Auditory localization in a horizontal or azimuthal plane is facilitated by interaural phase or time(...)
  • 6.04  auditory after-effect; auditory negative afterimage. Phenomenon in which a familiar sound appears to take on a modulated quality after listening to rapid high-intensity pulses for about one minute. Annotation         The after-effect, which varies with the sound pressure level and(...)
  • 3.01  acoustic nerve; auditory nerve; eighth cranial nerve; vestibulocochlear nerve. Nerve, consisting of two sets of fibers: the anterior branch or cochlear nerve and the posterior branch or vestibular nerve. Afferent fibers of the nerve conduct neural signals from the inner ear to the(...)
  • 3.04  auditory ossicles; ossicles. Three small bones in the middle ear; i.e., the malleus, the incus and the stapes. These bones transmit mechanical vibrations from the tympanic membrane to the oval window of the cochlea.
  • 6.08  auditory perception. Interpretation of auditory sensations as meaningful events through the sense of hearing.
  • 11.28  auditory sensation area. Region enclosed by the curves defining the hearing threshold and the threshold of pain as a function of frequency.
  • 6.09  auditory time error; time error; time order error. Error in judgment between some characteristic of two sound stimuli that occurs as a function of the time separation between the stimuli. Annotation         When the error enhances the first stimulus, it is called positive; when it(...)
  • 3.05  auditory tube; Eustachian tube. Tube that connects the middle ear with the nasal part of the pharynx. The auditory tube serves to equalize air pressure on the two sides of the tympanic membrane, i.e., middle ear pressure and ambient pressure.
  • 6.17  combination tone; aural combination tone. Secondary tone that can be perceived when two loud primary tones are presented simultaneously. The secondary tone may be a difference tone or a summation tone. Several combination tones may be produced from a single pair of primary tones. Unit,(...)
  • 11.38  aural harmonic. Harmonic of a given stimulus tone generated in the auditory system and perceived by a listener.
  • 3.06  auricle; pinna. The external and visible portion of the ear comprising an ovoid-formed, skin-covered, fibro-cartilaginous appendage that is attached to the head around the opening of the external auditory meatus. The auricle contributes to the localization of sounds in the front-back and(...)
  • 7.04  automatic gain control. Means (other than peak clipping) by which the gain is automatically controlled as a function of the level of the signal being amplified. Abbreviation, AGC.
  • 4.05  Békésy audiometer; automatic recording audiometer. Pure-tone audiometer where hearing level variations are under the subject’s control. Signal presentations, hearing level variation, frequency selection or variation, and recording of subject responses are implemented and recorded(...)
  • 2.27  wavelength. For a simple harmonic wave in an isotropic medium, perpendicular distance between two wavefronts in which the displacements have a difference in phase of 2π radians. Unit, meter (m); symbol λ,  λ = c / f.
  • 10.43  average sound pressure level in a room. Ten times the logarithm to the base ten of the ratio of the space and time average of squared sound pressure to the squared reference sound pressure, the space average being taken over the total volume of the room, except for the regions of the(...)
  • 11.43  average speech power. For a stated time interval, the arithmetic mean instantaneous speech power over that interval. Unit, watt (W).
  • 6.62  axial sensitivity. Of a microphone, for a specified frequency, free-field sensitivity to a plane progressive sound wave whose direction of propagation is toward the reference point and along the principal axis. Unit, volt per pascal (V/Pa).
  • 8.20  sonar source level; axial source level. Sound pressure level on the axis of the sound projector measured in the far-field and scaled back to a standard reference distance of 1 meter from the effective acoustic center of the projector. Unit, decibel (dB).
  • b

  • 9.45  B-scan. In ultrasonic testing, a method of data presentation which provides a cross-sectional view of the test piece.
  • 2.34  background noise. Total of all sources of interference in a system used for the production, detection, measurement, or recording of a signal, independent of the presence of the signal. Annotation 1          Ambient sound detected, measured, or recorded with the signal is part of the(...)
  • 11.48  backward masking. Condition in which the signal occurs before the masking sound.
  • 6.10  band audibility function. For a given frequency band, the effective proportion of speech dynamic range within the band that contributes to speech intelligibility under conditions that are less than optimal. Annotation 1      The value of the audibility function is limited to the(...)
  • 7.04  band-elimination filter; band-rejection filter; bandstop filter; notch filter. Filter having a single attenuation band, neither of the band-edge frequencies being zero or infinite.
  • 7.04  band-elimination filter; band-rejection filter; bandstop filter; notch filter. Filter having a single attenuation band, neither of the band-edge frequencies being zero or infinite.
  • 3.10  band sound pressure level. Sound pressure level for sound contained within a restricted frequency band. Unit, decibel (dB); abbreviation, BSPL; symbol, Lpb. Annotation              A band may be identified by its nominal lower and upper band-edge frequencies, or by its nominal midband(...)
  • 7.03  bandpass filter. Filter with a single transmission band or passband with relatively low attenuation extending from a lower band-edge frequency greater than zero to a finite upper band-edge frequency.
  • 7.04  band-elimination filter; band-rejection filter; bandstop filter; notch filter. Filter having a single attenuation band, neither of the band-edge frequencies being zero or infinite.
  • 9.43  base line. In ultrasonic testing, the distance trace (i.e., horizontal) across the A-scan.
  • 3.07  basilar membrane. Fibrous plate extending from the osseous spiral lamina to the spiral ligament on the outer wall of the cochlea. The basilar membrane separates the scala media from the scala tympani and supports the organ of Corti.
  • 8.45  bathythermograph. Plot of water temperature in the ocean as a function of depth. Alternatively, a device used to measure water temperature in the ocean as a function of depth.
  • 9.35  beam axis. Acoustic centerline of an ultrasonic search unit's beam pattern as described by the locus of points of maximum sound pressure in the far field and its extensions into the near field.
  • 9.36  beam spread. Divergence of the ultrasonic beam as a result of propagation through a medium.
  • 6.69  beam width. At a specified frequency, in a specified plane including the beam axis, the angle included between the two directions, one to the left and the other to the right of the axis, at which the angular deviation loss has a specified value. Unit, degree. Annotation             (...)
  • 2.43  beats. Periodic variations that result from the superposition of two sinusoidal quantities of different frequencies, f1 and f2. Beats involve the periodic increase and decrease of amplitude at the beat frequency, (f1 –f2), where f1 is greater than f2.
  • 6.11  beats. Periodic variations that result from the superposition of two simple harmonic quantities of different frequencies f1 and f2. Beats involve the periodic increase and decrease of amplitude at the beat frequency which is the absolute value of (f1 - f2). Annotation         As the(...)
  • 4.05  Békésy audiometer; automatic recording audiometer. Pure-tone audiometer where hearing level variations are under the subject’s control. Signal presentations, hearing level variation, frequency selection or variation, and recording of subject responses are implemented and recorded(...)
  • 3.02 bel. Unit of the level of a power or power-like quantity when the base of the logarithm is ten. Unit symbol, B.
  • 5.13  bending wave. In a plate or bar, transverse wave which is a combination of compressional and rotational waves.
  • 6.12  binaural. Hearing by use of two ears.
  • 6.13  binaural beats. Phenomenon in which two (primary) tones with frequencies f1 and f2, presented separately at each ear, produce a periodic shift in the degree of lateralization (see 6.06); the shift occurs at the beat frequency of the secondary tone, the absolute value of (f1 - f2). Unit,(...)
  • 6.14  binaural diplacusis. Otological condition in which a pure tone evokes a different pitch in each ear.
  • 5.04  binaural trading ratio. Arrival time difference, divided by the sound pressure level difference, of two sound stimuli presented dichotically at the ears such that the apparent sound is lateralized (see 6.06) at the medial plane of the head. Unit, microsecond per decibel (μs/dB).
  • 4.18  Bing test; occlusion test. Test used to distinguish between conductive and sensorineural hearing losses. A vibrating tuning fork is placed on the center of the subject’s forehead and the external acoustic meatus is gently occluded.  If the ear is normal or if there is a pure(...)
  • 8.11  biologic noise; biologics. Underwater acoustic noise generated by aquatic life.
  • 6.80  biological check; diagnostic listening. Listening and perceptually evaluating an acoustic signal to determine if the signal sounds normal. Annotation         Typically conducted on an audiometer on a daily basis. Involves listening for such extraneous signals as hum, static, frequency(...)
  • 8.11  biologic noise; biologics. Underwater acoustic noise generated by aquatic life.
  • 6.25  blocked impedance. For a transducer that converts an electric signal into a mechanical or an acoustic signal, input impedance when the output is connected to a load of infinite impedance.
  • 11.19  bone conduction. Transmission of sound to the inner ear primarily by means of mechanical vibration of the cranial bones.
  • 7.22  bone conduction microphone. Microphone actuated by contact with the cranial bone.
  • 4.19  bone conduction vibrator; bone vibrator. Electromechanical transducer intended to produce the sensation of hearing by vibrating the cranial bones.
  • 7.56  bone vibrator; bone-conduction vibrator. Electromechanical transducer intended to produce the sensation of hearing by vibrating the cranial bones.
  • 4.19  bone conduction vibrator; bone vibrator. Electromechanical transducer intended to produce the sensation of hearing by vibrating the cranial bones.
  • 7.56  bone vibrator; bone-conduction vibrator. Electromechanical transducer intended to produce the sensation of hearing by vibrating the cranial bones.
  • 7.24  close-talking microphone; boom-mounted headset microphone. Microphone designed particularly for use close to the mouth of a talker.
  • 3.08  border cells. A single row of supporting cells lying on the inward side of (medial to) the inner hair cells in the organ of Corti. The upper part (or superior surfaces) of the border cells form part of the reticular lamina. See 3.43.
  • 4.16  auditory brainstem response; brainstem auditory evoked response. Electrical response emanating from the auditory nerve and brainstem, which can be obtained from electrodes on the scalp.
  • 6.15  breathiness. Perception of audible air escape during phonation.
  • 9.33  bubbler. Device used to couple an ultrasonic beam to the test piece.
  • c

  • 9.46  C-scan. In ultrasonic testing, a method of data presentation which yields a plan view of a test object or any discontinuities.
  • 11.06  calculated loudness level. Loudness level calculated by a specified procedure. Unit, phon. Annotation              Such procedures are given in ANSI/ASA S3.4, American National Standard Procedure for the Computation of Loudness of Steady Sounds, and in ISO 532, Acoustics - Method for(...)
  • 7.25  electrostatic microphone; capacitor microphone; condenser microphone. Microphone that consists of a capacitor and whose operation depends upon interaction between its electric field and the change of its electrostatic capacitance when exposed to the pressure of a sound wave.
  • 7.23  carbon microphone. Microphone whose operation depends upon the variations in contact resistance between carbon granules.
  • 9.11  cavitation. Formation, growth, and collapse of gaseous and vapor bubbles in a liquid due to pressure fluctuations, e.g., intense sound waves.
  • 9.12  cavitation noise. Noise produced in a liquid by gaseous or vaporous cavitation.
  • 3.09  cells of Claudius. Cells that lie on the outward side of (lateral to) the cells of Hensen in the organ of Corti.
  • 3.10  cells of Deiter; outer phalangeal cells. Several rows of supporting cells that extend from the basilar membrane and support the outer hair cells in the organ of Corti. Processes from the cells of Deiter form part of the reticular lamina.
  • 3.11  cells of Hensen. Several rows of supporting cells lying on the outward side of (lateral to) the cells of Deiter in the organ of Corti. Processes from the cells of Hensen form part of the reticular lamina.
  • 12.24  cent. Unit denoting a fraction of a semitone. 1 semitone = 100 cents. Annotation              One octave is 1200 cents.
  • 4.20  central hearing loss; central deafness; central auditory processing disorder. Hearing impairment that occurs when there is damage to or dysfunction of the central auditory pathways. Annotation         Damage may occur due to tumors, arteriosclerosis, cerebral hemorrhage, chronic(...)
  • 4.20  central hearing loss; central deafness; central auditory processing disorder. Hearing impairment that occurs when there is damage to or dysfunction of the central auditory pathways. Annotation         Damage may occur due to tumors, arteriosclerosis, cerebral hemorrhage, chronic(...)
  • 4.20  central hearing loss; central deafness; central auditory processing disorder. Hearing impairment that occurs when there is damage to or dysfunction of the central auditory pathways. Annotation         Damage may occur due to tumors, arteriosclerosis, cerebral hemorrhage, chronic(...)
  • 11.47  central masking. Masking that occurs when a signal is presented to one ear and a masking sound to the other ear. Annotation              The site of central masking is thought to be in the higher auditory pathways of the brain.
  • 3.12  cerumen; ear wax. Secretion from the ceruminous glands of the external auditory meatus, typically brown-yellow-black in color.
  • 6.35  specific acoustic impedance, characteristic acoustic impedance. At a point in a medium, the complex ratio of sound pressure to particle velocity in the direction of the wave propagation. Unit, pascal per (meter per second) [Pa/(m/s)]. Annotation 1          In the MKS system of units,(...)
  • 6.39  characteristic impedance. Product of the equilibrium density and speed of sound in a medium. Unit, pascal per (meter per second) [Pa/(m/s)].
  • 12.28  chromatic scale. A series of tones, the core of which is comprised of the 12 semitones within the octave, denoted by the note names C, C#/Db, D, D#/Eb, E, F, F#/Gb, G, G#/Ab, A, A#/Bb and B. The scale repeats in successive octaves. Annotation              See Table 3.
  • 5.19  circularly polarized sound wave. Transverse wave in an elastic medium in which the displacement vector at any point rotates about a point with constant angular velocity and constant magnitude. Annotation              A circularly polarized wave is equivalent to two superposed(...)
  • 7.103  circumaural earphone. Earphone having a cavity large enough to cover the region of the head including and surrounding the pinna.
  • 7.16  circumaural earphone.  Earphone which encloses the pinna and rests on the surrounding surface of the head. Annotation            Contact with the head is normally maintained by compliant cushions. Circumaural earphones may touch but should not significantly compress the pinna.
  • 6.16  click. Acoustic signal typically produced by exciting an earphone or speaker with a brief duration electrical pulse. Annotation 1      For an appropriately brief pulse (e.g., 0.1 ms), the acoustic spectrum of a click is primarily determined by the transfer function of the(...)
  • 7.24  close-talking microphone; boom-mounted headset microphone. Microphone designed particularly for use close to the mouth of a talker.
  • 6.61  close-talking sensitivity. Of a microphone, for a specified frequency, quotient of the output open-circuit voltage by the sound pressure, at the position previously occupied by the microphone reference point, in the undisturbed sound field produced by a specified source that simulates(...)
  • 3.13  cochlea. Spirally coiled, tapered cavity within the temporal bone consisting of about two and three-eighths turns in humans. The cochlea contains the receptor organs essential to hearing. Part of the osseous labyrinth, see 3.44.
  • 3.14  cochlear duct. That portion of the membranous labyrinth contained within the cochlea. The cochlear duct is an endolymph-filled duct following the spiral shape of the cochlea, and contains the organ of Corti. The cavity of the cochlear duct is called the scala media.
  • 7.06  cochlear implant. Prosthesis which is surgically implanted in the temporal bone in patients with profound or total hearing loss in order to restore hearing by direct electrical stimulation of the surviving nerve fibers.
  • 4.21  cochlear microphonic. Alternating current electric potential generated within the cochlea in response to a sound stimulus. At low and moderate sound pressures the amplitude of the cochlear microphonic is proportional to the instantaneous sound pressure of the stimulus, and is a faithful(...)
  • 3.15  cochlear nerve. Anterior branch of the vestibulocochlear (8th cranial) nerve. The cochlear nerve arises from the nerve cells of the spiral ganglion of the cochlea and terminates in the dorsal and ventral cochlear nuclei in the brain stem. It contains both afferent and efferent nerve fibers.
  • 3.16  cochlear structures. Term used collectively to describe the partitions of the scala media, the most significant of which are the basilar membrane, the tectorial membrane, Reissner’s membrane, and the organ of Corti.
  • 7.09  coherence. Number calculated from the cross spectrum, ranging from 0 to 1 indicating to what degree the output of a system is linear, causal, and time invariant, with respect to the input. Annotation 1      The coherence, γ2, is calculated as where GAB is the cross spectrum, GAA is(...)
  • 10.61  coincidence effect. Greater sound transmission when sheet material bending (flexural) wave wavelength equals wavelength of airborne sound waves of the same frequency and incident at an acute angle as projected onto the sheet surface.
  • 9.31  collimator. Device used to control the size and direction of an ultrasonic beam.
  • 6.17  combination tone; aural combination tone. Secondary tone that can be perceived when two loud primary tones are presented simultaneously. The secondary tone may be a difference tone or a summation tone. Several combination tones may be produced from a single pair of primary tones. Unit,(...)
  • 3.20  community noise equivalent level. On a given day of the week, twenty-four hour-average A-weighted sound level after addition of 5 decibels to sound levels from 1900 hours to 2200 hours, and after addition of 10 decibels to sound levels from midnight to 0700 hours and from 2200 hours to(...)
  • 6.18  comodulation masking. In simultaneous masking, the difference in decibels in detectability of a signal when the masker envelope is not coherent relative to when the masker envelope is coherent. Unit, decibel (dB).
  • 6.04  complex parameter.
  • 12.03  complex tone. Sound wave containing sinusoidal components of different frequencies.
  • 2.57  compliance. Reciprocal of stiffness. Unit, meter per newton (m/N). Annotation              See also 6.33.
  • 6.33  compliance. Reciprocal of stiffness. Unit, meter per newton (m/N). Annotation              See also 2.57.
  • 5.09  compressional wave. Wave in an elastic medium that causes an element of the medium to change its volume without undergoing rotation. Annotation 1          Mathematically, a compressional wave is one for which the velocity field has zero curl. Annotation 2          A compressional(...)
  • 7.07  compressor. Amplifier in which the amount of amplification (gain) decreases automatically as the signal level increases. See also 7.05.
  • 4.06  computer-controlled audiometer. Audiometer in which the test procedure is controlled by a computer or microprocessor.
  • 3.17  concha. A depression in the external ear surrounding the meatus bounded in front by the tragus (3.69) and above, to the rear and below by cartilaginous ridges. It and the boundary ridges contribute to sound source direction and location. It also aids fitting and retention of ear molds in(...)
  • 7.25  electrostatic microphone; capacitor microphone; condenser microphone. Microphone that consists of a capacitor and whose operation depends upon interaction between its electric field and the change of its electrostatic capacitance when exposed to the pressure of a sound wave.
  • 4.22  conductive hearing loss. Impairment of hearing that occurs when there is interference with the transmission of sound waves through the external or middle ear. Annotation 1      Some causes of conductive hearing loss are: wax (cerumen) accumulation in the external ear; inflammation in(...)
  • 7.84  cone loudspeaker. Loudspeaker unit in which the radiating unit has the shape of a cone.
  • 7.99  conical horn. Horn whose cross-sectional area increases as the square of the axial length.
  • 6.16  conjugate impedances. Impedances for which the real components (resistances) are equal and for which the imaginary components (reactances) are equal but of opposite sign. Annotation              Conjugate impedances are expressed by conjugate complex quantities.
  • 6.19  consonance. Phenomenon in which tones presented together produce a blended or pleasant sensation. See 6.28.
  • 6.20  continuity effect. Sound pressure level at which the perception of a signal changes from just interrupted to just continuous. Unit, decibel (dB).
  • 6.21  continuous discourse. Speech produced continuously without significant pauses.
  • 2.50  continuous spectrum. Spectrum of a wave for which the components are continuously distributed over a frequency region.
  • 4.16  continuous system; distributed system. System considered to have an infinite number of possible independent displacements. The configuration is specified by a function of a continuous spatial variable or variables, in contrast to a discrete or lumped-parameter system which requires only(...)
  • 9.47  continuous wave. Continuous flow of ultrasonic waves as opposed to pulsed waves.
  • 6.23  contralateral masking. Decrease in the detectability of a signal presented to one ear due to the simultaneous presentation of a sound to the other ear.
  • 3.18  contralateral reflex; crossed reflex. Middle-ear muscle reflex that is elicited in the ear contralateral (opposite) to the stimulus ear. When the stimulus is acoustic, the resulting middle-ear muscle reflex is sometimes called contralateral acoustic reflex or crossed acoustic reflex.
  • 9.40  control echo. Reference signal from a constant reflecting surface such as a back reflection.
  • 8.32  convergence zone. Region of the sea close to the surface where sound rays are concentrated at great range from the source by upward refraction over great depths.
  • 6.22  conversational speech. Speech produced with significant pauses, as is likely to occur in a conversation involving two or more people.
  • 5.05  correct detection. A “signal-plus-noise” response (output) following a “signal-plus-noise” stimulus (input) occurring in a detection task during a specified observation interval. Annotation 1      A correct detection and a false dismissal are mutually exclusive for a(...)
  • 5.06  correct dismissal; true dismissal. A “noise-alone” response (output) following a “noise-alone” stimulus (input) occurring in a detection task during a specified observation interval. Annotation 1      A correct detection and a false alarm are mutually exclusive for a “noise-alone”(...)
  • 4.31  Coulomb damping; dry friction damping. Dissipation of energy that occurs when the motion of a particle in a vibrating system is resisted by a force for which the value is constant and independent of displacement and the magnitude of velocity, and for which the direction is opposite to(...)
  • 9.32  couplant. Substance used between the search unit and the test surface to permit or improve the transmission of acoustical energy.
  • 4.22  coupled modes. Modes of vibration that are not independent but which influence one another because of energy transfer from one mode to the other.
  • 6.77  transducer voltage coupling loss; coupling loss. Of an electroacoustic transducer consisting of a sound-receiving element (e.g., microphone or hydrophone) and associated electrical system, for a specified frequency, ten times the logarithm to the base ten of the ratio of the(...)
  • 7.10  crest factor. Ten times the logarithm to the base ten of the square of the wideband peak amplitude of a signal to the time-mean-square amplitude over a stated time period. Abbreviation, CF; unit, decibel (dB). Annotation 1      The amplitude of an electrical signal is usually(...)
  • 6.24  critical band. (a) For loudness, that frequency band within which the loudness of a band of continuously distributed sound of constant sound pressure level is independent of its bandwidth (see also 6.31); (b) For masking (see C11.31), that frequency band of sound, being part of a(...)
  • 4.34  critical damping. Minimum viscous damping that will allow a displaced system to return to its initial position without oscillation. Annotation              The quality factor of a critically damped system is ½ and the damping ratio is 1.
  • 10.23  critical distance. Distance at which the sound pressure level of the direct and the reverberant sound fields are equal when dealing with a directional source. Unit, meter (m). Annotation                      The critical distance is dependent on the geometry and absorption of the(...)
  • 6.25  critical ratio for masking (Fletcher critical bandwidth). For a pure tone just audible in the presence of a continuous noise of constant spectral density, the ratio of the mean-square sound pressure of the tone to the noise spectral density . Unit, hertz (Hz). See also 6.24 and(...)
  • 4.08  critical speed. Speed of a rotating system that corresponds to a resonance frequency of a system.
  • 7.29   cross correlation; interaural cross correlation. Covariance of signals vs. time shift; i.e., the degree of similarity of a reference signal and a time-shifted signal, as a function of time shift or delay. Annotation 1      The cross correlation is calculated as where T is the(...)
  • 3.19  cross hearing. Phenomenon in which sounds presented to one ear are coupled around or through the head and are heard in the other ear.
  • 3.18  contralateral reflex; crossed reflex. Middle-ear muscle reflex that is elicited in the ear contralateral (opposite) to the stimulus ear. When the stimulus is acoustic, the resulting middle-ear muscle reflex is sometimes called contralateral acoustic reflex or crossed acoustic reflex.
  • 7.94  crossover frequency. Of an electrical dividing network, frequency at which equal electric powers are delivered to each of the adjacent electrical circuits when all circuits are terminated in specified impedances. Unit, hertz (Hz).
  • 8.34  crossover range. Range within which the propagation loss caused by divergence is equal to that caused by absorption.
  • 2.11  cycle. Complete sequence of values of a periodic quantity that occurs during a period.
  • 5.16  cylindrical wave. Wave in which the wavefronts are coaxial cylinders.
  • d

  • 5.07  detectability index; d′. The square root of the ratio of the difference between the mean signal plus noise power and the mean noise power to the variance of the noise at the point where the detection decision is made. The detector may be a subject or any other decision-making(...)
  • 4.23  damage risk criteria. For hearing, the level of sound exposure which a population may receive for a specified time with a specified risk of hearing loss. See also 4.32 and 4.45. Annotation         Damage risk criteria are often specified in terms of a frequency-weighted sound pressure(...)
  • 4.10  damped natural frequency. Frequency of free vibration of a damped linear system. Unit, hertz (Hz). Annotation              Oscillation of a damped system may be considered periodic (see 2.09) in the limited sense that the time interval between zero crossings in the same direction is(...)
  • 2.74  damping. Dissipation of energy with time or distance. Respective unit, joule per second (J/s) or watt (W), or joule per meter (J/m).
  • 4.30  damping ratio. For a system with viscous damping, the ratio of viscous damping to critical damping, as defined in 4.32 and 4.34, respectively.
  • 3.17  day-average sound level. Time-average frequency-weighted sound level between 0700 and 2200 hours. Unit, decibel (dB); abbreviation, DL; symbol, Ld.  
  • 3.19  day-night-average sound level. On a given day of the week, twenty-four-hour average frequency-weighted sound level after addition of 10 decibels to levels from midnight to 0700 hours and from 2200 hours to midnight. Unit, decibel (dB); abbreviation, DNL; symbol, Ldn.  
  • 10.27  dead room. Room characterized by a relatively large amount of sound absorption and a relatively short reverberation time.
  • 9.41  dead zone. Distance from the surface of the test object to the nearest depth that can be inspected.
  • 4.24  deafness. Condition caused by a hearing loss which results in a person’s inability to use auditory information effectively for communication or other daily activities, even with amplification.
  • 12.18  decay. Term used to designate the final portion of a sound.
  • 10.08  decay rate. At a stated frequency, time rate at which sound pressure level decreases in a room. Unit, decibel per second (dB/s). Annotation              Decay rate, d, in a reverberant room is related to reverberation time T by T = 60 dB / d.  
  • 12.20  decay time. The amount of time taken for the amplitude of a sound to fall from one specified value to another. Annotation              The tem is generally used to refer to the final portion of a sound.
  • 3.03 decibel. Unit of the level of a power or power-like quantity when the base of the logarithm is the tenth root of ten. Unit symbol, dB.  
  • 8.36  deep isothermal layer. Ocean layer below the thermocline extending to the bottom of the sea which has a nearly constant temperature and in which the velocity of sound increases with depth.
  • 8.37  deep scattering layer. Layer of scatterers, generally biological, located at a certain depth and which returns echoes.
  • 9.07  sonic cleaning; degreasing. Cleaning of contaminated materials by the action of intense sound in the liquid in which the material is immersed, usually involving cavitation, see 9.11.
  • 4.13  degrees of freedom. Minimum number of generalized coordinates required to define completely the positions of all parts of a mechanical system at any instant in time. In general, the number of degrees of freedom is equal to the number of independent generalized linear or angular(...)
  • 9.38  delay line. Liquid or solid material placed in front of a projecting transducer to introduce a time delay to an ultrasonic wave train.
  • 9.39  delayed sweep. Horizontal sweep whose start is delayed in order to prevent the appearance of unwanted early response information on the screen.
  • 11.33  detection. In acoustics, determination of the presence of an acoustic signal.
  • 5.08  detection-choice experiment. Trials in which a stimulus is presented in exactly zero or one of the n observation intervals and the subject either identifies the interval during which the stimulus was perceived or reports that no stimulus was perceived. Annotation         Usually the(...)
  • 11.34  detection differential; recognition differential. For a specified aural detection system, that amount by which the signal level exceeds the noise level presented to the ear for a stated probability of detection. Unit, decibel (dB). Annotation              The bandwidth of the system(...)
  • 6.80  biological check; diagnostic listening. Listening and perceptually evaluating an acoustic signal to determine if the signal sounds normal. Annotation         Typically conducted on an audiometer on a daily basis. Involves listening for such extraneous signals as hum, static, frequency(...)
  • 5.09  dichotic. Condition in which the sound stimulus presented at one ear differs from the sound stimulus presented at the other ear. Annotation         Stimuli may differ in sound pressure, frequency, phase, time, duration, and bandwidth.
  • 5.10  differential threshold; difference limen; just-noticeable difference; JND. Minimum change in a stimulus that can be correctly judged as different from a reference stimulus in a specified fraction of trials. Annotation         Instead of the method of constant stimuli, which is implied(...)
  • 11.35  difference limen for loudness. For an individual listener and a sound of specified frequency under specified conditions, the minimum change of sound pressure level and frequency that is just noticed as a change in loudness. Unit, decibel (dB).
  • 11.36  difference limen for pitch. For an individual listener and a sound of specified frequency under specified conditions, the minimum fractional change of the frequency and sound pressure level that is just noticed as a change of pitch.
  • 6.26  difference tone. Combination tone (see 6.17) with a frequency equal to the difference between the frequencies of two primary tones or their harmonics. Unit, hertz (Hz).
  • 5.48 differential scattering cross section. Ratio of the free-field scattered sound power per unit solid angle in a specified direction to the intensity of the incident propagating plane sound wave. Unit, meter squared per steradian (m2/sr) Symbol, Sd.
  • 5.10  differential threshold; difference limen; just-noticeable difference; JND. Minimum change in a stimulus that can be correctly judged as different from a reference stimulus in a specified fraction of trials. Annotation         Instead of the method of constant stimuli, which is implied(...)
  • 5.46  diffracted wave. Wave for which the front has been changed in direction by an obstacle or other nonhomogeneity in a medium, other than by reflection or refraction.
  • 5.45  diffraction. Process that produces a diffracted wave.
  • 6.54  diffraction factor. For a specified frequency and a specified direction of sound incidence, ratio of sound pressure acting on the part of the transducer designed to receive sound, to the free-field sound pressure that would exist at the same place in the absence of the transducer.
  • 10.22  diffuse-field distance. Distance from the acoustic center of a sound source at which the time-mean-square sound pressure of the direct field in a specified direction is equal to the time-mean-square sound pressure of the reverberant sound in the room containing the source. Unit, meter (m).
  • 6.64  diffuse-field sensitivity. Of an electroacoustic transducer for sound reception, for a specified frequency, quotient of the root-mean-square open-circuit output voltage due to sound waves arriving more or less simultaneously with equal probability from all directions, by the(...)
  • 10.21  diffuse sound field. Sound field in which the time average of the mean-square sound pressure is everywhere the same and the flow of acoustic energy in all directions is equally probable, such that the time-averaged acoustic intensity is zero.
  • 5.12  diotic. Condition in which the sound stimulus presented at each ear is identical.
  • 6.27  diplacusis. See 6.14 (binaural diplacusis) and 6.48 (monaural diplacusis).
  • 10.20  direct sound field. That portion of a sound field, in an enclosure, which arrives from the sound source without having undergone any reflection.
  • 10.24 direct-to-reverberant ratio. At a given location, the ratio of the sound pressure level of a direct sound from a directional source to the reverberant sound pressure level simultaneously incident to the same location. Unit, decibels (dB). Annotation              The critical distance(...)
  • 6.67  directional gain; directivity index. Of a transducer, ten times the logarithm to the base ten of the directivity factor. Unit, decibel (dB). Annotation              Directional gain may be given for directions other than that of the principal axis if the direction is specified.
  • 7.12  directional hearing aid. Hearing aid equipped with a fixed or adaptive directional microphone system. See also 7.13.
  • 7.17  directional microphone. Microphone whose response is dependent on the direction of the incident sound.
  • 7.13  directional microphone. Microphone whose response is dependent on the direction of the incident sound.
  • 6.65  directional pattern. Description, presented graphically in polar coordinates or in a table, of the sensitivity level of an electroacoustic transducer as a function of the direction of propagation of the radiated or incident sound, in a specified plane and at a specified frequency.
  • 6.66  directivity factor. (a) Of an electroacoustic transducer for sound emission, at a specified frequency, ratio of the time-mean-square free-field sound pressure at a fixed point on the principal axis, to the time-mean-square sound pressure over the surface of a sphere concentric with the(...)
  • 6.67  directional gain; directivity index. Of a transducer, ten times the logarithm to the base ten of the directivity factor. Unit, decibel (dB). Annotation              Directional gain may be given for directions other than that of the principal axis if the direction is specified.
  • 5.11  discrimination. (1) Process or ability to differentiate one signal from another. (2) Distinction between signal quantities of different values.
  • 2.51  displacement. Vector quantity that specifies the change of position of a body or particle. Units, meter (m), radian (rad).
  • 10.36  dissipation. Conversion of sound energy into heat.
  • 10.37  dissipation coefficient. Ratio of sound energy dissipated as heat to the energy of the incident sound wave.
  • 6.28  dissonance. Phenomenon in which tones presented together produce a harsh or unpleasant sensation. See 6.19.
  • 2.44  distortion. (a) Any change in the shape of a waveform not resulting from linear addition of another waveform. (b) Undesired change of waveform. Annotation 1          Multiplication by a constant and time delay do not constitute distortion. Noise and certain desired changes in(...)
  • 4.16  continuous system; distributed system. System considered to have an infinite number of possible independent displacements. The configuration is specified by a function of a continuous spatial variable or variables, in contrast to a discrete or lumped-parameter system which requires only(...)
  • 5.38  divergence loss; spreading loss. That part of the transmission loss resulting from divergence of sound waves in accordance with the geometry of the system (e.g., spherical waves emitted by a point source). Unit, decibel (dB). Annotation              For spherical waves,(...)
  • 7.93  dividing network. Electrical device to control the ranges of frequency applied to various loudspeakers.
  • 7.85  dome loudspeaker. Loudspeaker unit in which the radiating unit is in the form of a spherical segment.
  • 5.28  Doppler effect. Phenomenon evidenced by the change in the observed frequency of a wave in a transmission system caused by a time rate of change in the effective length of the path of travel between the source and the point of observation.
  • 5.29  Doppler shift. Change in the observed frequency of a wave as a result of the Doppler effect Unit, hertz (Hz).
  • 11.50  remote masking; downward masking. Phenomenon in which an intense band of noise raises the threshold of hearing for sounds lower in frequency than that of the noise.
  • 9.13  drilling. Process of cutting or shaping materials with an abrasive slurry driven by a reciprocating tool usually attached to an electromechanical transducer.
  • 7.80  loudspeaker unit; driver. Electroacoustic transducer used as a loudspeaker without its associated acoustical enclosure or baffle.
  • 6.19  driving point impedance. Quotient of a dynamic acoustic variable (e.g., force, sound pressure) at one point in a system by the resulting kinematic acoustic variable (e.g., vibration velocity or particle velocity) at the same point.
  • 3.71  tympanic membrane; eardrum; drum membrane. Conical shaped translucent membrane that separates the external acoustic meatus from the middle ear cavity. The handle of the malleus is attached to the tympanic membrane medially. Annotation            The amplitude of displacement of the(...)
  • 4.31  Coulomb damping; dry friction damping. Dissipation of energy that occurs when the motion of a particle in a vibrating system is resisted by a force for which the value is constant and independent of displacement and the magnitude of velocity, and for which the direction is opposite to(...)
  • 9.30  dual search unit. Search unit containing two side-by-side elements with one serving as a transmitter and the other as a receiver.
  • 10.64  duct lining. Layer of porous material placed on the inner surface of a duct to attenuate sound that propagates through the duct.
  • 4.47  duration of shock pulse. Time required for the acceleration of a pulse to rise from some stated fraction of the maximum value and decay to the same value.
  • 6.76  dynamic range (of a spectrum). The ratio of the maximum to minimum values that occur across the bandwidth of the spectrum.
  • 6.75  dynamic range (of an acoustic quantity). The ratio of the maximum to minimum values that occur.
  • 6.74  dynamic range (of an electroacoustic transducer). The range in operating amplitude between a minimum threshold level that is set by the inherent noise and an upper limit set by tolerance in nonlinearities or distortion. Unit, decibel (dB). Annotation 1                  The dynamic(...)
  • 4.26  dynamic vibration absorber; tuned damper. Device for reducing vibration of a primary system by the transfer of energy to an auxiliary system which is tuned to the frequency of the vibration. The force exerted by the auxiliary system is opposite in phase to the force acting on the primary(...)
  • e

  • 3.20  ear. Organ of hearing. In humans it consists of three parts: external (outer) ear, middle ear or tympanic cavity, and the inner ear or labyrinth.
  • 3.23  external acoustic meatus; external auditory meatus; external auditory canal; ear canal. Canal that directs sound from the auricle to the tympanic membrane.
  • 7.52  ear simulator. Device which approximates the acoustic transfer impedance of the inner part of the human ear canal, from the tip of an ear mold to the eardrum. Annotation 1          Ear simulators fall into the following four categories: acoustic couplers; artificial(...)
  • 3.12  cerumen; ear wax. Secretion from the ceruminous glands of the external auditory meatus, typically brown-yellow-black in color.
  • 3.71  tympanic membrane; eardrum; drum membrane. Conical shaped translucent membrane that separates the external acoustic meatus from the middle ear cavity. The handle of the malleus is attached to the tympanic membrane medially. Annotation            The amplitude of displacement of the(...)
  • 7.14  earmold. Device used to couple an electroacoustic transducer to the ear. An earmold is a prefabricated device that fits in the ear canal of the listener that is typically used with hearing aids.
  • 7.62  earmuff. Hearing protector worn over the pinna of an ear.
  • 7.45  earphone. Electroacoustic transducer intended to be closely coupled acoustically to the ear.
  • 7.15  earphone. Electroacoustic transducer intended to be closely coupled to the ear.
  • 7.61  earplug. Hearing protector that is inserted into the ear canal.
  • 6.29  ears covered. Listening situation where both ears are covered simultaneously by earphones mounted in supra-aural cushions held in place by a headband.
  • 6.30  ears not covered. Listening situation where one or both ears are not covered with an earphone in a supra-aural cushion. Annotation         An example of a situation where both ears are not covered would be for determining hearing threshold levels for acoustic stimuli. An example where(...)
  • 2.41  echo. Wave that has been reflected or otherwise returned with sufficient magnitude and delay to be detectable as a wave distinct from that directly transmitted. Annotation 1                  An echo is separately localizable. Annotation 2          See also 10.29.
  • 10.29  echo. Reflected sound wave that arrives with sufficient magnitude and time delay as to be distinguishable as a repetition of the direct sound. Annotation              See also 2.41.
  • 6.51  effective acoustic center; virtual acoustic center. For an electroacoustic transducer used for sound emission, in a specified direction, for a specified frequency and range of distances, position of the effective or virtual point source from which sound pressure varies inversely as(...)
  • 4.25  effective masking. Occurs when the masker is just strong enough to prevent a listener from hearing the test stimulus when stimulus and masking signals are presented simultaneously to the same ear.
  • 4.26  effective masking level for pure tones. Sound pressure level of a band of noise whose geometric center frequency coincides with that of a specific pure tone that masks the pure tone to 50% probability of detection. The hearing level reference of the masking signal is equal to that of the(...)
  • 4.27  effective masking level for speech. Sound pressure level of a specified masking noise that masks a speech signal to 50 percent probability of recognition. The hearing level reference of the masking signal is equal to that of a speech recognition threshold. Annotation         On a(...)
  • 11.17  effective perceived noise level. Level of the time integral of the antilogarithm of one-tenth of tone-corrected perceived noise level over the duration of an aircraft flyover, the reference duration being 10 s. Unit, decibel (dB); abbreviation, EPNL; symbol,(...)
  • 2.61  effective sound pressure. At a point, root-mean-square of a sound pressure signal at the time of observation. Unit, pascal (Pa).
  • 2.72  efficiency. Ratio of the useful output of a device to its total input. Output and input are physical quantities that may be stored, transferred, or transformed. Annotation              Unless specifically stated otherwise, the term "efficiency" means efficiency with respect to energy(...)
  • 5.13  efficiency (of a detector). Signal-to-noise energy ratio required by an ideal detector divided by the signal-to-noise energy ratio required by the detector under study when the performance of the detectors is the same. Symbol, η. Annotation         The detector may be a human subject(...)
  • 3.01  acoustic nerve; auditory nerve; eighth cranial nerve; vestibulocochlear nerve. Nerve, consisting of two sets of fibers: the anterior branch or cochlear nerve and the posterior branch or vestibular nerve. Afferent fibers of the nerve conduct neural signals from the inner ear to the(...)
  • 7.26  electret microphone; prepolarized microphone. Electrostatic microphone in which the electric field results from an internal permanent charge on one of the capacitor’s electrodes.
  • 6.48  electroacoustic transducer. Transducer designed to receive an electric input signal and to supply acoustic output signals, or vice versa.
  • 7.86  electrodynamic loudspeaker; moving-conductor loudspeaker; moving-coil loudspeaker. Loudspeaker designed to operate by the motion of a conductor or a coil carrying a varying current in a steady magnetic field.
  • 7.27  electrodynamic microphone; moving-conductor microphone. Microphone whose operation depends upon the generation of an induced current in a conductor moving in a magnetic field.
  • 7.87  electromagnetic loudspeaker. Loudspeaker designed to operate by the variations in the reluctance of a magnetic field.
  • 7.28  electromagnetic microphone. Microphone whose operation depends upon the variation in the reluctance of a magnetic field.
  • 6.34  electromechanical transducer. Transducer designed to receive an electric input signal and to furnish a mechanical output signal, or vice versa.
  • 7.29  electronic microphone. Microphone whose operation depends upon the variations of electronic flux by the motion of one of the electrodes of a transistor.
  • 11.40  electrophonic effect. Sensation of hearing produced when an alternating current of suitable frequency and magnitude from an external source is passed through a person.
  • 7.49  electrostatic actuator. Device comprising an auxiliary electrode that allows the application of an electrostatic force to the diaphragm of a microphone in order to obtain a calibration over a limited range of frequencies.
  • 7.88  electrostatic loudspeaker. Loudspeaker designed to operate by electrostatic forces.
  • 7.25  electrostatic microphone; capacitor microphone; condenser microphone. Microphone that consists of a capacitor and whose operation depends upon interaction between its electric field and the change of its electrostatic capacitance when exposed to the pressure of a sound wave.
  • 9.18  electrostriction. Phenomenon wherein some dielectric materials experience an elastic strain when subjected to an electric field, this strain being independent of the polarity of the field.
  • 5.18  elliptically polarized sound wave. Transverse wave in an elastic medium in which the displacement vector at any point rotates about the point, and has a value that varies as the radius vector of an ellipse. Annotation              An elliptically polarized wave is equivalent to two(...)
  • 3.21  endocochlear potential. Resting electric polarization of the endolymph of the scala media, positive relative to the perilymph in the scala vestibuli and scala tympani and also to tissues outside of these canals. Abbreviation, EP. Unit, volt. Symbol, V.
  • 3.22  endolymph. Fluid, high in potassium and low in sodium content, contained within the membranous labyrinth.
  • 9.03  ensonify. To expose an object to a sound field in a process or analysis.
  • 11.08  equal-loudness-level contour. Curve that shows, as a function of frequency, the sound pressure level required to produce a given loudness for a listener having normal hearing, listening to a specified kind of sound in a specified manner.
  • 12.12  equal temperament. The division of the octave into equal increments of pitch. The term usually refers to the division of the octave into twelve equal parts, called semitones. See 12.23.
  • 12.29  equally tempered scale; scale of equal temperament. Musical scale formed by division of the octave into a number (usually twelve) of equal intervals. Annotation              See Table 2.
  • 6.40  equilibrium density.
  • 10.06  equivalent absorption area. Of a surface or of an object, area of a surface having a sound power absorption coefficient of unity that would absorb the same amount of sound power in a reverberation room with a diffuse sound field as the object or the surface. In the case of a surface,(...)
  • 3.15  time-averaged sound level; equivalent continuous sound level.    
  • 6.72  equivalent noise pressure; inherent noise pressure. Of an electroacoustic transducer or system used for sound reception, the root-mean-square sound pressure of a sinusoidal plane progressive wave, which, if propagated parallel to the principal axis of the transducer, would produce an(...)
  • 6.73  equivalent noise pressure level. Of a transducer, twenty times the logarithm to the base ten of the ratio of the root-mean-square of the equivalent noise pressure to the stated reference pressure. Unit, decibel (dB).
  • 6.31  ERB; equivalent rectangular bandwidth. The bandwidth of an ideal rectangular filter which has the same maximum height and area of the assumed critical band filter. See also 6.24. Annotation 1         The Equivalent Rectangular Bandwidth (ERB) of the auditory filter for normally(...)
  • 4.28  equivalent speech recognition threshold sound pressure level. For a specified speech signal, type of transducer, and manner of signal presentation, the sound pressure level produced by a transducer in a specified acoustic coupler when the voltage applied to the transducer is that which(...)
  • 4.12  equivalent system. One that may be substituted for another system for the purposes of analysis. Annotation              Many types of equivalence are common in vibration and shock technology: (1) equivalent stiffness; (2) equivalent damping; (3) torsional system equivalent to a(...)
  • 4.30  equivalent threshold force level (monaural listening). At a specified frequency, for a specified configuration of a bone vibrator on a specified mechanical coupler, the vibration force level set up by the bone vibrator in a specified coupler when the bone vibrator is activated by that(...)
  • 11.23  equivalent threshold level. For monaural listening, at a specified frequency, for a specified type of transducer, and for a stated force of application of the transducer to the human head, the vibration level, or sound pressure level set up by that transducer in a specified coupler or(...)
  • 4.29  equivalent threshold level (monaural earphone listening). At a specified frequency, for a specified type of earphone, and for a stated force of application of the earphone to a human ear, the sound pressure level set up by the earphone in a specified coupler or artificial ear when the(...)
  • 4.33  equivalent viscous damping. Value of viscous damping assumed for the purpose of analysis of a vibratory motion, such that the dissipation of energy per cycle at resonance is the same for either the assumed or actual damping force.
  • 6.31  ERB; equivalent rectangular bandwidth. The bandwidth of an ideal rectangular filter which has the same maximum height and area of the assumed critical band filter. See also 6.24. Annotation 1         The Equivalent Rectangular Bandwidth (ERB) of the auditory filter for normally(...)
  • 5.14  error of anticipation; error of expectation. Constant error resulting from the tendency of a subject to change a given response to a sequence of stimuli in a monotonically increasing or decreasing magnitude before it is appropriate.
  • 5.14  error of anticipation; error of expectation. Constant error resulting from the tendency of a subject to change a given response to a sequence of stimuli in a monotonically increasing or decreasing magnitude before it is appropriate.
  • 5.15  error of habituation. Constant error resulting from the tendency of a subject to continue a given response to a sequence of stimuli of monotonically increasing or decreasing magnitude longer than is appropriate.
  • 3.05  auditory tube; Eustachian tube. Tube that connects the middle ear with the nasal part of the pharynx. The auditory tube serves to equalize air pressure on the two sides of the tympanic membrane, i.e., middle ear pressure and ambient pressure.
  • 4.31  evoked otoacoustic emission. See C11.39.
  • 4.32  exchange rate. The level in decibels that the sound level of daily noise exposure must increase (decrease) to preserve a selected measure hearing damage risk when the exposure duration is halved (doubled). See also 4.23, 4.45 and A3.22.
  • 2.81  excitation; stimulus. External force (or other input) applied to a system that causes the system to respond in some way.
  • 7.100  exponential horn. Horn whose cross sectional area increases exponentially with axial length.
  • 3.12  exponential-time-weighted sound level.
  • 7.69  exponential-time-weighting sound level meter. Device for measuring the level of an exponential-time-weighted and frequency-weighted sound pressure signal at a stated instant of time. Annotation 1          See also 3.12. Annotation 2          Exponential-time-weighting sound level(...)
  • 4.07 extended high-frequency audiometer. Audiometer providing test frequencies in the 8000 to 16000 Hz range.
  • 3.23  external acoustic meatus; external auditory meatus; external auditory canal; ear canal. Canal that directs sound from the auricle to the tympanic membrane.
  • 3.23  external acoustic meatus; external auditory meatus; external auditory canal; ear canal. Canal that directs sound from the auricle to the tympanic membrane.
  • 3.23  external acoustic meatus; external auditory meatus; external auditory canal; ear canal. Canal that directs sound from the auricle to the tympanic membrane.
  • 3.24  external ear; outer ear. Combination of the auricle and the external acoustic meatus.
  • 6.32  externalization. Perception that allows a subject to identify a sound as being away from the head and toward the sound source. Annotation         When the apparent position of the sound source is determined, the perception is called localization; see 6.07.
  • 10.09  Eyring absorption coefficient. Sound power absorption coefficient attributed to a surface as by the Eyring reverberation-time equation.
  • f

  • 2.77  fall time. Time for a physical quantity to fall from 90% of its initial value to 10% of its initial value. Annotation              For an exponentially decaying quantity the fall time is approximately 2.1972 times the time constant.
  • 5.16  false alarm. Event that occurs in a detection situation, during a specified observation interval, when a “signal-plus-noise” response (output) follows a “noise-alone” stimulus (input). Annotation 1      A false alarm and a correct dismissal are mutually exclusive for a “noise-alone”(...)
  • 5.17  false decision. Either a false alarm or a false dismissal in a detection situation, or an incorrect identification in a (multiple) forced-choice situation.
  • 5.18  false rejection; miss. Event that occurs in a detection situation, during a specified observation interval, when a “noise-alone” response (output) follows a “signal-plus-noise” stimulus (input). Annotation         A false dismissal and a correct detection are mutually exclusive for a(...)
  • 6.33  falsetto. The vocal register occupying the frequency range just above the modal voice register produced by the vibration of the ligamentous edges of the vocal cords, in whole or in part. See 6.87.
  • 4.17  auditory fatigue; fatigue; post-stimulatory fatigue. Temporary decrease in hearing sensitivity resulting from a previous auditory stimulus. Auditory fatigue produces a temporary threshold shift that may last for seconds, minutes, hours, or days. Annotation            Auditory fatigue(...)
  • 7.01  filter; wave filter. Device for separating waves on the basis of their frequency. Annotation 1          The device can be a mechanical, electrical or acoustical filter. Annotation 2          A filter is typically all-pass, low-pass, high-pass, bandpass or band-stop. Annotation(...)
  • 7.12  filter attenuation band. Continuous range of frequencies over which the filter introduces an attenuation equal to or less than a specified maximum. Unit, hertz (Hz).
  • 7.10  filter bandwidth. Difference between the upper and lower exact band-edge frequencies and, hence, the nominal width of the passband. Unit, hertz (Hz).
  • 7.06  filter bandwidth designator. 1/b, the reciprocal of a positive integer (including 1) that specifies a fraction of an octave band for a set of contiguous bandpass filters.
  • 7.11  filter bandwidth quotient. Figure-of-merit measure of the relative bandwidth, or sharpness, of a bandpass filter, described by the ratio of the nominal midband frequency to the nominal bandwidth.
  • 7.09  filter exact band-edge frequencies. Frequencies of the lower and upper edges of an ideal bandpass filter, symbols f1 and f2, respectively. Band-edge frequencies are determined from  
  • 7.07  filter exact midband frequency. Geometric mean of the lower and upper band-edge frequencies, symbols f1 and f2, respectively, of a bandpass filter. Midband frequency, symbol fm, has a specified relationship to a reference frequency, fr, such that the ratio of the exact midband(...)
  • 7.08  filter nominal midband frequency. For a set of contiguous one-third octave bandpass filters, frequency of a specified series such as the preferred-frequency series that includes 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, and 1000 hertz, extended by successive multiplication or(...)
  • 10.54   flanking sound transmission. In sound transmission measurement, the transmission of sound from the sound source room to the receiving room by paths other than through the partition under test. Annotation              Flanking transmission may include structure-borne sound transmission.
  • 9.05  flaw detection. Process of locating imperfections in solid materials by observing internal reflections or a variation in transmission through the materials as a function of sound-path location.
  • 2.35  flow noise. Noise due to flow-excited vibration or noise directly radiated from flow turbulence. Annotation              See also 8.12.
  • 8.12  flow noise. Noise due to flow over any object. Annotation              See also 2.35.
  • 10.40  flow resistance. Ratio of the difference of air pressure across a sheet of porous material to the volume velocity of airflow through the sheet. Unit, pascal per (cubic meter per second) [Pa/(m3/s)].
  • 10.42  flow resistivity. Ratio of specific flow resistance to the thickness of the porous sheet. Unit, pascal per (square meter per second) [Pa/(m2/s)].
  • 10.31  flutter echo. Rapid but nearly even succession of echoes originating from the same sound source when the sound is reflected between parallel or nearly parallel surfaces.
  • 9.37  focused beam. Converging energy of a sound beam at a specified distance.
  • 5.19  forced-choice experiment. Trials in which a stimulus is presented in exactly one of n observation intervals and the subject identifies the interval during which the stimulus was perceived. The subject must identify an interval even if the subject did not perceive the(...)
  • 4.01  forced oscillation; forced vibration. Response of a system caused by external excitation. Annotation                      For a linear system, if the response is periodic and continuing, the oscillation is steady-state.
  • 4.01  forced oscillation; forced vibration. Response of a system caused by external excitation. Annotation                      For a linear system, if the response is periodic and continuing, the oscillation is steady-state.
  • 11.44  formant. Of a complex sound, a range of frequencies in which there is an absolute or relative maximum in the sound spectrum. Unit, hertz (Hz). Annotation              The frequency at the maximum is the formant frequency.
  • 11.49  forward masking. Condition in which the signal occurs after the masking sound.
  • 4.38  foundation. Structure that supports the gravity load of a system. It may be fixed in space or it may undergo a motion that provides excitation for the supported system.
  • 5.08  free sound field; free field. Field in a homogeneous medium free from boundaries. In practice, the effects of boundaries on a free field are negligible over the region of interest. Annotation              The actual pressure impinging on an object (e.g., on an electroacoustic(...)
  • 6.55  free-field current sensitivity. Of an electroacoustic transducer for sound reception, for a specified frequency and a specified direction of incidence, quotient of the short-circuit current at the output terminals of the transducer by the sound pressure in the undisturbed plane(...)
  • 4.33  free-field equivalent bone vibrator output level. At a given frequency, vibration force level produced by the bone vibrator on a mechanical coupler plus a correction consisting of the difference between the free-field sensitivity level and the coupler sensitivity level for the specified(...)
  • 4.34  free-field equivalent earphone output level. At a given frequency, sound pressure level produced by the earphone in an acoustic coupler or ear simulator plus a correction consisting of the difference between the free-field sensitivity level and the coupler sensitivity level for the(...)
  • 6.53  free-field sensitivity. Of an electroacoustic transducer for sound reception, for a specified frequency and a specified direction of sound incidence, quotient of the output open-circuit voltage by the sound pressure in the undisturbed plane progressive free field. Unit, volt per pascal(...)
  • 4.35  free-field sensitivity (of an earphone). At a given frequency, for at least 10 otologically normal subjects, quotient of sound pressure level, or force level, of a frontally incident plane progressive wave by input voltage to an earphone, or bone vibrator, such that on average the(...)
  • 6.22  free impedance. For a transducer which converts an electric signal into a mechanical or acoustic signal, input impedance when the output is connected to a load of zero impedance.
  • 4.02  free oscillation. Phenomenon that occurs in a system when it oscillates in the absence of forced oscillation as the result of an initial displacement or velocity.
  • 5.07  free progressive wave; free wave. Wave in a medium free from boundary effects. Annotation              A free wave in a steady state can only be approximated in practice.
  • 5.08  free sound field; free field. Field in a homogeneous medium free from boundaries. In practice, the effects of boundaries on a free field are negligible over the region of interest. Annotation              The actual pressure impinging on an object (e.g., on an electroacoustic(...)
  • 5.07  free progressive wave; free wave. Wave in a medium free from boundary effects. Annotation              A free wave in a steady state can only be approximated in practice.
  • 2.12  frequency. The rate of change with time of the instantaneous phase of a sine function divided by 2π, with the dimensions of cycles per second or hertz (Hz).
  • 3.05  frequency level. Logarithm of the ratio of a given frequency to an appropriate reference value. The base of the logarithm and reference value should be indicated. Annotation 1          If the base of the logarithm is 2, the unit of frequency level is the octave. The reference value is(...)
  • 6.11  frequency response; frequency response function. For a linear system, quotient of the Fourier transform of an output signal by the Fourier transform of the input signal. Abbreviation, FRF. Annotation              The frequency response function is often referred to as a transfer function.
  • 6.11  frequency response; frequency response function. For a linear system, quotient of the Fourier transform of an output signal by the Fourier transform of the input signal. Abbreviation, FRF. Annotation              The frequency response function is often referred to as a transfer function.
  • 7.18  frontal-incidence microphone. Microphone having essentially a constant sensitivity, as a function of frequency, over a specified frequency range, when in a free field and pointed at the source of the sound; i.e., at frontal incidence. Annotation              Sensitivity that is(...)
  • 6.34  frontal plane. Vertical plane that divides the body into front and back. It is perpendicular to the medial plane. See 6.46.
  • 6.35  frontally incident-free sound field. Sound field in which the subject directly faces the single source of a free progressive wave.
  • 12.04  fundamental. The frequency of a harmonic complex tone whose period is the same as that of the entire complex. Annotation              See also 2.13 and 4.20.
  • 2.13  fundamental frequency. For a function periodic in time, the reciprocal of the period. Unit, hertz (Hz). Annotation              See also 4.20 and 12.04.
  • 4.20  fundamental frequency. (a) For a periodic quantity, frequency of a sinusoidal quantity that has the same period as the periodic quantity. (b) Lowest natural frequency of an oscillating system. Annotation   See also 2.13 and 12.04.
  • 4.19  fundamental mode of vibration. Vibration of a system at the lowest natural frequency.
  • g

  • 7.22  gain. Amount of amplification provided between the input signal and the output signal of a system. The parameter for which the gain is being specified, i.e., time-mean-square sound pressure, sound power, electrical voltage, or electrical power, should be stated. Annotation        (...)
  • 7.23  Gaussian noise. Noise for which the amplitudes of some measure such as instantaneous voltage in an electrical signal or instantaneous sound pressure in an acoustical signal may be described by a normal (Gaussian) distribution.
  • 6.88  vocal fry; glottal fry. Syncopated vocal fold vibration typically heard as a low pitch “bubbling” or “cracking” type of sound and at times described as a pulsating type of voice. Lowest voice register. See also 6.87.
  • 5.34  group velocity. Velocity of propagation of the crest of a group of interfering waves where the component wave trains have slightly different individual frequencies and phase velocities. Unit, m/s; symbol, vg.
  • h

  • 3.25  hair cells in organ of Corti. Sensory receptor cells for hearing. Hair cells are ciliated epithelial cells located within the organ of Corti and are subdivided into the inner hair cells and the outer hair cells. Nerve innervation is different for the inner and outer hair cells. Several(...)
  • 3.36  malleus; hammer. Outermost of the three ossicles that are located in the middle ear. The shape of the malleus resembles a club. The handle of the malleus (manubrium) is attached to the tympanic membrane; the head of the malleus (capitulum) is attached to the body of the incus.
  • 2.29  harmonic. Sinusoidal quantity having a frequency which is an integral multiple of the fundamental frequency. Annotation              See also 12.06.
  • 12.06  harmonic. Partial whose frequency is an integral multiple of the fundamental frequency. Annotation 1          The term "overtone" has frequently been used in place of "harmonic," the nth harmonic being called the (n – 1) overtone. The term "overtone" is now deprecated to reduce(...)
  • 12.07  harmonic complex tone. A complex tone that consists of partials that are integral multiples of the fundamental frequency.
  • 6.36  head-related transfer function; HRTF. Magnitude and phase of the transfer function defined by the body, head, and pinna of the listener as a sound travels from its source to the outer ear canal. Annotation         The head-related transfer function contains spacial cues and(...)
  • 4.36  hearing. Sense by which sound is perceived.
  • 7.59  hearing aid; hearing instrument. Portable instrument, fitted individually to the listener, intended to aid the hearing of a person with impaired hearing, usually consisting of a microphone, an amplifier, and an earphone (receiver) or bone vibrator.
  • 7.17  hearing aid. Any wearable sound amplification device designed for, offered for the purpose of, or represented as aiding persons with hearing loss. Annotation         In general, hearing aids have internal microphones, amplifiers, and output transducers that are housed in the same(...)
  • 7.18  hearing assistance devices/systems (HADS).  A group of instruments intended to facilitate hearing by providing amplification of an acoustic signal and/or improving the signal-to-noise ratio by means of a non-acoustic signal transmission method. HADS include personal assistive listening(...)
  • 4.38  hearing loss; hearing impairment; hypoacusis; impairment of hearing. Term denoting an impairment of auditory sensitivity. Can range from mild, to moderate, to profound. See also C11.24.
  • 7.59  hearing aid; hearing instrument. Portable instrument, fitted individually to the listener, intended to aid the hearing of a person with impaired hearing, usually consisting of a microphone, an amplifier, and an earphone (receiver) or bone vibrator.
  • 11.26  hearing level; hearing threshold level. For a specified signal, amount in decibels by which the hearing threshold for a listener, for either one or two ears, exceeds a specified reference equivalent threshold level. Unit, decibel (dB). Annotation              Use of the term "hearing(...)
  • 4.37  hearing level for speech. For a specified speech signal, for a specified type of transducer, and for a specified manner of presentation, the speech level minus the appropriate speech recognition threshold signal. Unit, decibel (dB). Annotation         Typically, a speech audiometer is(...)
  • 4.38  hearing loss; hearing impairment; hypoacusis; impairment of hearing. Term denoting an impairment of auditory sensitivity. Can range from mild, to moderate, to profound. See also C11.24.
  • 7.60  hearing protector; hearing protection device. Personal device worn to reduce harmful auditory or annoying subjective effects of sound.
  • 7.60  hearing protector; hearing protection device. Personal device worn to reduce harmful auditory or annoying subjective effects of sound.
  • 4.41  hearing screening. A component of an audiological test program that attempts to determine which individuals of a total population may have a hearing problem and therefore require further audiological evaluation.
  • 11.21  hearing threshold; threshold of audibility. For a given listener and specified signal, the minimum (a) sound pressure level or (b) force level that is capable of evoking an auditory sensation in a specified proportion of trials. Sound reaching the ears from other sources is assumed to(...)
  • 11.26  hearing level; hearing threshold level. For a specified signal, amount in decibels by which the hearing threshold for a listener, for either one or two ears, exceeds a specified reference equivalent threshold level. Unit, decibel (dB). Annotation              Use of the term "hearing(...)
  • 4.42  hearing threshold level for pure tones. Hearing threshold of a given ear at a specified frequency and for a specified type of transducer when measured with an audiometer calibrated to reference equivalent threshold levels for air or bone conduction. See also C11.26. Unit, decibel (dB).
  • 3.26  helicotrema. Narrow aperture within the apex of the cochlea that allows communication between the scala vestibuli and the scala tympani.
  • 7.77  Helmholtz resonator.  Acoustical device consisting of a rigid-walled chamber open to an orifice or tube, in which all internal dimensions are much smaller than a wavelength of sound. Thus, the chamber serves as an acoustic compliance and the orifice or tube as an acoustic mass, which in(...)
  • 10.26  hemi-anechoic room. Test room with a hard, reflecting floor whose other surfaces absorb essentially all the incident sound energy over the frequency range of interest, thereby affording nominally free-field conditions above a reflecting plane.
  • 5.20  heterophasic. Condition in which the signal plus noise, or noise alone, presented to one ear is statistically independent from the signal plus noise, or noise alone, presented to the other ear. Annotation         An example of a heterophasic condition is one in which the noise at each(...)
  • 7.13  high-pass filter. Filter having a single transmission band from some band-edge frequency greater than zero up to infinite frequency.
  • 5.21  homophasic. Condition in which the phase or time difference of the signal presented at each ear is the same as the phase or the time difference of the noise presented at each ear. Annotation         An example of a homophasic condition is one in which the signal and the noise are both(...)
  • 7.97  horn; acoustic horn. Tube of varying cross section intended to achieve an acoustic impedance match and to produce a directional effect.
  • 7.98  horn loudspeaker. Loudspeaker in which the radiating element is coupled to the medium by means of a horn.
  • 7.101  horn mouth. End of a horn where the cross-sectional area is largest.
  • 7.102  horn throat. End of a horn where the cross-sectional area is smallest.
  • 7.43  thermal microphone; hot wire microphone. Microphone designed to operate by the changes in the resistance of a hot wire resulting from the cooling or heating effects associated with the presence of a sound wave.
  • 6.36  head-related transfer function; HRTF. Magnitude and phase of the transfer function defined by the body, head, and pinna of the listener as a sound travels from its source to the outer ear canal. Annotation         The head-related transfer function contains spacial cues and(...)
  • 9.24  hydrodynamic oscillator. Transducer for generating sound waves in fluids, in which a continuous flow through an orifice is modulated by a reciprocating valve system controlled by acoustic feedback.
  • 8.21  hydrophone. Transducer that produces electric signals in response to waterborne acoustic signals. Annotation              Transducer may initially convert waterborne acoustic signals to electrical signals or to optical signals.
  • 4.38  hearing loss; hearing impairment; hypoacusis; impairment of hearing. Term denoting an impairment of auditory sensitivity. Can range from mild, to moderate, to profound. See also C11.24.
  • i

  • 5.22  ideal detector. Device, the design of which is given by the likelihood ratio (see 5.25), that is used as a standard for comparison with experimental results.
  • 9.26  image convertor. Device for making acoustic field configurations optically visible.
  • 9.53  immersion testing. In ultrasonic testing, an inspection method where the search unit and the material are submerged in a liquid.
  • 6.18  immittance. A general term denoting either impedance or admittance which is commonly used in evaluation of middle ear function. Annotation              Use of immittance in electroacoustics is strongly deprecated.
  • 4.50  impact. Single collision of one mass in motion with a second mass which may be in motion or at rest.
  • 10.58  impact insulation class. Single-number rating of impact sound insulation derived by fitting a reference rating curve to the normalized impact sound pressure level values measured for the 16 contiguous one-third octave frequency bands with nominal midband frequencies of 100 Hz to 3150(...)
  • 10.55  impact sound pressure level. In a specified frequency band, the average sound pressure level produced by the operation of the standard tapping machine on a floor-ceiling assembly as measured in a room below or near the room or space containing the tapping machine, averaged over each of(...)
  • 4.38  hearing loss; hearing impairment; hypoacusis; impairment of hearing. Term denoting an impairment of auditory sensitivity. Can range from mild, to moderate, to profound. See also C11.24.
  • 6.15  impedance. At a specified frequency, quotient of a dynamic acoustic variable (e.g., force, sound pressure) by a kinematic acoustic variable (e.g., vibration velocity, particle velocity), or quotient of a voltage by a current. Annotation 1          The term impedance is normally(...)
  • 4.49  impulse. Product of a force and the time during which the force is applied: more specifically, impulse is the time integral of force from an initial time to a final time, the force being time dependent and equal to zero before the initial time and after the final time.
  • 7.25  in situ. In the assessment of the performance of hearing aids, refers to measurements made on the hearing aid while it is in the position of normal use, i.e., on the ear of the wearer. Annotation         Derived from the Latin words meaning “in position.”
  • 3.27  incus; anvil. Middle member of the three auditory ossicles that are located in the middle ear. The body of the incus is attached to the head of the malleus and the rounded projection at the lower end of the incus (lenticular process) is attached to the head of the stapes.
  • 6.44  acoustic mass; inertance. At a frequency for which inertial forces are dominant, quotient of sound pressure by the resulting in-phase volume acceleration during sinusoidal motion. Unit, pascal per (cubic meter per second squared) [Pa/(m3/s2)], or kilogram per (meter to the fourth power)(...)
  • 2.17  infrasonic frequency. Frequency less than 20 Hz, the nominal lower range of human hearing. Unit, hertz (Hz). Annotation 1          The term "infrasonic" may be used to indicate a device or system intended to operate at an infrasonic frequency. Annotation 2          The frequency(...)
  • 2.03  infrasound. Sound at frequencies less than 20 Hz. Annotation   A frequency of 20 Hz is the approximate lower limit of the range of human hearing.
  • 12.08  inharmonic complex tone. A complex tone that consists of partials that are not integral multiples of the fundamental frequency.
  • 6.72  equivalent noise pressure; inherent noise pressure. Of an electroacoustic transducer or system used for sound reception, the root-mean-square sound pressure of a sinusoidal plane progressive wave, which, if propagated parallel to the principal axis of the transducer, would produce an(...)
  • 3.31  inner ear; internal ear. Portion of the ear, located within the petrous part of the temporal bone, containing the cochlear and vestibular systems. Consists of the osseous labyrinth and the membranous labyrinth.
  • 3.28  inner hair cells. See 3.25, 3.43.
  • 3.29  inner phalangeal cells. Single row of supporting cells lying on the inward side of (medial to) the inner rods of Corti in the organ of Corti. The upper part of the inner phalangeal cells partially encloses  the cuticular plates of the inner hair cells.
  • 3.30  inner tunnel of Corti; tunnel of Corti. Fluid-filled space of approximately triangular cross-section formed by the inner and outer rods of Corti and the basilar membrane, extending the length of the organ of Corti from base to apex.
  • 7.24  input-output characteristic. For a hearing aid, a single frequency plot of coupler SPL on the ordinate as a function of input SPL on the abscissa with equal decibel scale divisions on each axis.
  • 7.46  insert earphone. Small earphone designed to be fitted into the outer ear or to be attached directly to a connecting element such as an ear mold.
  • 7.19  insert earphone. An earphone coupled by a tube and/or ear mold with the output tip inside the ear canal.
  • 7.27  insertion gain. Difference between the earphone-presented sound pressure level and the open-ear sound pressure level, both measured at the eardrum. Unit, decibel (dB). Annotation         Insertion gain may be measured with a probe microphone on a human listener or using a manikin(...)
  • 6.71  insertion loss. Ten times the logarithm to the base ten of the ratio of the power delivered to that part of the system that will follow the transducer, before insertion of the transducer, to the power delivered to the same part of the system after insertion of the transducer. Unit,(...)
  • 10.65  insertion loss. Of a silencer or other sound-reducing element in a specified frequency band, the decrease in sound pressure level measured at the location of the receiver when a sound insulator is inserted in the transmission path between the sound source and the receiver. Unit, decibel(...)
  • 2.24  instantaneous phase.
  • 11.41  instantaneous speech power. Rate at which sound energy is radiated by a speech source at any given instant. Unit, watt (W).
  • 7.70  integrating-averaging sound level meter. Device for measuring the level of time-mean-square frequency-weighted sound pressure signal during a stated time interval. Annotation 1          See also 3.15. Annotation 2          Integrating-averaging sound level meters may be described(...)
  • 11.45  articulation; intelligibility. Percentage of speech units correctly received out of those transmitted. Annotation 1          The word "articulation" is used when the units of speech material are syllables or other meaningless fragments of speech; the word "intelligibility" is used(...)
  • 7.29   cross correlation; interaural cross correlation. Covariance of signals vs. time shift; i.e., the degree of similarity of a reference signal and a time-shifted signal, as a function of time shift or delay. Annotation 1      The cross correlation is calculated as where T is the(...)
  • 5.23  interaural phase. Phase (or time) difference between the signal and noise, or noise alone, parameter presented at one ear and the signal and noise, or noise alone, parameter presented at the other ear. Phase is usually referenced to one ear. Unit, radian. Annotation 1      Typical(...)
  • 7.66  interferometer. Device used to produce and measure the interference between two or more wave trains.
  • 3.31  inner ear; internal ear. Portion of the ear, located within the petrous part of the temporal bone, containing the cochlear and vestibular systems. Consists of the osseous labyrinth and the membranous labyrinth.
  • 6.37  internalization. Perception of sound as being within or near the head. Sound is typically presented to the subject by earphones. Annotation         A subject’s inability to alter interaural sound pressure level and phase differences by moving the head when wearing earphones is(...)
  • 6.38  intertone. Subjective tone perceived when two (primary) tones, closely spaced in frequency, produce beats. The pitch of the intertone lies between the pitches of the two primary tones. Unit, hertz (Hz).
  • 12.21  interval. Spacing in pitch or frequency, as indicated by context, between two tones. The frequency spacing is expressed by the ratio of the frequencies or by a specified logarithm of this ratio. Annotation              See Table 2.
  • 3.32  intra-aural muscles. See 3.61 stapedius and 3.68 tensor tympani.
  • 3.33  intracellular potentials. Negative polarizations of cells with respect to surrounding extracellular fluid.
  • 6.39  ipsilateral masking. Situation where one acoustic signal (the masker) is presented to the same ear as a second signal (the stimulus) in order to reduce the audibility (e.g., elevate the threshold or reduce the loudness) of the second signal.
  • 3.34  ipsilateral reflex; uncrossed reflex. Middle-ear muscle reflex that is elicited in the stimulus ear. When the reflex is acoustical, the resulting middle-ear reflex is sometimes called the ipsilateral acoustical reflex or uncrossed acoustical reflex.
  • 5.24  isophonic contour. Curve that shows the related values of sound pressure level and frequency required to produce a given magnitude of a specific auditory sensation for the typical subject for a stated manner of listening to the sound. The specific sensation, such as loudness, pitch,(...)
  • j

  • 2.54  jerk. Vector that specifies the time rate of change of acceleration; jerk is the third derivative of translational displacement with respect to time. Unit, meter per second-cubed (m/s3).
  • 9.25  jet-edge generator. Fluid dynamic transducer, involving vortex formation, in which stabilization is achieved by hydrodynamic feedback between a jet and an edge.
  • 6.40  jitter. Slight cycle-to-cycle variations in vibratory period of a sustained vocalization.
  • 5.10  differential threshold; difference limen; just-noticeable difference; JND. Minimum change in a stimulus that can be correctly judged as different from a reference stimulus in a specified fraction of trials. Annotation         Instead of the method of constant stimuli, which is implied(...)
  • 11.12  judged perceived noise level. Sound pressure level of a frontally presented octave band of pink noise centered on 1000 Hz having a duration of 2 s that is subjectively judged equally noisy to a given sound. Unit, decibel (dB).
  • 5.10  differential threshold; difference limen; just-noticeable difference; JND. Minimum change in a stimulus that can be correctly judged as different from a reference stimulus in a specified fraction of trials. Annotation         Instead of the method of constant stimuli, which is implied(...)
  • 12.27  Just scale; scale of Just intonation. Musical scale formed by setting the major triads to have the frequency ratio 4:5:6 and the minor triads to have ratios of 10:12:15, which makes major and minor triads as consonant (smooth) as possible. Annotation 1                  See Table(...)
  • l

  • 7.30  lapel microphone. Microphone designed for positioning on the clothing of a user.
  • 3.35  lateral inhibition. Mechanism which is thought to increase the frequency selectivity of neurons in the central nervous system. Neurons with a specific best frequency inhibit neurons with different best frequencies, thus reducing response to frequencies remote from best frequency,(...)
  • 6.06  auditory lateralization; lateralization. Determination by a subject that the apparent direction of a sound is either left or right of the frontal-medial plane of the head. Annotation 1      Auditory lateralization is usually restricted to sounds presented by earphones or by bone(...)
  • 3.01  level. In acoustics, logarithm of the ratio of a variable quantity to a corresponding reference value of the same units. The base of the logarithm, the reference value, and the kind of level are to be specified.  
  • 11.29  sensation level; level above threshold. For an individual listener and a specified sound signal, amount by which a sound pressure level or force level exceeds the hearing threshold for that sound. Unit, decibel (dB). Annotation              The conditions of the measurements are to(...)
  • 10.44  level difference; noise reduction; sound isolation between rooms. Between two rooms in a specified frequency band, difference between the space-time average sound pressure levels in the two enclosed spaces when one or more sound sources operates in one of the rooms. Unit, decibel (dB);(...)
  • 5.25  likelihood ratio. For an event, the ratio of the probability density of that event given “signal-plus-noise,” to the probability density of that event given “noise-alone.”
  • 5.26  likelihood ratio criterion. That value of the likelihood ratio at or above which a “signal-plus-noise” response is made and below which a “noise-alone” response is made.
  • 8.29  limiting ray. Ray tangential to a horizontal plane at which the propagation velocity goes through a maximum or a step change.
  • 10.60  limp wall. Panel or wall whose sound transmission properties are determined by the mass per unit area of the panel without stiffness.
  • 7.31  line microphone; shotgun microphone   . Directional microphone consisting of an array of transducers arranged in a straight line or the acoustical equivalent.
  • 2.48  line spectrum. Spectrum for which the components occur at a number of discrete frequencies.
  • 6.01  linear system. System in which, for every element in the system, the response is proportional to the excitation. Superposition applies. Annotation              The time-dependent properties of each element of a linear system can be represented by a set of linear differential equations(...)
  • 7.32  lip microphone. Microphone designed for use in contact with the lips of a user.
  • 6.41  listening region. Region of a listening room where predetermined acoustical conditions for optimum listening are met; region of listening room designated for listeners.
  • 6.42  listening room. Room designed specifically to provide optimum conditions for listening to reproduced sound (see IEC/TR 60268-13).
  • 10.17  live room. Room characterized by a relatively small amount of sound absorption.
  • 6.23  loaded impedance. For a transducer, input impedance of stated kind, electric, mechanical or acoustic, when the output is connected to a stated load.
  • 6.07  auditory localization; localization. Determination by a subject of the apparent direction and distance, direction alone, or distance alone, of a sound source. Annotation         Auditory localization in a horizontal or azimuthal plane is facilitated by interaural phase or time(...)
  • 4.29  logarithmic decrement. Natural logarithm (base e) of the ratio of any two consecutive peak instantaneous values about equilibrium of like sign, in the decay of a single-frequency oscillation.
  • 6.43  logatom. Meaningless sequence of phonemes connected together according to the rules of a given language. Synonym: nonsense syllable.
  • 6.44  Lombard effect. Involuntary raised speech level and increased vocal effort resulting from a speaker’s efforts to compensate for background noise and enhance audibility.
  • 5.10  longitudinal wave. Wave in which the direction of displacement at each point of the medium is normal to the wavefront.
  • 9.52  loss of back reflection. In ultrasonic testing, absence of or significant reduction of an ultrasonic signal from the back surface of the object under test.
  • 11.03  loudness. That attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from soft to loud. Unit, sone. Annotation              Loudness depends primarily upon the sound pressure although it also depends upon the frequency, waveform, and duration of(...)
  • 11.05  loudness level. Of a sound, the median sound pressure level in a specified number of trials of a free progressive wave having a frequency of 1000 Hz that is judged equally loud as the unknown sound when presented to listeners with normal hearing who are facing the source. Unit,(...)
  • 6.45  loudness summation. Perceived increase in loudness as the bandwidth of a sound stimulus is increased even though the stimulus power is held constant.
  • 7.79  loudspeaker; speaker. Electroacoustic transducer intended to radiate acoustic power into the air, the acoustic waveform being essentially equivalent to the electric input within the device’s frequency range of operation.
  • 7.80  loudspeaker unit; driver. Electroacoustic transducer used as a loudspeaker without its associated acoustical enclosure or baffle.
  • 7.14  low-pass filter. Filter having a single transmission band extending from a frequency near zero to some finite band-edge frequency.
  • m

  • 9.04  macrosonics. Technology of sound at signal amplitudes so large that linear approximations are not valid. Annotation              Processing of materials by sonic techniques usually involves macrosonics.
  • 9.19  magnetostriction. Phenomenon wherein ferromagnetic materials experience an elastic strain when subjected to an external magnetic field.
  • 7.89  magnetostriction loudspeaker. Loudspeaker that operates by deformation of a magnetostrictive material.
  • 7.33  magnetostriction microphone. Microphone whose operation depends upon the magnetostrictive properties of a material.
  • 3.36  malleus; hammer. Outermost of the three ossicles that are located in the middle ear. The shape of the malleus resembles a club. The handle of the malleus (manubrium) is attached to the tympanic membrane; the head of the malleus (capitulum) is attached to the body of the incus.
  • 4.08  manual audiometer. Pure-tone audiometer in which the signal presentations, frequency, hearing level selection, and recording of results are performed manually.
  • 3.37  manubrium. See 3.36.
  • 7.34  mask microphone. Microphone designed for use inside an oxygen mask or other respiratory mask.
  • 11.22  masked threshold. Hearing threshold for a specified sound in the presence of another sound. Unit, decibel (dB).
  • 11.31  masking. (a) The process by which the threshold of hearing for one sound is raised by the presence of another (masking) sound. (b) The amount by which the threshold of hearing for one sound is raised by the presence of another (masking) sound, expressed in decibels.
  • 5.27  masking level difference. Any decrease (improvement) in the masked threshold obtained when two ears are used instead of one. Masking level difference is usually referred to the masked threshold determined under monotic listening conditions. Abbreviation, MLD; unit, decibel (dB).
  • 10.59  mass-law sound transmission loss. Phenomenon whereby doubling the mass per unit area of a panel, or doubling the frequency for a given mass per unit area, increases the sound transmission loss by six decibels.
  • 7.53  artificial mastoid; mastoid simulator. Device simulating the mechanical impedance of an average human mastoid which is used to calibrate bone vibrators.
  • 3.13  maximum sound level. Greatest frequency-weighted and exponential-time-weighted sound level within a stated time interval. Unit, decibel (dB); abbreviation for F time weighting and A frequency weighting, for example, is MXFA; symbol LAFmx (or C and S).
  • 10.18  mean-free path. Average distance traveled by sound waves in an enclosure between successive reflections, where the average is over a large number of reflections and all initial directions of propagation. Unit, meter (m).
  • 7.48  mechanical coupler. Device to present a specified mechanical impedance to a vibrator applied with a specified static force and equipped with an electromechanical transducer to measure the mechanical force level at the surface of contact between vibrator and mechanical(...)
  • 7.28  mechanical coupler. Device that presents a specified mechanical impedance to a vibrator applied with a specified static force and equipped with an electromechanical transducer to measure the alternating force level at the surface of contact between vibrator and mechanical(...)
  • 6.28  mechanical impedance (at a point). In a linear mechanical system, quotient of a force applied at a point by the resulting component of velocity in the direction of the force. Unit, newton per (meter per second) [N/(m/s)]. Annotation              In the case of torsional mechanical(...)
  • 6.30  mechanical reactance. Imaginary part of a mechanical impedance. Unit, newton per (meter per second) [N/(m/s)].
  • 6.29  mechanical resistance. Real part of a mechanical impedance. Unit, newton per (meter per second) [N/(m/s)].
  • 4.40  mechanical shock. Motion that occurs when the position of a system is significantly changed in a relatively short time and in a nonperiodic manner. Mechanical shock is characterized by suddenness and large displacements resulting in significant internal forces in the system.
  • 4.11  mechanical system. System comprising a defined configuration of mass, mechanical stiffness, and mechanical resistance.
  • 6.03  mechanical system. System capable of generating, transmitting, or receiving mechanical signals.
  • 5.02  mechanical transmission system. Assembly of mass, mechanical stiffness, and mechanical resistance, adapted to the transmission of mechanical power.
  • 6.46  medial plane. Vertical plane that divides the body into left and right. It is perpendicular to the frontal plane.
  • 9.22  sonic applicator, medical. Self-contained electromechanical transducer designed for local application of sound for therapeutic purposes.
  • 11.02  mel. Unit of pitch height. By definition, a simple tone of frequency 1000 hertz, 40 decibels above a listener's threshold, produces a pitch height of 1000 mels. The pitch height of any sound that is judged by the listener to be n times that of a 1-mel tone is n mels.
  • 3.38  membranous cochlea. Scala media and its walls, including the basilar membrane, Reissner’s membrane, and the stria vascularis.
  • 3.39  membranous labyrinth; otic labyrinth. Closed system of endolymph-filled ducts and sacs contained within the inner ear. The membranous labyrinth takes the same general shape as the osseous labyrinth. Consists of the cochlear duct, utricle, saccule, and semicircular ducts.
  • 5.28  method of adjustment; method of average error; method of equivalent stimuli; method of reproduction. Test method whereby a subject varies some dimension of the stimulus until that stimulus appears equal to or just noticeably different from a reference stimulus. Used primarily to(...)
  • 5.28  method of adjustment; method of average error; method of equivalent stimuli; method of reproduction. Test method whereby a subject varies some dimension of the stimulus until that stimulus appears equal to or just noticeably different from a reference stimulus. Used primarily to(...)
  • 5.31  method of equal sense distances; method of bisection; method of equal appearing intervals; method of equidistances; method of mean gradation; method of mean stimuli; method of supraliminal differences. Test method whereby a subject adjusts a set of stimuli until the elements of the set(...)
  • 5.29  method of constant stimuli; method of right and wrong cases. Test method whereby a subject responds to each presentation of a stimulus: “yes-no,” “same-different,” “greater than-equal to-less than.” Stimuli, ranging from rarely to almost always perceivable (or rarely to almost always(...)
  • 5.30  method of cross-modality matching. Test method whereby a subject adjusts a stimulus along some dimension until that stimulus appears equal to another stimulus received by a different sense modality. Used primarily to scale sensations.
  • 5.31  method of equal sense distances; method of bisection; method of equal appearing intervals; method of equidistances; method of mean gradation; method of mean stimuli; method of supraliminal differences. Test method whereby a subject adjusts a set of stimuli until the elements of the set(...)
  • 5.31  method of equal sense distances; method of bisection; method of equal appearing intervals; method of equidistances; method of mean gradation; method of mean stimuli; method of supraliminal differences. Test method whereby a subject adjusts a set of stimuli until the elements of the set(...)
  • 5.31  method of equal sense distances; method of bisection; method of equal appearing intervals; method of equidistances; method of mean gradation; method of mean stimuli; method of supraliminal differences. Test method whereby a subject adjusts a set of stimuli until the elements of the set(...)
  • 5.28  method of adjustment; method of average error; method of equivalent stimuli; method of reproduction. Test method whereby a subject varies some dimension of the stimulus until that stimulus appears equal to or just noticeably different from a reference stimulus. Used primarily to(...)
  • 5.38  method of ratio production; method of fractionation; method of matching; method of multiplication; method of sensation ratios. Test method whereby a subject adjusts a stimulus along some dimension until that stimulus appears to be a specific fraction or multiple of a reference stimulus.(...)
  • 5.32  method of limits; method of just-noticeable differences; method of least-noticeable differences; method of least-noticeable stimuli; method of minimal changes; method of serial exploration. Test method whereby some dimension of a stimulus, or of the difference between two stimuli, is(...)
  • 5.32  method of limits; method of just-noticeable differences; method of least-noticeable differences; method of least-noticeable stimuli; method of minimal changes; method of serial exploration. Test method whereby some dimension of a stimulus, or of the difference between two stimuli, is(...)
  • 5.32  method of limits; method of just-noticeable differences; method of least-noticeable differences; method of least-noticeable stimuli; method of minimal changes; method of serial exploration. Test method whereby some dimension of a stimulus, or of the difference between two stimuli, is(...)
  • 5.32  method of limits; method of just-noticeable differences; method of least-noticeable differences; method of least-noticeable stimuli; method of minimal changes; method of serial exploration. Test method whereby some dimension of a stimulus, or of the difference between two stimuli, is(...)
  • 5.33  method of magnitude estimation. Test method whereby a subject assigns, to a set of stimuli, numbers which are proportional to some subjective dimension of the stimuli. Used primarily to scale sensations.
  • 5.34  method of magnitude production. Test method whereby a subject adjusts a stimulus along some dimension until the magnitude of the stimulus appears equal to some specified magnitude. Used primarily to scale sensations.
  • 5.38  method of ratio production; method of fractionation; method of matching; method of multiplication; method of sensation ratios. Test method whereby a subject adjusts a stimulus along some dimension until that stimulus appears to be a specific fraction or multiple of a reference stimulus.(...)
  • 5.31  method of equal sense distances; method of bisection; method of equal appearing intervals; method of equidistances; method of mean gradation; method of mean stimuli; method of supraliminal differences. Test method whereby a subject adjusts a set of stimuli until the elements of the set(...)
  • 5.31  method of equal sense distances; method of bisection; method of equal appearing intervals; method of equidistances; method of mean gradation; method of mean stimuli; method of supraliminal differences. Test method whereby a subject adjusts a set of stimuli until the elements of the set(...)
  • 5.32  method of limits; method of just-noticeable differences; method of least-noticeable differences; method of least-noticeable stimuli; method of minimal changes; method of serial exploration. Test method whereby some dimension of a stimulus, or of the difference between two stimuli, is(...)
  • 5.38  method of ratio production; method of fractionation; method of matching; method of multiplication; method of sensation ratios. Test method whereby a subject adjusts a stimulus along some dimension until that stimulus appears to be a specific fraction or multiple of a reference stimulus.(...)
  • 5.35  method of paired comparisons. Test method whereby a subject compares, along some dimension, stimuli that are presented in pairs. Used primarily to scale sensations.
  • 5.36  method of rank order; order of merit method; scaling method. Test method whereby a subject orders, along some dimension, a series of stimuli. Used primarily to scale sensations.
  • 5.37  method of ratio estimation. Test method whereby a subject makes ratio judgments of some dimension of a set of stimuli. Used primarily to scale sensations. Annotation         A special case of the method of ratio estimation is the constant sum method; here, the subject makes ratio(...)
  • 5.38  method of ratio production; method of fractionation; method of matching; method of multiplication; method of sensation ratios. Test method whereby a subject adjusts a stimulus along some dimension until that stimulus appears to be a specific fraction or multiple of a reference stimulus.(...)
  • 5.28  method of adjustment; method of average error; method of equivalent stimuli; method of reproduction. Test method whereby a subject varies some dimension of the stimulus until that stimulus appears equal to or just noticeably different from a reference stimulus. Used primarily to(...)
  • 5.29  method of constant stimuli; method of right and wrong cases. Test method whereby a subject responds to each presentation of a stimulus: “yes-no,” “same-different,” “greater than-equal to-less than.” Stimuli, ranging from rarely to almost always perceivable (or rarely to almost always(...)
  • 5.38  method of ratio production; method of fractionation; method of matching; method of multiplication; method of sensation ratios. Test method whereby a subject adjusts a stimulus along some dimension until that stimulus appears to be a specific fraction or multiple of a reference stimulus.(...)
  • 5.32  method of limits; method of just-noticeable differences; method of least-noticeable differences; method of least-noticeable stimuli; method of minimal changes; method of serial exploration. Test method whereby some dimension of a stimulus, or of the difference between two stimuli, is(...)
  • 5.39  method of single stimuli; rating scale method. Test method whereby a subject rates, along some dimension, stimuli presented singly. Used primarily to scale sensations.
  • 5.31  method of equal sense distances; method of bisection; method of equal appearing intervals; method of equidistances; method of mean gradation; method of mean stimuli; method of supraliminal differences. Test method whereby a subject adjusts a set of stimuli until the elements of the set(...)
  • 10.11  metric sabin. Unit of Sabine absorption, equivalent to square meter (m2).
  • 7.15  microphone. Electroacoustic transducer that produces electric signals when excited by acoustic signals.
  • 3.40  middle ear; tympanic cavity; tympanum. Air-filled cavity within the mastoid portion of the temporal bone typically described as containing the eardrum and ossicles and inner opening of the Eustachian tube.
  • 3.41  middle-ear muscle reflex. Change in the tonus of muscles of the middle ear in response to an acoustical or tactile stimulus. Annotation            Contractions of the middle-ear muscles may be monitored by measurement of the change in acoustic impedance or acoustic admittance within(...)
  • 12.31  MIDI; Musical Instrument Digital Interface. A widely accepted communications protocol enabling music synthesizers, computers, and related equipment to communicate, control, and synchronize with each other.
  • 4.43  minimum audible field. Sound pressure level of a tone at the threshold of audibility measured in a free sound field for a subject listening with both ears and facing the sound source. The magnitude of the sound pressure level at the position to be occupied by the center of the subject’s(...)
  • 4.44  minimum audible sound pressure level. Sound pressure level of a tone at the threshold of audibility that is presented by an earphone and measured or inferred at the tympanic membrane. Abbreviation, MAP; unit, decibel (dB).
  • 5.18  false rejection; miss. Event that occurs in a detection situation, during a specified observation interval, when a “noise-alone” response (output) follows a “signal-plus-noise” stimulus (input). Annotation         A false dismissal and a correct detection are mutually exclusive for a(...)
  • 4.39  mixed hearing loss. Impairment of hearing caused by combined conductive and sensorineural components.
  • 6.79  mobility. Analogous mobilities are defined as the ratios of the following complex amplitudes: mechanical mobility              ZM =velocity across/force through; rotational mobility    ZR =angular velocity across/torque through; acoustic mobility      ZA =volume velocity(...)
  • 6.78  mobility analogy. Analogy in which some of the quantities in the following groups are considered analogous: (1) mechanical, rotational, and acoustic mobilities, and electric impedance; (2) velocity across, angular velocity across, volume velocity across; (3) force through,(...)
  • 6.91  vocal register, chest; modal register. Sound produced by vibration of the entire vocal folds, resulting in the natural vocal pitch for a given individual. See also 6.87.
  • 4.17  mode of vibration. Characteristic pattern assumed by a system undergoing vibration in which the motion of every particle is simple harmonic or damped harmonic with the same frequency and with a fixed-phase relationship. Two or more modes may exist concurrently in a(...)
  • 3.42  modiolus. Conical shaped central core of the cochlea. The modiolus contains the spiral ganglion of the cochlear nerve and forms the inner wall of the scala vestibuli and the scala tympani.
  • 2.42  modulation. Variation of some parameter characterizing a periodic oscillation. Thus amplitude modulation of a sinusoidal oscillation is a variation in the amplitude of the sinusoidal oscillation.
  • 9.16  molecular relaxation. Equalization of energy among the degrees of freedom of a molecule following a disturbance that produces deviations from the equilibrium distribution law.
  • 6.47  monaural. Hearing by only one ear.
  • 6.48  monaural diplacusis. Otological condition in which a monotically presented pure tone is perceived as a group of tones and noise, or noise alone.
  • 2.80  point source; monopole. Source that radiates sound isotropically as if from a single point.
  • 5.40  monotic. Condition in which a sound stimulus is presented to only one ear.
  • 6.49  most comfortable level. Sound pressure level of a given acoustic signal considered by the listener as optimal in the specified environment. Abbreviation, MCL. Unit, decibel (dB).
  • 6.27  motional admittance. Of a transducer, the loaded electric admittance minus the electric admittance when mechanically blocked. Annotation              This definition is best applied to transducers with transformer coupling.
  • 6.26  motional impedance. Of a transducer, loaded electric impedance minus the electric impedance when mechanically blocked (open circuit). Annotation              This definition is best applied to transducers with gyroscopic (anti-reciprocal) coupling.
  • 7.54  artificial mouth; mouth simulator. Device consisting of a loudspeaker unit mounted in a baffle or an enclosure shaped to produce a radiation pattern similar to that of an average human mouth.
  • 7.86  electrodynamic loudspeaker; moving-conductor loudspeaker; moving-coil loudspeaker. Loudspeaker designed to operate by the motion of a conductor or a coil carrying a varying current in a steady magnetic field.
  • 7.35  moving-coil microphone. Electrodynamic microphone in which the transducing element has the form of a coil.
  • 7.86  electrodynamic loudspeaker; moving-conductor loudspeaker; moving-coil loudspeaker. Loudspeaker designed to operate by the motion of a conductor or a coil carrying a varying current in a steady magnetic field.
  • 7.27  electrodynamic microphone; moving-conductor microphone. Microphone whose operation depends upon the generation of an induced current in a conductor moving in a magnetic field.
  • 7.78  silencer; muffler. Duct designed to reduce the level of sound. The sound reducing mechanisms may be either absorptive or reactive, or a combination.
  • 7.08  multi-band compressor. Amplifier in which the input signal is separated into two or more frequency bands, with compression applied differently to each band, with the output of the bands then recombined.
  • 7.92  multicellular loudspeaker. Horn loudspeaker in which the radiating element is coupled to the medium by means of two or more juxtaposed horns.
  • 7.95  multichannel loudspeaker. Assembly consisting of two or more loudspeakers, usually with one or more dividing networks, designed to radiate simultaneously in their respective frequency bands. Annotation              A loudspeaker divided into two bands is called a two-way loudspeaker.(...)
  • 7.96  multichannel loudspeaker system. A loudspeaker system composed of three or more loudspeakers often used in cinemas and home theater surround sound systems.  The system may be composed of left and right, front and back, center and subwoofer channels.
  • 4.15  multiple-degree-of-freedom system. System which requires two or more coordinates to define completely the configuration of the system at any instant.
  • 10.30  multiple echo. Succession of separate echoes originating from a single sound.
  • 7.36  multiple microphone. Device containing two or more associated microphones in order to obtain directional effects.
  • 12.31  MIDI; Musical Instrument Digital Interface. A widely accepted communications protocol enabling music synthesizers, computers, and related equipment to communicate, control, and synchronize with each other.
  • n

  • 6.50  nasality. Noticeable quality of sound produced by the coupling of resonating cavities of the nasal passages to the oral and pharyngeal cavities.
  • 4.09  natural frequency; undamped natural frequency. Frequency of free, undamped oscillation for a system. For a multiple-degree-of-freedom system, the natural frequencies are the frequencies of the normal modes of vibration. Unit, hertz (Hz). Annotation              For the equation of(...)
  • 5.05  near field. The acoustic field so close to an extended source that the effects of the source size are manifest in measurements. Annotation              For a source of extent L, the "Fresnel range" can be used to define the maximum distance for the near field: L2/λ, where λ is the(...)
  • 3.04  neper. Logarithmic unit for the amplitude ratio of two sinusoidal acoustic variables of the same kind, or for the ratio of a sinusoidal amplitude to a reference value of the same kind, when the logarithm is on the Napierian (natural) base e ≈ 2.7183. Annotation 1          A(...)
  • 3.18  night-average sound level. Time-averaged frequency-weighted sound level between 0000 and 0700 hours and 2200-2400 hours. Unit, decibel (dB); abbreviation, NL; symbol, Ln.  
  • 5.24  node. Point, line, or surface in a standing wave where some characteristic of the wave field has essentially zero amplitude. Annotation              The appropriate modifier should be used before the word "node" to signify the type that is intended; e.g., displacement node, velocity(...)
  • 2.32  noise. (a) Undesired sound. By extension, noise is any unwanted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device. (b) Erratic, intermittent, or statistically random oscillation. Annotation 1          If ambiguity exists(...)
  • 7.37  noise canceling microphone. Microphone capable of discriminating against ambient sound arriving from specified directions or distances.
  • 4.45  noise dose. Percentage of a maximum permissible daily exposure to noise; typically, the combination of the A-weighted Leq and corresponding exposure duration. See also 4.23, 4.32, and A3.22.
  • 4.46  noise-induced hearing loss. Permanent hearing loss which progressively increases (at most frequencies the loss will eventually approach an asymptote) due to cumulative daily or regular exposures to hazardous noise not intense enough to cause immediate acoustic trauma. See also C11.53.
  • 11.53  noise-induced permanent threshold shift. Permanent hearing loss resulting from noise exposure. Abbreviation, NIPTS.
  • 11.52  noise-induced temporary threshold shift. Temporary hearing loss occurring as a result of noise exposure. Abbreviation NITTS.
  • 10.50  noise isolation class. Single-number rating of the 16 level differences between two rooms obtained according to ASTM E336 and rated by use of the ASTM E413 reference rating curve defined for sound transmission class. Unit, decibel (dB); abbreviation, NIC.
  • 8.15  noise-limited condition. Condition in which detection is limited by sonar background noise other than reverberation.
  • 10.44  level difference; noise reduction; sound isolation between rooms. Between two rooms in a specified frequency band, difference between the space-time average sound pressure levels in the two enclosed spaces when one or more sound sources operates in one of the rooms. Unit, decibel (dB);(...)
  • 10.14  noise reduction coefficient. Arithmetic mean of sound absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05 metric sabin per square meter. Abbreviation, NRC.
  • 11.14  perceived noisiness; noisiness. Prescribed function of sound pressure levels in the 24 one-third octave bands with nominal mid-band frequencies from 50 Hz to 10 kHz that is used in the calculation of perceived noise level. Unit, noy; abbreviation, n. Annotation              The(...)
  • 7.05  non-linear signal processing hearing aid. A hearing aid having output that is not linearly related to the input. Annotation            This term is also used to refer to devices having output which is time varying with respect to the input.
  • 4.47  nonacoustic reflex. Middle-ear muscle reflex elicited by a nonacoustic stimulus.
  • 4.48  nonacoustic reflex activating stimulus. Electric, tactile, pneumatic, photic, or other nonacoustic signals used to elicit a middle-ear muscle reflex.
  • 2.83  nonlinear acoustics. Science of sound in which the amplitude of particle displacement is sufficiently large that linear approximations to the properties of the medium and/or the equations of motion are no longer sufficient and consequently higher order effects need to be(...)
  • 4.35  nonlinear damping. Damping resulting from a damping force that is not proportional to velocity.
  • 7.30  nonstationary noise.  Noise signal, with or without audible tones, for which the level and average statistical properties vary during the period of observation.
  • 5.41  normal-receiver-operating characteristic. See 5.52 and 5.07, Annotation 1.
  • 3.21  normalized 8-hour average sound level. Level of time-mean-square, A-weighted sound pressure during a normalization period Tn of eight hours, such that the sound exposure therefrom is equal to the sound exposure of a time-varying sound at a place where total sound exposure occurs within a(...)
  • 10.57  normalized impact sound index. Single-number rating of impact sound insulation derived by fitting a reference rating curve to the normalized impact sound pressure level values measured for the 16 contiguous one-third octave frequency bands with nominal midband frequencies of 100 Hz to(...)
  • 10.56  normalized impact sound pressure level. For a specified frequency band, average sound pressure level in decibels due to the standardized impact sound source, plus ten times the logarithm to the base ten of the ratio of the Sabine absorption in the receiving room to the reference Sabine(...)
  • 10.45  normalized level difference; normalized noise reduction. Level difference between rooms plus ten times the logarithm to the base ten of the ratio of the reverberation time in the receiving room to reference reverberation time of 0.5 s. Unit, decibel (dB); abbreviation, NNR.
  • 10.51  normalized noise isolation class. Single-number rating of the 16 normalized level differences, obtained by use of the reference rating curve defined for sound transmission class. Unit, decibel (dB); abbreviation, NNIC.
  • 10.45  normalized level difference; normalized noise reduction. Level difference between rooms plus ten times the logarithm to the base ten of the ratio of the reverberation time in the receiving room to reference reverberation time of 0.5 s. Unit, decibel (dB); abbreviation, NNR.
  • 7.04  band-elimination filter; band-rejection filter; bandstop filter; notch filter. Filter having a single attenuation band, neither of the band-edge frequencies being zero or infinite.
  • 12.14  Note. Conventional designation indicating pitch and duration; also the tone sensation itself or the sound pressure causing the sensation. The word serves when no distinction is desired among the symbol, the sensation, and the physical stimulus. Annotation 1          By convention,(...)
  • 11.15  noy. Unit of perceived noisiness, equal to the perceived noisiness of a one-third octave band of noise centered on 1 kHz and having an A-weighted sound pressure level of 40 dB.
  • o

  • 7.21  occluded ear simulator. Device which approximates the acoustic transfer impedance of the inner part of the human ear canal, from the tip of an ear mold to the eardrum.
  • 4.49  occlusion effect. Increase in loudness of conductive signals, usually at frequencies below 2000 Hz, when the external ear(s) are covered. Typically observed in bone-conduction audiometry as an improvement in threshold when an ear is covered with an earphone (occluded) or an external ear(...)
  • 4.18  Bing test; occlusion test. Test used to distinguish between conductive and sensorineural hearing losses. A vibrating tuning fork is placed on the center of the subject’s forehead and the external acoustic meatus is gently occluded.  If the ear is normal or if there is a pure(...)
  • 12.22  octave. The interval between two tones whose frequencies (or fundamental frequencies) stand in the ratio of 2:1. Annotation 1          In the musical scale, notes that are an octave apart are given the same name, and the octave in which a tone occurs is designated by(...)
  • 7.05 octave filter frequency ratio. Nominal frequency ratio of 2:1; general symbol G. Two options, designated base ten and base two, for determining an octave-band, or fractional-octave-band, frequency ratio are permitted. The base-ten system is preferred.
  • 11.11  offset. The deliberate turning off of a sound.
  • 7.16  omnidirectional microphone. Microphone whose response is essentially independent of the direction of the incident sound over a specified frequency range.
  • 2.78  simple sound source; omnidirectional sound source. Source that radiates sound uniformly in all directions under acoustic free-field conditions. Annotation   See 5.08 for definition of free field.
  • 3.16  one-hour-average sound level. Time-averaged frequency-weighted sound level during a time period of one hour. Unit, decibel (dB); abbreviation, 1 HL; symbol, L1h.
  • 11.10  onset. The deliberate turning on of a sound.
  • 6.24  open-circuit impedance. For a transducer which converts a mechanical or an acoustic signal into an electric signal, input impedance—mechanical or acoustic—when the output is connected to a load of infinite impedance (i.e., open circuit). Unit, newton per (meter per second) [N/(m/s)] or(...)
  • 5.36  method of rank order; order of merit method; scaling method. Test method whereby a subject orders, along some dimension, a series of stimuli. Used primarily to scale sensations.