reference equivalent threshold sound pressure level

4.57  reference equivalent threshold sound pressure level. At a specified frequency, the mean value of the equivalent threshold sound pressure levels of a sufficiently large number of ears of otologically normal persons of both sexes aged between 18 and 30 years inclusive, expressing the threshold of hearing in a specified acoustical coupler or artificial ear for a specified type of earphone. Abbreviation, RETSPL; unit, decibel (dB).