**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 approximately equal to 16.352 Hz for musical acoustics.

Annotation 2 If the base of the logarithm is 2^{1/12}, the unit of frequency level is the semitone. The reference value is approximately equal to 16.352 Hz for musical acoustics (see 12.23).

Annotation 3 If the base of the logarithm is 10, then the unit of frequency level is the decade; e.g., 20,000 Hz is three decades above 20 Hz.

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