Ear sensitivity depends jointly on frequency and on sound level

Evidence

The net result of these effects is that the sensitivity of the ear is a function of both frequency and amplitude. In other words, the frequency response of the ear is not flat and is also dependent on sound level. Therefore two tones of equal sound pressure level will rarely sound equally loud. For example, a sound at a level that is just audible at 20 Hz would sound much louder if it was at 4 kHz. Tones of different frequencies therefore have to be at different sound pressure levels to sound equally loud, and their relative loudness will also be a function of their absolute sound pressure levels. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 625

The second effect is an amplitude dependence of sensitivity which is due to the way the ear transduces and interprets the sound and, as a result, the frequency response is a function of amplitude. This effect is particularly noticeable at low frequencies but there is also an effect at higher frequencies. The net result of these effects is that the sensitivity of the ear is a function of both frequency and amplitude. — David Howard, Jamie Angus, coustics and Psychoacoustics, 614

Categorization

• XREF: This is the audio-engineering basis for loudness equalization in mastering; ties to Fletcher-Munson curves in recording contexts. — connects to: The ear’s just-noticeable amplitude difference depends on signal type, level, and frequency — David Howard, Jamie Angus, coustics and Psychoacoustics, 636
• DEFINE: Explains why loudness judgment can never be read directly off a meter — sensitivity is amplitude-dependent.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 625; David Howard, Jamie Angus, coustics and Psychoacoustics, 614