Aurally compensated volume

The human ear does not perceive sounds of differing frequencies as being equally loud even if they are actually set at the same volume level. A bass sound at 50 Hz and at a sound pressure level of 50 dB is perceived to be just as loud as a 4 kHz sound at 12 dB. Our ears are most sensitive at 4 kHz. Towards the bass end and the higher frequencies, our hearing sensitivity tails off. To ensure that the sounds produced are at different frequencies are perceived to be equally loud even if the amplifier is set to a low volume, amplifiers have a loudness knob.