Loudness compensation is a setting found on some hi-fi equipment and equalisers that increases the level of the high and low frequencies.[1] This is intended to be used at low listening levels, to compensate for the fact that as the volume of audio decreases, the ear's lower sensitivity to extreme high and low frequencies may cause these signals to fall below threshold.[2] As a result audio material may seem to become 'thin' sounding at low volumes, losing bass and treble, the 'Loudness compensation' button (often just labelled 'Loudness' or 'Loud') is intended to rectify this situation. See the section on Loudness for a more detailed explanation of the ears' sensitivity to sound as a function of frequency.
Loudness compensation is difficult to implement, since the audio level at a listener's ears depends on speaker sensitivity as well as amplifier gain. Loudness controls are rarely present on modern equipment, although such controls were common on 70s and 80s amplifiers. One reason for its apparent popularity could be that older speakers needed a boost in bass and the high frequencies to balance losses in the speakers, and sometimes the media itself.