"LOUDNESS simulates the loudness button on old HiFi
amps, which alters the frequency response curve to
correspond roughly with the equal loudness
characteristic of the ear at lower volumes.
The level of high and low frequencies is increased
at low listening levels, to compensate for the fact
that as the volume of audio decreases, the ears
lower sensitivity to extreme high and low
frequencies may cause these signals to fall below
threshold. 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. LOUDNESS is
available for free as a 32bit VST plugin for Windows
(sometimes a nagscreen pops up)."