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 which may cause these signals to fall below the 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 labeled ‘Loudness’ or ‘Loud’) is intended to rectify this situation.
Loudness is available as a free VST effect for Windows with 32-bit support. It can be downloaded from WOK.