Loudness on Music System

Ever noticed that "loudness" button on your music system, home theatre, on car audio system. Do you know what it does? What is it meant for? This option is quite misunderstood amongst the users. Here is some information that I hope would help to clear things.

Loudness is the correction required during playback of an audio source...
that is reproduced at a 'playback level' different from its 'recording level'. The intent of this compensation is to ensure that the spectral balance of the audio source is same as that intended by the artist or the composer, when played back at any volume level.

Put differently, the essence of loudness compensation is to maintain the perceived spectral balance of audio content irrespective of playback volume level. When in use, loudness compensation corrects for our reduced sensitivity at very low and high frequencies regions when playing back at a low volume levels. For instance, while watching a movie at low volume levels as it may be late at night, one might have noticed the sound becomes "dialogue only" and other environmental sounds like footsteps, thunder are not audible.

The need for this correction in an audio playback system arises due to the inherent non-linearity in the human aural perception. This fact has been established in Equal Loudness Contours (ELC), which is standardized in ISO 226:2003.

Reference: Prasad, "A low complexity approach for Loudness Compensation", 129th Audio Engineering Society (AES) Convention, vol. 1, pp. 310-319, (no. 8263) Nov 4-7, 2010, San Francisco

Link: A Low Complexity Approach for Loudness Compensation