I am using Decibel Audio Player 1.02, on Ubuntu 9.04. (I don't know what gstreamer version DAP uses). I first filed this bug with Decibel Audio Player on Launchpad, and was directed to file it here instead. Because I am using gstreamer with Decibel Audio Player, the behaviour and replication descriptions are necessarily DAP-specific.

The following behaviour applies both to regular mode, and to gapless mode using the "--playbin2" switch. Also, it applies to both FLAC files and MP3 files.

When all of the equalizer sliders are set to the -24 dB position, any vocals or other sounds that are in the centre of the soundstage, (i.e. sounds that are equal in amplitude and phase in both stereo channels), are virtually lost. The effect is very much like what is heard when listening to the 'difference' signal that can be heard by, for example, connecting a speaker between the '+' output of one channel and the '+' output of the other channel of most stereo amplifiers. (I suspect that a flaw in the Equalizer algorithm is in fact somehow causing it to extract the difference between the two channels).

To replicate, open the equalizer and click the Center button to 'zero' all of the sliders. Play a song with strong 'phantom centre' information, (most stereo recordings with vocals fall into this category), and note that the vocal is centred between the two speakers. Then, adjust all of the equalizer sliders to the very bottom, and note that the vocal is almost not present, (the degree of this disappearance will vary from song to song), and also note that the music has a 'spacey' quality. Click on the 'Center' button to restore normal playback.

In a correctly functioning equalizer, moving all the sliders to the bottom should result in considerably reduced volume, but the character, imaging, and tonal balance of the music should remain substantially unchanged.