" What a wah does is clear - it is either a bandpass filter or an overcoupled lowpass filter that exhibits a resonant peak just at its lowpass rolloff frequency. The resonant peak can be moved up and down in frequency by the player, and this makes for a striking emulation of the human voice making a "waaaah" tone, or its tonal inverse, "aaaooow". "
From : http://www.geofex.com/article_folders/wahpedl/wahped.htm
