Originally Posted by Martian

Point of interest: the loss in the bottom end comes more from a stereo technique than from bitrates. The vast majority of mp3's are encoded using 'joint stereo,' which mashes the stereo signals together into a monaural one below a certain frequency threshold. The theory is that since low frequencies are hard to locate anyway, the listener doesn't really lose very much. In practice it depends on the quality of the codec used.