Bark

The bark is the standard unit corresponding to one critical band width of human hearing. Human hearing critical bands are narrow at low frequencies but become wider at higher frequencies.

Figure: Human hearing critical bands versus mp3's 32 equal width subbands

Widths of the critical bands gets wider towards the higher frequencies. MP3's 32 equal width subbands (shown on top) cover the 22.05kHz bandwidth. The width of 1 MP3 subband is about 689Hz (32*689=22.05kHz).