Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
Listen to the sound to test your hearing
Figure 4.9 Creating a single-frequency tone in Adobe Audition
188.8.131.52 Critical Bands
One part of the ear's anatomy that is helpful to consider more closely is the area in the inner ear
called the basilar membrane. It is here that sound vibrations are detected, separated by
frequencies, and transformed from mechanical energy to electrical impulses sent to the brain.
The basilar membrane is lined with rows of hair cells and thousands of tiny hairs emanating from
them. The hairs move when stimulated by vibrations, sending signals to their base cells and the
attached nerve fibers, which pass electrical impulses to the brain. In his pioneering work on
frequency perception, Harvey Fletcher discovered that different parts of the basilar membrane
resonate more strongly to different frequencies. Thus, the membrane can be divided into
frequency bands, commonly called critical bands. Each critical band of hair cells is sensitive to
vibrations within a certain band of frequencies. Continued research on critical bands has shown
that they play an important role in many aspects of human hearing, affecting our perception of
loudness, frequency, timbre, and dissonance vs. consonance. Experiments with critical bands
have also led to an understanding of frequency masking, a phenomenon that can be put to good
use in audio compression.
Critical bands can be measured by the band of frequencies that they cover.
Fletcher discovered the existence of critical bands in his pioneering work on the cochlear
response. Critical bands are the source of our ability to distinguish one frequency from another.
When a complex sound arrives at the basilar membrane, each critical band acts as a kind of
bandpass filter, responding only to vibrations within its frequency spectrum. In this way, the
sound is divided into frequency components. If two frequencies are received within the same