How does the basilar membrane help the brain identify sound frequency?

The basilar membrane functions as a frequency analyzer by exhibiting continuous physical changes along its length, which allows the brain to identify sound frequency through a process known as "place theory."1

As sound vibrations enter the cochlea, they create traveling waves along the basilar membrane. The membrane’s physical properties—specifically its width, mass, and stiffness—vary from its base (near the oval window) to its apex (the tip of the cochlea):1

• High-frequency sounds cause the basal end of the membrane to vibrate most vigorously, as this area is stiffer and has less mass.12
• Low-frequency sounds cause the apical end to vibrate, as this region has decreased stiffness and increased mass.12

When a specific region of the membrane vibrates most intensely, it stimulates the corresponding hair cells in the organ of Corti. These cells send nerve impulses to the brain via the auditory nerve. The brain identifies the pitch of the sound by recognizing which specific group of nerve fibers is activated. For lower frequencies, the brain also utilizes the rate of nerve stimulation to further refine its perception of pitch.1