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Why do different regions of the basilar membrane respond to different sound frequencies?

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Cross-section diagram of the human ear showing the outer ear, middle ear, and inner ear, with labeled parts including the auricle, auditory canal, tympanic membrane, ossicles, cochlea, semicircular canals, and nerves. — human ear Structure of the human ear.

The basilar membrane distinguishes sound frequencies through its physical properties, which change continuously along its length from the base (near the oval window) to the apex (at the tip of the cochlea).

These regional differences in response are determined by variations in stiffness, width, and mass:

Basal end: This region is characterized by greater stiffness and less mass. Consequently, it is more responsive to high-frequency sounds.12
Apical end: This region has decreased stiffness and increased mass, making it more responsive to low-frequency sounds.12

When sound vibrations enter the cochlea, they create traveling waves along the membrane. These waves reach their maximum amplitude at a specific location corresponding to the sound's frequency—the higher the frequency, the shorter the distance the wave travels before peaking at the base. The brain identifies the pitch of a sound by recognizing which specific region of the membrane and its associated hair cells are most vigorously stimulated.1