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Why do high-frequency sounds stimulate the base of the cochlea?

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Diagram showing auditory mechanisms in insects, including labeled illustrations of a scolophore organ, the mosquito ear with antenna shaft and scolophores, the cicada ear with auditory capsule and tympanic membrane, and the grasshopper ear with sensory body and tympanic membrane. — sound reception Auditory mechanisms in insects. (Left) A scolophore organ. (Top right) The mosquito ear. (Centre right) The ear of the cicada *Magicicada septendecim*. (Bottom right) The ear of the grasshopper.

High-frequency sounds stimulate the base of the cochlea due to the physical properties of the basilar membrane. This membrane varies in its structural characteristics along its length, specifically increasing in width and mass while decreasing in stiffness from the base (near the oval window) to the apex (at the tip of the cochlea).1

When sound vibrations enter the cochlea, they travel along the basilar membrane as waves. Because the basal end is stiffer, it is most responsive to high-frequency vibrations, which cause the membrane to reach its maximum amplitude in that region before the waves subside. Conversely, lower-frequency sounds travel further along the membrane to reach their peak amplitude at the more flexible apical end. This tonotopic organization allows the brain to distinguish pitch based on the specific location of the stimulated hair cells along the basilar membrane.1