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Why do low-frequency sounds stimulate the tip 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.

Low-frequency sounds stimulate the apical end (the tip) of the cochlea due to the physical properties of the basilar membrane.

The basilar membrane is not uniform; it changes in width, mass, and stiffness along its length. Specifically, the membrane increases in width and mass while decreasing in stiffness from its base (near the oval window) to its apex (the tip). Because of these structural characteristics, different regions of the membrane are tuned to vibrate most vigorously at specific frequencies—a concept known as the "place theory" of hearing.1

High-frequency sounds cause the stiffer, narrower basal end to vibrate, while low-frequency sounds cause the more flexible, wider, and more massive apical end to vibrate. As sound vibrations travel through the cochlear fluids, they create traveling waves that reach their maximum amplitude at the specific location on the basilar membrane corresponding to the sound's frequency. The region that vibrates most vigorously stimulates the greatest number of hair cells in the organ of Corti, which then send nerve impulses to the brain to signal the pitch.12