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How do different parts of the basilar membrane detect different pitches?

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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.

The basilar membrane distinguishes pitch through a process known as "place theory," which relies on the membrane's physical properties that change along its length from the base (near the oval window) to the apex (at the tip of the cochlea).

As sound vibrations travel through the cochlear fluids, they create traveling waves along the basilar membrane. Because the membrane increases in width and mass while decreasing in stiffness from the base to the apex, different regions are tuned to respond to specific frequencies:12

High-frequency sounds: These cause the basal end of the membrane to vibrate most vigorously, as this area is stiffer and has less mass.12
Low-frequency sounds: These cause the apical end to vibrate, as this region is wider, has more mass, and is less stiff.12

When a specific region vibrates, it stimulates the hair cells in the organ of Corti at that location, which then send nerve impulses to the brain. The brain identifies the pitch based on which specific group of nerve fibers is activated.1