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Why do low-frequency sounds stimulate the apex of the basilar membrane?

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musical sound The universality and innate nature of music, and the pentatonic scale, as demonstrated by singer Bobby McFerrin.

Low-frequency sounds stimulate the apex of the basilar membrane because of the membrane's physical properties, which change along its length from the base to the apex.12

The basilar membrane is not uniform; it is narrow and stiff at its basal end (near the oval window) and becomes wider, more massive, and less stiff toward its apical end. These structural characteristics create a gradient of resonance:12

High-frequency sounds cause the stiffer, less massive basal end to vibrate most vigorously.12
Low-frequency sounds travel further along the membrane as waves, reaching their peak amplitude at the more flexible, wider, and more massive apical end.1

This "place theory" of hearing explains that the cochlea acts as a frequency analyzer by mapping specific sound frequencies to distinct locations along the membrane. When a low-frequency sound reaches the apex, the resulting vibration stimulates the hair cells in that region of the organ of Corti, which then transmit nerve impulses to the brain to signal the perception of a low-pitched tone.12