How does the brain distinguish the pitch of a sound?

The brain distinguishes the pitch of a sound primarily through the mechanical properties of the basilar membrane within the cochlea.1

As sound waves enter the cochlea, they create traveling waves along the basilar membrane, which varies in width, mass, and stiffness from its base to its apex. Because of these physical gradients, different regions of the membrane vibrate most vigorously in response to specific frequencies:1
* High-frequency sounds cause the basal end of the membrane to vibrate.1
* Low-frequency sounds cause the apical end to vibrate.1

The brain identifies the pitch by recognizing which specific group of nerve fibres is activated by the peak vibration on the membrane, a concept known as "place theory". For lower frequencies (up to approximately 3,000 hertz), the brain also utilizes the rate of nerve impulses, which can reflect the timing or phase of the sound wave. In these cases, the auditory nerve provides the brain with information about both the location of maximum vibration and the temporal frequency of the stimulus. For higher frequencies, the place of vibration on the basilar membrane is the decisive factor.12