Fourier in the ear

The cochlea is a coiled cavity within the inner ear that is primarily responsible for much of our auditory system. Within the cochlea is the basilar membrane, a filmy structure that divides the cochlea and vibrates with incoming sound energy. The width of the basilar increases from the base (entrance) to apex (end), while the width of the cochlear cavity decreases.

The basilar membrane has thousands of basilar fibers embedded within. These fibers affect the local stiffness in the membrane. The membrane starts thick and stiff, but becomes thinner and more flexible toward its apex. When sound waves travel along the basilar membrane, sound energy is dissipated at the place along the membrane which has the same natural resonant frequency. The stiff fibers will resonate with high frequencies, and the more flexible fibers resonate at lower frequencies.

The hair cells along the length of the basilar membrane detect this vibration and convert it into electrical potentials for transmission to the brain. There are also outer hair cells that contract in response to signals from the brain. This allows the brain to adjust or tune the stiffness of the membrane, thus providing a feedback mechanism to enhance the resolution of frequency content.

Together, the basilar membrane and hair cells give a continuum of natural resonance from high frequencies at the base to low frequencies at the apex. This distributes the energy over space as a function of frequency. (the distribution is logarithmic, which also accounts for why we hear sound on a log scale). Thus, the cochlea acts as a filterbank, performing a Fourier decomposition of the incoming sound.

Interestingly, this is a reversal of the normal frequency representation, since the highest frequencies appear at the entrance to the cochlea, and the lowest frequencies at the rear.