Chapter 20 - The ear

Abstract

The outer, middle, and inner ear work together to make us hear sound, and instantaneously measure its pitch, loudness, and timbre, and, using both ears for sound localization, also the direction and distance from which it arrives. The outer and middle ear work in air, and function to focus and amplify the airborne sound waves; the inner ear's cochlea is filled with liquid, and thus the sound pressure (force per unit area) needs to be amplified before it can be transmitted through the cochlea. All along the cochlea are sensory receptor cells called hair cells, equipped with hair bundles that move with the cochlear liquid, and detect the passage of a pressure wave. When a hair bundle moves, its motion is converted into a nerve pulse, which is sent from the hair cell to the brain via the cochlear nerve. Hair bundles in different positions along the cochlea detect different frequencies, inaccurately, starting from high frequencies at the larger-diameter beginning of the spiral, and ending with low frequencies at the smaller-diameter central end. Loudness and pitch are interpreted by the brain when it receives nerve pulses from the hair cells. The nerve pulses are cleverly encoded so that the average number of pulses per second provides the brain with loudness, and the time-sequence of nerve pulses provides the accurate pitch. Multiple frequencies and their amplitudes in music or voice, e.g., the fundamental frequency and all the partials, are all measured simultaneously, thus the ear-brain system is a Fourier analyzer. Like a Fourier analyzer, the ear is not sensitive to phase.