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HUMAN EXPERIENCE IS ENRICHED by the ability to distinguish a remarkable range of sounds—from the intimacy of a whisper to the warmth of a conversation, from the complexity of a symphony to the roar of a stadium. Hearing begins when the sensory cells of the cochlea, the receptor organ of the inner ear, transduce sound energy into electrical signals and forward them to the brain. Our ability to recognize small differences in sounds stems from the cochlea’s capacity to distinguish among frequency components, their amplitudes, and their relative timing.

Hearing depends on the remarkable properties of hair cells, the cellular microphones of the inner ear. Hair cells transduce mechanical vibrations elicited by sounds into electrical signals, which are then relayed to the brain for interpretation. The hair cells can measure motions of atomic dimensions and transduce stimuli ranging from static inputs to those at frequencies of tens of kilohertz. Remarkably, hair cells can also serve as mechanical amplifiers that augment auditory sensitivity. Each of the paired cochleae contains approximately 16,000 of these cells. Deterioration of hair cells and their innervation accounts for most of the hearing loss that afflicts about 10% of the population in industrialized countries.

The Ear Has Three Functional Parts

Sound consists of alternating compressions and rarefactions propagated by an elastic medium, the air, at a speed of approximately 340 m/s. This wave of pressure changes carries mechanical energy that stems from the work produced on air by our vocal apparatus or some other sound source. The mechanical energy is captured and transmitted to the receptor organ, where it is transduced into electrical signals suitable for neural analysis. These three tasks are associated with the external ear, the middle ear, and the cochlea of the inner ear, respectively (Figure 26–1).

Figure 26–1

The structure of the human ear. The external ear, especially the prominent auricle, focuses sound into the external auditory meatus. Alternating increases and decreases in air pressure vibrate the tympanum. These vibrations are conveyed across the air-filled middle ear by three tiny, linked bones: the malleus, the incus, and the stapes. Vibration of the stapes stimulates the cochlea, the hearing organ of the inner ear.

The most obvious component of the human external ear is the auricle, a prominent fold of cartilage-supported skin. The auricle acts as a reflector to capture sound efficiently and focus it into the external auditory meatus, or ear canal. The ear canal ends at the tympanum, or eardrum, a diaphragm approximately 9 mm in diameter and 50 μm in thickness.

The external ear is not uniformly effective at capturing sound from all directions; the auricle’s corrugated surface collects sounds best when they originate at different, but specific, positions with respect to the head. Our capacity to localize sounds ...

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