Chapter 44

Mechanical events resulting from sound, gravitational forces, and rotational acceleration are detected by the cochlea and vestibular organs within the inner ear. Sound is a mechanical vibration (eg, as produced by a vibrating piano string). This vibration sets up small oscillations of air molecules that, in turn, cause adjacent molecules to oscillate as the sound propagates away from its source. Sound is called a pressure wave because when the molecules of air come closer, the pressure increases (compression); as they move further apart, the pressure decreases (rarefaction).

A sound is characterized by its frequency and intensity. The frequency of a sound is its pitch. Middle C on a piano has a frequency of 256 cycles per second, whereas high C (seven white keys to the right) has a frequency of 512 cycles per second (Figure 44–1). People with normal hearing can tell the difference between two sounds that differ in frequency by less than 0.5%. To appreciate how small a difference this is, one needs only to realize that middle C differs from C sharp by more than 5%. Human hearing is limited to sound waves between 20 and 20,000 Hz. Many other mammals can hear ultrasound (>20,000 Hz), and some, such as whales, approach up to 100,000 Hz.

Figure 44–1.

The pressure waves of sound are represented by the advancing concentric lines radiating away from the vibrating source. Middle C has a frequency of 256 cycles per second, while upper C (one octave higher) has a frequency of 512 cycles per second.

The intensity of a sound determines its loudness and reflects how tightly packed the molecules of air become during the compression phase of a sound wave. The ear can detect sounds in which the vibration of the air at the tympanic membrane is less than the diameter of a hydrogen molecule (< 0.24 nm). The mammalian ear has the ability to discriminate a wide range of intensities—over a 100,000-fold difference in energy (120 dB).

To maximize the transfer of sound energy from the air-filled environment to the fluid-filled inner ear, land animals evolved external ears as sound collectors and middle ears as mechanical force amplifiers (Figure 44–2).

Figure 44–2.

Anatomy of the ear. The external ear collects sound pressure waves and funnels them toward the tympanic membrane. The middle ear ossicles transmit the sound waves to the inner ear (cochlea). The middle ear acts to match the impedance difference between the air of the external environment to the fluid within the cochlea. This permits maximal sound transmission.

The task of the cochlea is to analyze environmental sounds and transmit the results of that analysis to the brain. The inner ear first determines how much energy is present at different frequencies that make up a specific sound. The cochlea can do this because of its tonotopic organization, ...

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