The Ears and Hearing

Sound causes vibrations of the tympanic membrane.These vibrations, in turn, produce movements of the middle-ear ossicles, which press against a membrane called the oval window in the cochlea. Movements of the oval window produce pressure waves within the fluid of the cochlea, which in turn cause movements of a membrane called the basilar membrane. Sensory hair cells are located on the basilar membrane, and the movements of this membrane in response to sound result in the bending of the hair cell processes.This stimulates action potentials that are transmitted to the brain in sensory fibers and interpreted as sound.

Sound waves are alternating zones of high and low pressure traveling in a medium, usually air or water. (Thus, sound waves cannot travel in space.) Sound waves travel in all directions from their source, like ripples in a pond where a stone has been dropped. These waves are characterized by their frequency and intensity. The frequency is measured in hertz (Hz), which is the modern designation for cycles per second (cps). The pitch of a sound is directly related to its frequency—the greater the frequency of a sound, the higher its pitch.

The intensity, or loudness, of a sound is directly related to the amplitude of the sound waves and is measured in units called decibels (dB). A sound that is barely audible—at the threshold of hearing—has an intensity of zero decibels. Every 10 decibels indicates a tenfold increase in sound intensity; a sound is ten times louder than threshold at 10 dB, 100 times louder at 20 dB, a million times louder at 60 dB, and 10 billion times louder at 100 dB.

The ear of a trained, young individual can hear sound over a frequency range of 20 to 20,000 Hz, yet still can distinguish between two pitches that have only a 0.3% difference in frequency. The human ear can detect differences in sound intensities of only 0.1 to 0.5 dB, while the range of audible intensities covers twelve orders of magnitude (1012), from the barely audible to the limits of painful loudness.

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