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The Search for the Mechanisms of Hearing
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# : |
11064 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
6 / 1986 |
3,597 Words |
| Author
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Peter Dallos
Dr. Peter Dallos was born in Hungary and came to the United
States in 1956. He received his doctorate in electrical
engineering from Northwestern University, where he is
presently professor of neurobiology and audiology |
Human beings are able to process sounds--from the quietest whisper of forest to the roar of a close-up engine--with remarkable precision. The extremes of loudness and softness that we encounter possess energies whose ratio is some hundred-million-million to one. At the lowest limit, the softest sound we can hear, the eardrum may move a thousand-millionth of an inch; at the top of the range, the movement is so great that the hearing sensation gives way to pain. And across this astounding span, we are also able to detect minute changes in intensity.
We can, moreover, hear very small changes in both the frequency of sound (corresponding to its pitch) and its duration. A normal young adult human can hear sounds whose frequencies extend from a rumble to a squeak, from approximately 20 to 20,000 Hz (the name of the scientific unit for cycles--per second is Hertz, Hz for short). And humans can reliably tell a difference in frequency as small as 0.1 percent. That is, we recognize that a sound of 1,000 Hz is not the same as one whose frequency is 1,001 Hz.
To get a hint of the auditory system's remarkable capacities, consider a single microscopic hair, or cilium, on one of the sensory-receptor cells in the ear. The movement of cilia is important, since they act as microlevers to transmit sound-related movements to their sensory-receptor cells--which in turn convey the implied information to the brain. If you scale the dimension of that one cilium to the height of Chicago's Sears Tower, the movement of the tip of the cilium at the threshold of hearing is equal to a two-inch displacement of the top of the Tower!
Such auditory abilities pose a formidable challenge to our understanding. What sort of mechanism did Nature devise that can perform these feats? What happens when parts of the mechanism go awry? How can we remedy such defects?
The Auditory System
The cross-section (Figure) shows the external ear and ear canal, which funnel sound waves arriving from the environment onto the eardrum. This flexible membrane is set into vibration by sound and, in turn, couples its vibrations to three interconnected bones in the middle ear. These bones form a link between the drum and the inner ear. The inner ear, or labyrinth, is a maze of interconnecting fluid-filled channels hollowed out of bone. Suspended within these channels is a closed system of membraneous labyrinth, and within these tubes and sacs are specialized regions that serve as sense
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