von Bekesy and the Place Theory
Excerpt from "Sound and Hearing", Stevens, S. S., & Warshofsky, Fred,eds., Time-Life Books, NY, 1965. p54
"The molder of the modern theory of basilar-membrane "resonance" is Georg von Bekesy. In 1928 Bekesy was a communications engineer in Budapest, studying the mechanical and electrical adaptation of telephone equipment to the demands of the human hearing mechanism. One day, in the course of a casual conversation, an acquantance asked him whether a major improvement would soon be forthcoming in the quality of telephone systems. The idle remark strarted a chain of thought that eventually posed to Bekesy a more fundamental question: "How much better is the quality of the human ear than that of any telephone system?" His search for the answer has added volumes to our present-day knowledge of hearing."
"Bekesy studied the inner ear by building mechanical models of the cochlea- e.g., a metal tube filled with water. Along the length of the tube ran a narrow slot covered by a stretched membrane, which served as the basilar membrane of the model. When the fluid was set in motion, he observed, it caused a bulge which swept like an undulating wave along the membrane. By adjusting the tension of the membrane along the slot he was able to confine the biggest part of the bulge to a particular region on the membrane. The undulation traveled down the length of the membane, but its amplitude - the bigness of the bulge- varied with position: the bulge was slight everywhere except in one area where it was large."
"He also detected the same wave movement in the cochlea itself. Using first animal ears and then the ears of human cadavers, he carefully cut out the cochlea and bored a tiny opening in the bone. Working under a microscope with "microtools" of his own invention (one pair of scissors had blades only a few thousandths of an inch long), he laid open part of the basilar membrane. The cochlear fluid was drained and replaced with a salt solution containing a suspension of powdered aluminum and coal. By scattering flashes of intense light off the powder suspension, Bekesy was able to follow events within the interior of the cochles. Under the microscope he saw a bulging undulation sweep over the basilar membrane when a sound was introduced into the cochlea. It was the same traveling wave he had seen coursing along the artificial membrane of his model."
"From this work Bekesy evolved his traveling-wave theory: a sound impulse sends a wave sweeping along the basilar membrane. As the wave moves along the membrane, its amplitude increases until it reaches a maximum, then falls off sharply until the wave dies out. That point at which the wave reaches its greatest amplitude is the point at which the frequency of the sound is detected by the ear. And as Helmholtz had postulated, Bekesy found that the high-frequency tones were perceived near the base of the cochlea and the lower frequencies toward the apex."
"No mammalian ear was beneath Bekesy's scrutiny. He observed the traveling wave in animals ranging from mouse to elephant. In the early 1940s he read that an elephant had died in the Budapest zoo. Immediately he went to the zoo to ask for the ears, but learned that the body had been sent to Budapest University. The University authorities reported that they had shipped the huge carcass to a glue factory. Finally, at the factory, he found the head still intact."
"That evening Bekesy sent his assistant to saw the portions of the skull containing the inner ears. The young man proudly returned with two large ears and chunks of bone. But when Bekesy peered into the ear canals, he found to his dismay that he could see clear through them; the prized inner ears were missing. Because an elephant's ear canal is about eight inches long, the assistant had not sawed in far enough. Bekesy sent him back to the glue factroy and this time he returned with the elephantine cochleas. To Bekesy's delight, the traveling wave phenomenon was clearly visible in the elephant."
"For his studines of the traveling wave, Georg von Bekesy received the Nobel Prize in 1961. His incredible delicate and elegant experiments had traced sound to the very threshold of sensation. ..."
Place theory concepts
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