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2004-3-16
发表于 2004-3-22 18:45 | 显示全部楼层
原文题目为《The power of hearing》,发表于英文专业杂志《物理世界》2002年5月。据一家英国著名音响公司前首席工程师讲,同音响有关的人都应该读一读,因为它完全改变了我们所知的——或者说听觉形成机理根本就不是以前我们所知的那么回事。呵呵。。。

我没时间看(可能看不懂耶),有兴趣的自己先看看。
http://physicsweb.org/article/world/15/5/8

The power of hearing
Feature: May 2002

Physicists are exploring the ear's ability to hear both faint whispers and loud cries, and to distinguish between similar musical notes.

We naturally think of our ears as receivers for sound, so it came as a major surprise when, in 1979, David Kemp of University College London found that ears can also emit sounds. A sensitive microphone placed close to the eardrum typically records a faint hum, but in many human subjects clear whistles can be picked up on top of the background buzz. In rare pathological cases, these sounds can be loud enough to be heard by passers-by!

Kemp's experiments clearly showed that something within the ear was vibrating, and they heralded a new era of hearing research. Since then, researchers have thought of the ear as an active receiver. This research is now entering an exciting phase. An understanding of the cellular basis of the ear's power source is emerging, and the fundamental physics of active-signal detection is being worked out.

The occurrence of "otoacoustic emissions" was not a complete surprise to everyone, however. As long ago as 1948 Tommy Gold, a young researcher then at Cambridge, had foreseen that the ear employs an active process. He pointed to a problem with the classical theory of hearing that had been formulated by the German physicist Hermann von Helmholtz in the middle of the 19th century.

Helmholtz believed that the ear responded to sounds in much the same way that a harp string resonates when a singer hits the right note. He supposed that the inner ear contained a set of "strings", each of which vibrated at a different frequency. We now know, however, that the detection apparatus resides within the cochlea, a fluid-filled duct that is coiled like a snail's shell. Given the microscopic size of the putative strings, the viscous damping of the fluid would prevent the build up of a resonant response.

Gold argued that an active process must somehow counteract the friction, so that sharp frequency tuning and high gain can both be achieved. Without these capabilities, the ear would not be able to distinguish similar frequencies or to hear faint sounds. He therefore proposed that the ear operates rather like a regenerative radio receiver. Invented by the radio pioneer Edwin Armstrong, this device works by adding energy at the very frequency that it is trying to detect. It was clear to Gold that such a mechanism must be very delicate, however, as it would require a positive feedback of precisely the right magnitude to cancel the damping. Any less and the ear would be insensitive; any more and it would ring spontaneously.


Wave mechanics


Elegant though it was, Gold's hypothesis fell on deaf ears. No doubt disenchanted by the inertia of scientific thought, he turned to cosmology, where the inventiveness of his ideas was better appreciated. The attention of hearing researchers shifted, instead, to the fluid mechanics of the cochlea, attracted by the results from a series of remarkable experiments conducted in the 1930s and 1940s by Georg von Békésy at the laboratories of the Hungarian Post Office.



Figure 1

With great technical prowess, Békésy succeeded in imaging the minute displacements of the "basilar membrane", the flexible partition that extends along almost the entire length of the cochlea, dividing it into two separate channels (see figure 1). He discovered that a sound stimulus entering the inner ear causes a wave-like distortion to propagate along the basilar membrane. As the wave advances, its amplitude increases and its wavelength decreases until it reaches a place of maximal disturbance, after which it decays rapidly. Crucially, the location of the maximum
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