Discuss how sound perceived by human ear
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The ear contains three sections, the outer, middle, and inner ears. The outer ear consists of the lobe and ear canal, structures which serve to protect the more delicate parts inside.
The outer boundry of the middle ear is the eardrum, a thin membrane which vibrates in sympathy with any entering sound. The motion of the eardrum is transferred across the middle ear via three small bones named the hammer, anvil, and stirrup. These bones are supported by muscles which normally allow free motion but can tighten up and inhibit the bones' action when the sound gets too loud. The leverages of these bones are such that rather small motions of the ear drum are very efficiently transmitted.
The boundry of the inner ear is the oval window, another thin membrane which is almost totally covered by the end of the stirrup. The inner ear is not a chamber like the middle ear, but consists of several tubes which wind in various ways within the skull. Most of these tubes, the ones called the semicircular canals, are part of our orientation apparatus. (They contain fine particles of dust-the location of the dust tells us which way is up.) The tube involved in the hearing process is wound tightly like a snail shell and is called the cochlea.
The cochlea is filled with fluid and is divided in two the long way by the basilar membrane. The basilar membrane is supported by the sides of the cochlea but is not tightly stretched. Sound introduced into the cochlea via the oval window flexes the basilar membrane and sets up traveling waves along its length. The taper of the membrane is such that these traveling waves are not of even amplitude the entire distance, but grow in amplitude to a certain point and then quickly fade out. The point of maximum amplitude depends on the frequency of the sound wave.
The basilar membrane is covered with tiny hairs, and each hair follicle is connected to a bundle of nerves. Motion of the basilar membrane bends the hairs which in turn excite the associated nerve fibers. These fibers carry the sound information to the brain. This information has two components. First, even though a single nerve cell cannot react fast enough to follow audio frequencies, enough cells are involved that the aggregate of all the firing patterns is a fair replica of the waveform. Second, and probably most importantly, the location of the hair cells associated with the firing nerves is highly correlated with the frequency of the sound. A complex sound will produce a series of active loci along the basilar membrane that accurately matches the spectral plot of the sound.
The amplitude of a sound determines how many nerves associated with the appropriate location fire, and to a slight extent the rate of firing. The main effect is that a loud sound excites nerves along a fairly wide region of the basilar membrane, whereas a soft one excites only a few nerves at each locus.
The outer boundry of the middle ear is the eardrum, a thin membrane which vibrates in sympathy with any entering sound. The motion of the eardrum is transferred across the middle ear via three small bones named the hammer, anvil, and stirrup. These bones are supported by muscles which normally allow free motion but can tighten up and inhibit the bones' action when the sound gets too loud. The leverages of these bones are such that rather small motions of the ear drum are very efficiently transmitted.
The boundry of the inner ear is the oval window, another thin membrane which is almost totally covered by the end of the stirrup. The inner ear is not a chamber like the middle ear, but consists of several tubes which wind in various ways within the skull. Most of these tubes, the ones called the semicircular canals, are part of our orientation apparatus. (They contain fine particles of dust-the location of the dust tells us which way is up.) The tube involved in the hearing process is wound tightly like a snail shell and is called the cochlea.
The cochlea is filled with fluid and is divided in two the long way by the basilar membrane. The basilar membrane is supported by the sides of the cochlea but is not tightly stretched. Sound introduced into the cochlea via the oval window flexes the basilar membrane and sets up traveling waves along its length. The taper of the membrane is such that these traveling waves are not of even amplitude the entire distance, but grow in amplitude to a certain point and then quickly fade out. The point of maximum amplitude depends on the frequency of the sound wave.
The basilar membrane is covered with tiny hairs, and each hair follicle is connected to a bundle of nerves. Motion of the basilar membrane bends the hairs which in turn excite the associated nerve fibers. These fibers carry the sound information to the brain. This information has two components. First, even though a single nerve cell cannot react fast enough to follow audio frequencies, enough cells are involved that the aggregate of all the firing patterns is a fair replica of the waveform. Second, and probably most importantly, the location of the hair cells associated with the firing nerves is highly correlated with the frequency of the sound. A complex sound will produce a series of active loci along the basilar membrane that accurately matches the spectral plot of the sound.
The amplitude of a sound determines how many nerves associated with the appropriate location fire, and to a slight extent the rate of firing. The main effect is that a loud sound excites nerves along a fairly wide region of the basilar membrane, whereas a soft one excites only a few nerves at each locus.
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