Physiology of the Human Ear
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"Physiology of Hearing and Psychoacoustics"
For more information, read the text by D.R. Raichel entitled "Science and Applications of ."
The ear consists of three principal sections: I) the outer ear or pinna which gathers the sound and funnels it into the canal leading to the ii) middle ear which consists of a bone structure containing the hammer (malleus) attached to the eardrum, the anvil (incus) which connects to the stirrup (stapes) which attaches to the oval window that constitutes the entrance to the iii) inner ear. The inner ear contains the cochlea, a complex system of fluid-filled channels buried in the temporal bone. Inside the cochlea are located sensory nerve endings which provide the sense of equilibrium and hearing. The cochlea itself is a snail-shaped structure of approximately 2¸ turns. An cochlea uncoiled would extend about 35 mm. The fibrous, flexible basilar membrane running the length of the cochlea contains the nerve excitation mechanism. The basilar membrane is activated by acoustic energy introduced into the cochlea through the oval window. High frequencies cause the greatest excitation of the basilar membrane in the region nearest the oval window; and lower frequencies affect more the other end of the basilar membrane. The basilar membrane connects to a complex structure---the organ of Corti---which contains the hair cells entailed in the nerve stimulation process.
Deafness is classified principally as being of two types: conductive loss of hearing and sensorineural deafness. Conductive loss occurs when the ossicular chain (the bones in the middle ear÷the hammer, anvil and stirrup) do not function properly. This type of loss can be surgically treated. Sensorineural deafness occurs from damaged nerves in the inner ear and can generally be treated only by the use of a hearing aid (if there is residual hearing), Sensorineural deafness can result from overly long exposure to excessive noise levels, diseases such as whooping cough or measles, or the aging process.
Testing of hearing are best done by certified audiologists who use audiometers that provide pure tone test signals in the range from 125 through 8000 Hz relayed through headphones. The audiologist records the thresholds for a series of frequencies, i.e. the lowest sound pressure levels at which the respective signals can be detected by the person undergoing the test In addition, a bone conduction vibrator can be placed behind the outer ear to determine any possible conductive loss. The audiometer may also contain an input for an external sound source, e.g. for speech. The person undergoing the gearing test should be acoustically isolated from noisy areas---this normally entails the use of an isolation booth.
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Support for the development of this module was provided by the National Science Foundation and The Cooper Union for the Advancement of Science and Art.
Please send questions or comments to Professor Ron Adrezin or Professor Daniel Raichel.