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KMID : 0381219710030110745
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Journal of RIMSK
1971 Volume.3 No. 11 p.745 ~ p.752
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MECHANISM OF HEARING
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Abstract
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Hearing is one of the main functions of the ear and it is indispensable for communication. Mechanism of hearing has been studied extensively and many articles regarding the detailed functions of the ear are available. However, there still remains many unresolved problems on hearing mechanism.
Corti (1851) described the detailed microstructure of the inner ear, Helmholtz (1857) studied the mechanism of the middle ear as a mechanical transformer, and Bekesy (1928) described the traveling wave theory for pitch discrimination. For a better understanding of the recent trend of research for the mechanism of hearing, it is desired to have a brief review of prior studies on hearing.
What We Hear?.
The sound which we hear is "a oscillations propagated in an elastic medium" and it is a longitudinal wave. The sound is defined with pitch, intensity and phase.
Pitch: The audible range of sound pitch is 16-20,000 H and the minimal acceptable distortion of frequency is less than 5. 6%. The minimal acceptable difference of pitch for human beings is 3 Hz and the optimal discrimination time for pitch is 1/4 sec.
Intensity: The intensity varies with frequency and particle velocity. The audible intensity is 130 dB within the most sensitive speech range and it varies with frequency. The minimal detectable increase of intensity is about 10%(0. 4dB) and the optimal discrimination time for intensity is 1/6 sec.
Phase: The sound has compression and rarefaction phase and it has some effect on the localization of a sound source.
How We Hear?
The hearing organ can he divided, functionally, into the following: vibration conduction-transductionnerve conduction-cognition and association. The vibration conduction is also subdivided into conversion of pneumatic energy into mechanical energy at ear drum boundary and conversion of mechanical energy into hydraulic energy at the level of stapes foot plate.
External Ear:
Auricle: Collector and localizes of sound.
External auditory meatus: Resonance and reflection phenomena are here. Middle Ear:
Acoustic Qualities of Air and Sea Water: Because the density and elasticity of air and sea water differ, so much 99 9% of the energy is reflected and only 0. 1% is absorbed which amounts to 30dB loss.
Theories of hearing:
Various theories on pitch discrimination by vibration of basilar membrane are proposed, such as: Resonance theory of Helmholtz (1857)
Telephone theory of Ruther toed t 1886)
Stand wave theory of Ewald (1890
Traveling wave theories of Bekesy 1928
Organ of Corti as a secondary impedance matching device;
The weak hydraulic energy of inner ear fluid is converted into stronger energy by shearing force between reticular lamina and tectorial membrane.
Electric responses of the inner ear:
Since 1,935, Wever and Bray reported their study of electrical responses of the inner ear, many investigators have examined the phenomena. Four different responses have been demonstrated, viz., cochlear microphonic (CM) of Adrian (1931), Action Potential (AP) of Derbyshire and Davis (1935), Summating Potential(SP) of Davis (1950) and Endocochlear DC Potantial (EP) of Bekesy (1951).
As the above electrical phenomena of the inner ear became clearer, analytic function of pitch and other hearing mechanism became much clear.
Transduction:
The final step of inner ear function, the excitatory function of the inner ear, for convertion of hydraulic energy to nerve energy, there are chemical mediator theory, piezoelectric theory and mechanoelectric theory.
Davis (1952) proposed the mechanoelectric theory as follow;
There are high positive EP in the scala media and negative potential in the hair cells with a definite boundary on reticular lamina. The vibrating wave of basilar membrane causes the change of resistance to electric conductivity on reticular lamina and the resulting alternation of electric current between alove two battery causes the nerve excitation.
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