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Chapter 11

chapter 11 - auditory system

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Matthias Niemeier

INTRODUCTION The sense of hearing is known as audition and the sense of balance is regulated by the vestibular systemTHE NATURE OF SOUND Sounds are audible variations in air pressure When an object moves toward a patch of air it compresses the air increasing the density of the molecules The air is rarefied when an object moves away on the other hand see Fig 111y The speed of sound is about 343 msec 767 mph for air at room temperatureFrequency of sound is the number of compressed or rarefied patches of air that pass by our ears each second One cycle of the sound is the distance between successive compressed patches and it is expressed in hertz Hz y Sound waves all propagate at the same speed so high frequency sound waves have more compressed and rarefied regions packed into the same space than lowfrequency waves see Fig 112 y Our auditory system can respond to pressure waves between 20 Hz to 20 000 Hz y Whether a sound is perceived to have a high or low tone ie pitch is determined by its frequencyIntensity is the difference in pressure between compressed and rarefied patches of air Fig 112b Sound intensity determines the loudness we perceiveloud sounds having higher intensityTHE STRUCTURE OF THE AUDITORY SYSTEM See Fig 113 The visible portion of the ear consists primarily of cartilage covered by skin forming a funnel called the pinna which collects sounds from a wide area y Its shape makes us more sensitive to sounds coming from ahead than from behind y Is more or less fixed in position The entrance to the internal ear is the auditory canal which extends about 25 cm inside skull and ends at the tympanic membrane eardrum Ossicles are connected to the tympanic membrane to its medial surface and they transfer movements of the tympanic membrane into movements of the oval window The cochlea is located behind the oval window and it contains the apparatus to transform physical motion into a neural response y Sound waves moves the tympanic membranetympanic membrane moves the ossiclesossicles move oval window membranemotion at the oval window moves fluid in the cochlea fluid movement causes sensory neurons to respond y The pinna to tympanic membrane make the outer ear the tympanic membrane and ossicles make up the middle ear and the apparatus medial to the oval window make up the inner ear
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