Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 146

When a sound wave reaches the ear, it travels down the auditory canal until it reaches the eardrum. It sets the eardrum in motion and this vibration is transmitted across the 2 mm gap to the oval window by the lever system comprised of the auditory ossicles. It is thought that this mechanical system is an impedance matching device. The characteristic impedance, ρc, of air is approximately one two‐thousandth of the impedance of the cochlea fluid. The area of the tympanic membrane is 20 or 30 times larger than that of the oval window. Some believe that it is not the area of the oval window which is important, but rather the area of the footplate of the stapes. The tympanic membrane is about 20 times greater in area than the footplate area. However, not all of the tympanic membrane vibrates because it is firmly attached at its periphery. The ratio of the part of the tympanic membrane that moves to the footplate area is about 14 to 1.

Also, the pivot of the ossicle system may be assumed closer to the oval window than the eardrum, hence providing a mechanical advantage of two or three times. The net result is that low‐pressure, high particle velocity amplitude air waves arriving at the eardrum are converted into high‐pressure low particle velocity amplitude fluid waves in the cochlea, approximately matching the air to fluid impedances. We probably remember from electrical theory that in order to obtain maximum power transfer, impedances must be matched.

There is a “safety” device built into the inner ear mechanism. Attached to the malleus and stapes are two muscles: the tensor tympani and the stapedius. If continuous intense sounds are experienced, the muscles contract and rotate the ossicles so that the force passed onto the oval window does not increase correspondingly to the sound pressure.

This effect is called the acoustic reflex and many types of experiments indicate that the reflex attenuates low‐frequency sound levels up to about 20 dB [9]. However, these muscles are rapidly fatigued by continuous narrow‐band intense noise. In addition, the muscles are relatively slow in their contraction making the reflex ineffective in presence of impulse or impact sounds. There seems to be some evidence that the muscles also contract when we speak, to prevent us hearing so much of our own speech.

The Eustachian tube is used to equalize the pressure across the eardrum by swallowing. This explains why our ears “pop” in airplanes when we ascend and the atmospheric pressure changes. We may experience some pain in an airplane when landing again if we have a cold; mucus, blocking the Eustachian tube, can prevent us from equalizing the pressure by swallowing. Movement of the footplate of the stapes which is connected to the oval window causes pressure waves in the fluid of the upper gallery of the cochlea (Figures 4.3 and 4.4). The fluid in the lower gallery is separated from that in the upper gallery by the cochlear duct containing the organ of Corti. The organ has about 35 000 sensitive hair cells distributed along its length which are connected in a complicated way to about 18 000 nerve fibers which are combined into the auditory nerve which runs into the brain. The pressure waves cause the basilar membrane to deflect and a shearing motion occurs between the basilar and tectorial membranes. The hair cells sense the shearing motion and if the stimulus is great enough the neuron to which each hair cell is attached sends an impulse along the nerve fiber to the brain cortex [8]. Each neuron takes about 1/1000th of a second to recharge and so individual neurons are limited to “firing” no more than 1000 times/second. With the neurons, a triggering level must be reached before they “fire” and so they have an all‐or‐nothing response. The brain must interpret the neural impulses to give us the sensation of hearing and, as we can imagine, the way in which this is done is not well understood.