Document 5jd6LDLNY1Xo5LqZod091R8e

m 665566 JEROME R. COX, JKR. passageways,0'12 and through mufflers10 are related topics and may be pursued the literature. >1 III. The Ear and the Measurement of Hearing Any discussion of industrial noise would be incomplete without a despri^P of how it affects human beings. The physical aspects of noise can be measuiwl many ways, but this information would be useless without knowledge of hii1 responses to noise. First, the operation of the auditory system will be revi quite briefly. ' vlf A. THE EAR The ear may be divided into three.parts called the external, middle, and-iiSj ear (Figure 22). Sound waves enter the external ear and pass along the audijf! Skin &ONE. SCMI-CIACULAA CANALS. MALLEUS Incus ri i A Sound wauls ARC TRANSMITTED BY THt STAPES TO THt FLUIO OF THE INNER EAR J SOUND Path o* "waves'" Nerve to brain /-vjv r?Prf v&ir Stapes Middle ear E.USTACHIAN TUBE Slnsorv cells or THE COCHLEA, STIMULATED BT THE SOUNO WAVES Figure 22. A schematic drawing of the ear. canal, which is terminated by the ear drum membrane. This is the boundar tween the external and the middle ear. Variations in the sound pressure a drum membrane cause it to vibrate back and forth, and its displacement normuHs'VdfyTreWly'pTOpTSrtmrJalToTte'souTld'^essuferTli'emotidn orfluTR-uf membrane is transmitted across the middle ear through a system of thregj bones called the ossicles. The first of the three small' bones is the m (hammer), which is attached to the inside of the drum membrane. The') (stirrup) parries the sound into the inner ear. The incus (anyill connef malleus to the stapes. The inner ear, or cochlea, is filled with fluid that|| R. H. Bolt, L. L. Beranek, and R. B. Newman, Handbook of acoustic noise edinnfi Physical acoustics, Supplement 1, Wright Air Development Center Tech. Rept. 5 Wright-Patterson Air Force Base, Ohio, 1, 1055. NOISE AND THE CONSERVATION OP HEARING 657 ltra forth as a result of the vibration of the footplate of the stapes. In this SiSa'jr-pressure variations produced by the sound source are changed into '||f|phe ossicles by the drum membrane and then into motion of the fluid in J>50c*buoIlea, Finally, the motion of the fluid excites many thousands of tiny sensory jj.'/ ' ' 'galled hair cells. After excessive' exposure to intense noise, these microscopic 1 perform properly or may even show physical damage. .5 [^^Fffc;7&:|ddle ear cavity, which contains the three ossicles, has an ophHipg in KfffP'lwiaktwfl.ll that voes to the mastoid area behind the ear. and an onp.nihiriimthe ?i*Jumpqualize the atmospheric pressure on the opposite sides of the drum inem- -iibeuochlea is a spiral channel in the skull. It may be visualized as a tube BifS lengthwise into three compartments, all filled with fluid. The wall separat|j!g?two of the compartments is called the cochlear partition, and embedded in it Mrfthjabout 20,000 hair cells that serve to transform the mechanical motion into i-lcefri'eai signals. Nerve endings located at the base of the hair cells are stimu"`^Bfethesc electrical signals and initiate neural impulses that travel along the to higher nervous centers. Bfielhair cells nearest the middle ear are stimulated most, effectively by Wcohtaining the highest audible frequencies. Sounds with),low frequency ilefrtS. stimulate most effectively the hair cells in the area near the tip of the fiMwaiid farthest-from-the-middle-ear:-The extremesUfThe audible frequency |lp|iibt sharply defined, but are related to the intensity of the sound. In |lf|HHfial ears these extremes are approximately 30 and 15,000 c.p.s. for inSwAwhich in the middle frequency range would correspond to that of the Try speaking voice. At greater intensities many listeners can hear sounds to 20,000 c.p.s. Since the cochlea is set in the skull, intense vibration of the head at audible jJplicieB will agitate the fluid in the cochlea, and cause the sensation of sound. ^ocess is called hearing by "hone conduction." . By contrast, the usual jpd', utilizing the drum -membrane and .ossicles,,..is^called hearing by "air BTOtiion." : ' '5' ' '' ' ^ ~ |r -r "^S^'rai^MEASuSjEtlS^lfoF HEARING Ipfe device used today to measure hearing is called an audiometer. Years tuning forks, watch ticks, coin clicks, whispers, and voice tefets were used lively. With the advent of the, vacuum tube more exact -methods-atoeasurA- jsbeetEmrKVatlEble. in the early 1930's the electronic audiometer developed llpand soon reached a state closely resembling today's models.. The-photo- |Mih Figure 23 show five of the commercially available models that have designed to meet the specifications of the American Standards Association ||e American Medical Association. Those shown are adequate for most in-