THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

220 10. Physiology of Hearing and Psychoacousticsmoves, these stereocilia sway under the impact. The deflections initiate a chain ofelectrochemical events that generate electrical spikes or action potentials in thecells of the spiral ganglion. These signals are transmitted to a relay station calledthe cochlear nucleus in the brain stem. The information flows through other nucleialong the path to the auditory cortex, the portion of the brain that processes auditoryinformation.Within this cellular circuitry, the frequency of a sound is encoded by two mechanisms.The first mechanism is a place code that indicates the location along thetapered basilar membrane that moves the greatest distance. Stereocilia on the haircells respond to this displacement and produce action potentials among the nearestspiral-ganglion neurons. The other mechanism is a temporal code that is createdwhen neurons become synchronized (or phase-locked) into the period of an acousticwave. In normal hearing, neural responses can readily match frequencies upto about 1 kHz, but this phase-locking capability declines progressively at higherfrequencies. It is conceivable that perception of frequency is based on some combinationof phase and temporal orders, with the temporal code being effective onlyat lower frequencies.10.3 Hearing LossNearly a quarter of the population between the 15 and 75 years of age suffer hearingimpairment. Impaired hearing, which is often caused by infectious diseases oroverexposure to loud noise or simply the process of aging, is common enough tobe on a par with the onset of poor vision. When hearing loss occurs in early childhood,its consequences become more obvious than when it occurs in adulthood. Achild’s progress in learning and developing social relationships may be hinderedand the child may even be deemed “not too bright” if professional help and guidanceare not forthcoming. The primary problem of hearing loss, regardless of theage of the affected individual, is a diminution of a person’s ability to understandspeech.Even milder forms of hearing loss early in life can generate great difficulty,particularly for children who developed within normal limits but are not doingwell in school, due to their being inattentive. Such moderate hearing losses arenot uncommon and may even be on the increase due to heightened exposure to“rock” music. When a mild hearing loss is corrected, the child often becomes“like a different person.” Fortunately, many of the hearing impaired can be helpedthrough the use of hearing aids.The gradual waning of hearing loss affects adults in a more underhand manner.Most people with age-induced or noise-induced hearing impairment first losehearing acuity at high frequencies, making it difficult for them to distinguish consonants,especially s versus f , and t versus z. Such persons must strain harder tounderstand conversations. Going to the movies, listening to lectures, conversingwith friends and other pleasures become stressful chores. This can result in anindividual’s becoming withdrawn from his friends and relatives. Some of these