For giving us speed and convenience, technology demands part, and sometimes all, of our sense of hearing.
It comes as a surprise to many that noise can cause deafness: not the trauma of an explosion, but the cumulative effect of prolonged exposure to noise below the levels produced by the Chicago and New York subway systems. This kind of noise exposure is deafening millions in industry, and unknown numbers on farms and in offices.
Hearing loss from noise exposure is called sensory-neural or nerve deafness. The effect is to disable the inner ear and prevent it from transmitting sound signals to the brain. Though damage to the organ of hearing is the one form of bodily harm that can be measured, to a degree, and viewed under the microscope, very little is known about the exact mechanism by which the sound-wave pressure destroys the ear.
There are no direct tests for measuring the destructive effect of noise. Not even hearing loss can be measured directly. The tuning fork can detect the ability to hear specific frequencies. The varieties of audiometers measure the ability to understand selected speech signals or the ability to detect a pure tone of select frequencies. In pure-tone testing each tone is presented to the listener in a variety of ascending and descending intensities until it is determined at what decibel level the listener responds. In other words, pure-tone testing measures the range of select frequencies which can be heard, and the intensity—measured in decibels—necessary for the signal to be heard.
Only a periodic series of hearing tests can detect the onset of hearing loss.
Dr. Rosen has postulated that one reason for nerve deafness is reduction of the blood supply to the nerve endings of the inner ear. Hardening of the arteries could be one reason for the reduced blood supply; vasoconstriction caused by noise could be another. In 1961, together with an international team of physicians and audiologists, Rosen conducted a study of the primitive Mabaans of the African Sudan. These people were found to have a keen sense of hearing and no evidence of coronary heart disease. They live in an environment almost free of noise—the typical level is 40 decibels—with few emotional stresses. There was evidence that their blood vessels enjoyed a normal elasticity even in old age. Industrialized man loses this elasticity; hardening occurs. Among the Mabaans, who live in an atmosphere of virtual silence, the hearing of even men in their seventies and eighties is the equal of healthy youngsters of ten.
Noise-stressed blood vessels that have lost their elasticity take a long time to return to normal size after the noise stimulus is removed. This means that the blood flow is diminished for a period long enough to cause damage to the cells fed by the circulatory system. Rosen suggested that continued exposure to excessive noise may eventually lead to a chronic state of blood deprivation and finally to death of the cells involved with hearing.
This scientific hypothesis has since been substantiated in work on monkeys. At the University of Michigan, monkeys were subjected to a noise signal and then put to death. It was found that the inner ear had indeed been damaged, and that the blood vessels were constricted.
Another theory for the cause of hearing loss is that the constant pressure of intense noise does physical damage to the nerve endings in the inner ear. The concept is analogous to the case of wear and tear on the pile of a rug.
In any event, noise can produce what ear specialists call a threshold shift, or hearing loss.
Even a few minutes of exposure to intense noise can cause temporary deafness. The users of noisy appliances, powered lawn mowers for example, experience significant hearing loss for a variable period of time after using such products. This loss is called noise-induced temporary threshold shift, or NITTS. NITTS is what the members of rock'n'roll bands experience wherever amplified music is played. Subjectively it may be observed as a muffled sensation and/or a ringing in the ears. One empirical method of detecting a NITTS is to listen to the mechanism of a watch before and after exposure. The degree of loss is indicated by the amount of time needed for recovery.
Researchers at the University of Minnesota measured hearing sensitivity of band members following a four-hour session of music having an over-all sound-pressure level ranging from 110 to 125 decibels. In 25 minutes there was a loss of from 10 to 30 decibels of hearing in the critical 2,000 cps speech frequency. Recovery in some cases took from 18 to 50 hours. The longer recovery time could be serious if the individual re-exposed himself before full recovery occurred. In fact, after suffering an undetermined amount of acoustic assaults that cause temporary deafness, the amplified music addict, or the factory worker, may end up with noise-induced permanent threshold shift, or NIPTS.
Our ears, like our hearts, work 24 hours a day. The excessive acoustic stimuli to which modern man is subjected-or subjects himself—so abuses his sense of hearing that medical men speak of two phases of "ear life": the length of time the ear will serve us for hearing a wide range of sounds, and the length of time the ear will serve us for hearing speech.
In her kindness to us, unselfish Nature has made it easier for us to lose the ability to hear the upper frequencies first. This means that the first penalty of excessive noise is the loss of the ability to enjoy pastoral sounds and the full range of musical tones. High-fidelity stereo systems reproduce sounds up to 15,000 cps or even higher. Most members of an industrialized society, by the time they reach senior citizenship, will not be able to hear 10,000 cps, let alone 15,000. The decline in hearing acuity for the male in industrialized societies begins somewhere between the ages of 25 and 30. Many millions of human beings are exposed to a lifetime of noise so intense that their frequency range drops to below 2,000 cps and they find it no longer possible to hear human speech sounds. It is believed that to understand English speech perfectly, one needs to hear all its sounds in the range from 200 to 6,000 cps. To legally qualify as having a "hearing impairment," a worker must show a substantial loss in the critical speech frequencies of 500, 1,000, and 2,000 cps.