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Loud noise also causes a number of unpleasant bodily sensations, such as vibration of the head and eyeballs, loss of vision, loss of equilibrium and heating of the skin. A noise of 160 decibels can kill rats and mice. Explains Knudsen: "The body temperature rises to a lethal level. It's the conversion of sound energy into heat that kills." In humans, at sounds near and above 160 decibels, the stirrup (one of three little bones in the middle ear) may be driven through the small "window" in the well of the inner ear. Possible result: meningitis, from infection of the fluid in the inner ear.
Even low-intensity tones can be dangerous if they are pure, i.e., if their frequency is limited. The reason: pure tones form a concentration of sound energy within the ear. The most "sensitive" frequencies—those which the body can least tolerate—occur, says Dr. Knudsen, within the octaves of 300-600 cycles per sec. and 600-1,200 cycles (middle C is 261.6 cycles). Warns Dr. Knudsen: "Anyone who lives in an environment where the intensity at these frequencies consistently reaches 85 decibels [for example, alongside a busy jet airport] should have hearing tests—because damage is possible. If the level stays at 90 decibels or above, damage will definitely result."
Psychological Effects. Workers in noisy surroundings often complain of such apparently psychogenic ailments as nausea, fatigue, headache, loss of neuromuscular coordination, and reduced sexual desire. "Noise can and does drive some people to distraction," says Dr. Knudsen. "If noise does nothing more than interfere with sleep —and this it does on a gigantic scale—it is a menace to good health." Knudsen carefully catalogued causes for his own middle-of-the-night awakenings, found that 75% were the result of noise. Most common culprits: auto horns, barking dogs, ambulance sirens, chirping birds. Dr. Knudsen's solution: earplugs. The plugs could not cope with sirens and extra-loud auto horns, but they attenuated all noise by about 30 decibels, cut his nightly awakenings in half.
Studies also show that noise adversely affects human efficiency. The Air Force's Dr. von Gierke says: "It impairs both manual dexterity and accuracy." A normally accurate, responsible aircraft mechanic may unconsciously rush, through his work, do a slipshod job, if he happens to be working in the neighborhood of a whining jet exhaust. When officials of Aetna Life Insurance Co. cut office noise levels 14.5% by installing acoustic wallboard, they found that typists' errors dropped 29%, machine operators' errors fell 52%, employee turnover decreased 47%, and absenteeism declined 37.5%.
What's Being Done? Noise costs U.S. industry an estimated $2,000,000 a day in workmen's compensation (for noise-related injuries), lost man-hours and decreased efficiency—but industry has been slow about putting in adequate controls. U.S. airlines, for example, balk at installing adequate jet noise suppressors, estimate that reduced engine power would cut payloads by 13 passengers per plane. Truck-line operators remove factory-installed mufflers in the mistaken belief that vehicle performance is sharply improved. Despite growing public pressure for noise abatement, few U.S. cities have adequate noise-control ordinances.