Specific physiological reactions begin at sound levels of 70 to 75 dB for a 1000 Hz pure tone. At the threshold of such response, the observable reaction is slow but definite after a few minutes. These reactions are produced by other types of stimulation, so they can be considered as reactions to general physiological stress. First the peripheral blood vessels constrict with a consequent increase in blood flow to the brain, a change in breathing rate, changes in muscle tension, and gastrointestinal motility and sometimes glandular reactions detectable in blood and urine. Increased stimulation causes an increase in the reaction, often with a change in form. These reactions are sometimes called N-reactions—nonauditory reactions. If the stimulus continues for long, adaptation usually occurs with the individual no longer conscious of the reaction, but with the effect continuing. Auditory responses occur as well as these nonauditory or vegetative ones. If exposure is continued long enough, TTS can occur, and a loss of some hearing acuity usually results with increasing age. Some workers refer to a "threshold of annoyance to intermittent noise" at 75 to 85 dB.
At a slightly higher level, and especially for intermittent or impulsive noise, another nonauditory response ap-pears—the startle effect. Pulse rate and blood pressure change, stored glucose is released from the liver into the bloodstream (to meet emergency needs for energy), and the production of adrenalin increases. The body experiences a fear reaction. Usually psychological adaptation follows, but with changed physiological conditions.
At noise levels above 125 dB, electroencephalographic records show distorted brain waves and often interference with vision.
Most of these nonauditory reactions are involuntary; they are unknown to the subject and occur whether he is awake or sleeping. They affect metabolism; and since body chemistry is involved, an unborn baby experiences the same reactions as its mother. Sounds above 95 dB often cause direct reaction of the fetus without the brief delay required for the chemical transfer through the common bloodstream.
Most people find that under noisy conditions, more effort is required to maintain attention and that the onset of fatigue is quicker.
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