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According to their effects on man, the author distinguishes between four different noise zones, the transition ta the next zone being characterised by the appearance of an additional effect. These are: psychic reactions, vege- tative reactions, injuries ta the internal ear, and, finally, mechanical de- struction of ganglion cells. The author gives examples of psychic reactions, depending not only an the nature of the noise but also an the mental attitude of the person per- ceiving the noise towards its source. The most characteristic reaction is a constriction of peripheral blood vessels, followed by a decrease in circula- tion in the whole body. The degree of this reaction depends an the intensity of the noise, but not an its frequency, though the width of the noi se band plays an important r8le. As ta the vegetative reactions, there is no adaptation to noise. In sleep they are stronger than in a wakeful state. Exposure ta noise for years causes persistent effects an the blood flow through the skin, but also an the heart action and the regulation of the equilibrium. Noise of more than 90 phones may prove injurious ta the auditory ap- paratus. Higher frequencies are more detrimental than lower ones. The audiometric diagram allows ta distinguish between traumatic injuries and those due ta old age. Extraaural effects of noise are only ta be found among operators of jets and rockets, especially at test-stands, and may be pre- vented by wearing protective devices.
