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According to their effects on man, the author distinguishes between four different noise zones, the transition to the next zone being characterised by the appearance of an additional effect. These are: psychic reactions, vege- tative reactions, injuries to the internal ear, and, finally, mechanical de- struction of ganglion ceIls. The author gives examples of psychic reactions, depending not only on the nature of the noise but also on the mental attitude of the person per- ceiving the noise towards its source. The most characteristic re action is a constriction of peripheral blood vessels, followed by a decrease in circula- tion in the whole body. The degree of this reaction depends on the intensity of the noise, but not on its frequency, though the width of the noise band plays an important r61e. As to the vegetative reactions, there is no adaptation to noise. In sleep they are stronger than in a wakeful state. Exposure to noise for years causes persistent effects on the blood flow through the skin, but also on the heart action and the regulation of the equilibrium. Noise of more than 90 phones may prove injurious to the auditory ap- paratus. Higher frequencies are more detrimental than lower ones. The audiometric diagram allows to distinguish between traumatic injuries and those due to old age. Extraaural effects of noise are only to be found among operators of jets and rockets, especially at test-stands, and may be pre- vented by wearing protective devices.
