Besides the well-known occupational deafness, a type with relatively acute onset should be included in the group of hearing impairment due to noise. Since 1954, 2628 cases with aural disturbance have been studied by the precise hearing test and 82 cases (3.12%) are found to be of the accidental type.
In Sep. 1958 the author first reported sudden hearing loss due to industrial noise. This type of acute hearing loss have been taken into consideration by the Compensation Administration as the item of the Workmen's Compensation Insurance of Japan, since Apr. 1959.
Acute hearing impairment, however, can not be recognized in every person under the same condition. It usually befalls to persons specifically susceptible to noise. Even in the case of the same person, development of hearing impairment depends on the psychosomatic and environmental conditions present at the time. This disease shows the wide range of individual susceptibility, therefore, such specifications as follows must be established: onset of subjective symptoms of aggravation of previously impaired hearing, while working under conditions of harmful noise above the safety criteria (about 85phon) in certain place and time.
It is difficult to grasp precisely the clinical characteristics of the disease, as in the case of other perceptive deafness, but its developmental specificity deserves attention as one type of hearing impairment due to industrial noise.

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This study was designed to clarify the psychological backgrounds influenced to listner's judgement hearing a speech sound, with respect to familiarity of Japanese monosyllables.
The familiarity is defined as the frequency of usage in conversation and letters, say, as a degree in whether a monosyllable is more or less frequently used.
Familiarity of Japanese monosyllables can be classified to four kinds of group due to the frequency of usage of Japanese monosyllables.
Discrimination scores of four groups were compared with each other. Next, the problem, say, whether Japanese monosyllables are meaningfull or meaningless, was studied and time needed for its judgment was measured.
The results mentioned above were as follows. 1) A monosyllable was the more familiar the more discriminable. 2) Under various condition of acoustic distortion due to low-pass or high-pass filtering, it will be clear that high familiar monosyllables will be more remarkablly discriminated.
3) Most of all Japanese monsyllables are considered to be meaningfull.
4) Mechanism of familiarity of Japanese monosyllables acted on auditory discimination was discussed from the view of information theory. It is theoretically seemed that familiarity will act on mechanism of auditory information production in cortex.

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This study was designed to find out physiological background of the critical band proposed by Fletcher (1937) through psychoacoustic procedures. In this report the critical band could be interpreted in terms of spatial summation on the assumption that the critical band was related to neural information sent to the brain by some of the nerve patches as physiological background. Especially, the authors tried to clarify on anatomical level where spatial summation is performed. In this study, post-stimulatory facilitation, monaural power summation and binaural power summation were used as psychoacoustic method of measurement, and were performed both in normal and impaired ears. The results obtained were as fallows:
1) It would be made clear that the frequency range over which spatial summation was performed coincided approximately with Fletcher's critical bandwidth.
2) It was found that binaural summation could be considered to be negligible in psycholological measurement.
3) It could be inferred that spatial summation phenomenon was reflected in form of vibration along the basilar membrane with sinusoidal stimulus.
4) It could be assumed that Fletcher's critical band as well as spatial summation was psychoacoustical phenomenon caused by the action of synapse at the lower anatomical level than the primary auditory neuron.

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