At the present time, how to extract acoustic features of voiceless stop consonants is oneof the most difficult problems remaining unsolved in the field of automatic speech recognition.This paper describes a study on characteristic features of the Japanese voicelessstop consonants /p/, /t/ and /k/. A multi-dimensional statistical analysis method isapplied to analyze their spectra. Analysis reveals that the principal characteristics discriminatingbetween them are reflected in the accumulated power from about 1kHz upto 5kHz and the existence of a spectral peak. It is also found that these feature parametersmake it possible to separate the voiceless stop consonants from each other. Experimentsof automatic recognition based on the maximum likelihood method are alsoperformed. They are carried out on condition that the following vowel is correctlyknown beforehand and that there are no errors in detection of the noise onset. It isfound that a recognition rate as high as about 97% can be attained for the training dataset of known utterances if the recognition algorithm designed to make the best use ofthe transition patterns of the feature parameters is adopted. It is also shown that theaccurate detection of the moment of noise burst is essential to attain a high recognition rate.

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Theoretical investigations are made of the sound radiation from a point source movingin parallel to a plane interface between two different fluids. An asymptotic expressionfor the reflected wave in the far-field region is obtained from the solution of a boundaryvalue problem in a whole space. A simple physical interpretation for the reflected wave is given.

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Theoretical investigations are made on the diffraction of sound radiated from a pointsource moving in parallel to the edge of a half-screen. The far-field diffraction patternis obtained with reasonable accuracy. A simple interpretation is given to the asymptotic solution for the field diffracted from the edge of the screen.

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Theoretical investigations are made of the problem of sound radiation from a pointsource moivng vertically to a plane interface between two different fluids. The farfieldsolution of the problem is obtained from the exact solution of a boundary valueproblem in a whole space. A simple physical interpretation is given for the approximate solution of the reflected wave.

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A new method for the estimation of reflection characteristics of a ground and a freefieldspectrum of noise is presented. The method utilizes informations of differencesof ripples created in power and cross spectra of the sound pressure signals that aresimultaneously observed by two microphones closely spaced along an axis normal tothe ground surface. Ground effects will change due to various factors in measuringconditions. Therefore it will be desirable to estimate ground effects simultaneously withthe measurement of sound pressure signals. The proposed method enables one to makean evaluation of ground effects using noise signals themselves even though their frequencycharacteristics are not known. Results of several practical performances in cases of an aircraft fly-over noise and others are also described.

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