This paper describes the noise generation mechanism of axial-flow fans built for heat exchangers and having 4-equally spaced blades. Generated sound power levels are observed in a field which approximates a semi free space test environment. The power levels are analyzed by means of an analytical function of flow coefficients for various blade tip configurations, tip clearances and blade pitch angles, and impeller position at inlet-bell. As a result, it is found that the proper choices of set-up position of impeller and blade tip configuration reduce the noise.

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The synthetic method of electrical passive networks is successfully applied to the development of an acoustic antialiasing filter for an electret condenser microphone. The Butterworth type lowpass filter of order nine is realized as a tandem connection of an acoustic filter of order five and an electronic filter of order four which is imbeded in the microphone amplifier. The merit of this synthetic design is that the filter can be so realized with only one acoustic resistance that makes the adjustment at the production easy.

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The present paper proposes composite complex sinusoidal modeling (CCSM) for the estimation of direction and spectrum of an incident sound using a linear array of microphones. Output signals of the microphones in the array are stored in a computer and separated by discrete Fourier transform (DFT) into different frequency components, for all of which spatial sequences are generated. If only one plane wave is present, a spatial complex sinusoidal wave is formed by the sequence, whereas in the case of a plural number of plane waves, the sequence is provided by the sum of the same number of spatial complex sinusoidal waves as that of the plane waves. Direction and spectrum of an incident plane wave are estimated by evaluating amplitude and a rate of phase change (PCR: phase change rate) of the corresponding sinusoidal wave. The directivity pattern obtained by CCSM has no side lobe and gives only a single line which indicates direction and power of the incident plane wave even if the length of the array is of the same order as the wave length of the incident plane wave. In addition, effectiveness of CCSM is confirmed in computer simulation.

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A frequency and reverberation response of a circular pipe resonator are studied by newly developed techniques of the signal compression method and the calculation method of reverberation response from impulse response. By these methods, reverberation decay curves for very narrow band input signals are obtained and dips in reverberation curves are observed. These dips appear when two frequency response peaks are influenced equally by narrow band input signals. These measuring methods would be useful for room acoustics when fine structure of reverberation decay curve is desired.

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The effect of repetition of impact sound on its loudness is investigated. The stimuli used in this experiment are the repeated model impact sounds which have triangular envelope after the actual impact sounds, and they are synthesized by amplitude modulation of sinusoidal carrier. The stimulus is repeated 1, 2, 4, or 8 times with the interval of 100, 200, or 400ms. As the results of this experiment, PSE for loudness of repeated impact sound increases by about 1dB for doubling of the number of repetition, and it shows a tendency of saturation along with the increase in the number of repetition. PSE for loudness corresponds to a lesser degree to the energy principle, since doubling of the number of repetition means 3dB increase of sound energy. The readings of SPL, measured by a sound level meter with a square circuit of 125ms rise time constant and 200-300 ms decay time constant, may well describe the loudness of repeated impact sound.

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