PURPOSE: To provide sound absorbing performance throughout a whole area ranging from a low frequency to a high frequency by a method wherein a sound absorbing material is installed in a duct length region equivalent to a frequency in a specified range.
CONSTITUTION: In a fibrous sound absorbing material, a sound is converted to vibration of a fiber and exchanged as heat energy and absorbed. Vibration of the fiber is effected in a region where the particle speed of a sound is highest. Namely, in such a way that the sound absorbing material is arranged in a position where the particle speed is maximized, sound absorbing performance is drawn to a maximum limit. Especially, in the frequency of noise desired to be reduced, the sound absorbing material is situated at a duct part having length equivalent to 1/4 wavelength, preferably 1/2 wavelength, of a frequency in a range of 100-500Hz and in a position where the particle speed is maximized. Further, in a space layer 3 formed between a small hole formed in the wall surface of the intake duct 1 and the sound absorbing material, since a resonance mechanism of a kind is formed, in addition to the effect of the sound absorbing material, a high damping effect is produced in a resonance frequency region determined by the diameter and the thickness of a hole and the volume of a space part.
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WATANABE KYOICHI
SUGAWARA HIROSHI
