19) ON THE NATURE OF STREET NOISE AND THEIR ATTENUATION BY DISTANCE FROM THE STREET ROAD

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  • 19 街路騒音とその伝播性状に関する研究
  • 街路騒音とその伝播性状に関する研究
  • ガイロ ソウオン ト ソノ デンパ セイジョウ ニ カンスル ケンキュウ

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Abstract

In recent years, the air velocity in an ari-conditioning duct system is going to become higher, so the fan noise is increasing because of the higher pressure and the higher horse power. The greater parts of the eneryg of the fan noise are contained in low frequencies. Therefore, it seems to be unsuitable to attenuate the fan noise with acoustic lining ducts, for ordinary sound absorptive materials, as linings, have small sound absorption coefficient at low frequencies, and have high pressure losses of friction in a high speed duct system. The box type sound attenuator has good attenuation characteristics at low frequencies by calculating the length and the enlarging ratio of the cross sectional area of the box to that of the straight duct. To apply the box type sound attenuator to the practical use of the high speed duct system, an experimental research of the pressure losses due to area changes of the box type sound attenuator has been done. This report shows, as the result of the experimental research of the practical scale of the box and air velocity, that: 1. The normal box type attenuator has a smaller total pressure loss coefficient "ξ_t" than the sum of the pressure loss coefficient of abrupt expansion and abrupt contraction. 2. "ξ_t" becomes 3. "ξ_t" becomes 4. By adding the normal type. 5. By adding the the normal type. 6. But it has not hausting side of smaller as the enlarging ratio "m" becomes larger, larger as the length of the box "l_0" becomes larger. cone type hopper to the entering side of the box, "ξ_t" becomes larger than "ξ_t" of the cone type hopper to the exhausting side of the box, "ξ_t" becomes smaller than "ξ_t" of so large effect on "ξ_t" to add cone type hopper either to the entering side or to the ex-the box.

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