Study on hhe Sound Insulation Efficiency of Ducts : Testing Methods and TL Characteristics of No Lugging Ducts

Abstract

There are few reports on the sound insulation efficiency of ducts, and the effect of the duct shape on Sound Transmission characteristics has been scarcely investigated in Japan. Therefore, we examined the measuring methods of the transmission loss (TL) through the duct walls, then we measured the TL for the various shaped ducts to investigate the TL chracteristics of no lugging ducts with rectangular and circular shapes. The measurements were made both directions (breakout and breakin of the sound energy) of propagation through the duct walls. As a result of these experiments, the followings were found: 1) To measure the TL of ducts, the hemi-free field method or the SI method should be applied. 2) For no lugging rectangular type ducts, TL values obtained from the prediction formulas by ASHRAE were similar to our experimental data, therefore the formulas are effective. 3) Duct with a perfectly circular cross-section have a very high TL at low frequencies, however by decrease of the impedance for the cross sectional direction, the TL of the distorted circular ducts are effected considerably.