Abstract:
Noise reduction in ducts with flow is a very useful project. With common antisound methods, electronic instruments are required, so its application is restricted. By using bypass added to the main duct with flow, downstream noise reduction can also be obtained if suitable bypass parameters are selected: This simple method is completely in accord with antisound principle. But sound field analysis in this case is quite a complicated problem. The numerical and experimental research was completed by the authors. The numerical work included formulating acoustic relations and joint conditions and calculating the transmission loss for various noise sources. The experimental research on a duct with single bypass and outlet absorbing wedge under the conditions of different noise sources and flow velocities was carried out in a national lab. The measured data were in good agreement with the numerical results and considerable noise reduction was obtained in both dB(A) and 1/3 octave band. Finally, a spot test of noise reduction using double bypasses was carried out. It was shown that a noise reduction of 5 dB(A) was obtained even though the situation was not in optimization. [Work supported by NNSF of China.]