(A) study on narrowband noise reduction characteristics of acoustic screens in a rectangular duct

Abstract

Noise reduction characteristics and scattering of sound due to installation of an acoustic screen inside a rectangular duct was studied. Transmission loss was measured for various acoustic screens with different geometry. It was observed that sound path asymmetry, which arises when an acoustic screen is installed so as to divide the main duct section into two asymmetric sound paths, gives rise to large narrowband sound attenuation at certain frequencies. Particular attention was paid to understanding the narrowband sound attenuation mechanism. The scattered sound fields were visualized by experimental and numerical means for detailed information on the physical phenomenon involved. Plane wave model was found to be insufficient for describing narrow band sound attenuation due to sound paths asymmetry. An enhanced analytical model which takes acount of the evanescent mode generation was developed. The applicability of acoustic screens as an alternative noise reduction device was investigated.