Noise attenuation characteristics of acoustic screens installed in a straight section of a rectangular duct is studied. It was observed that sound path asymmetry*which arises when an acoustic screen is installed so as to divide the original duct into two asymmetric channels and recombine*gives rise to large narrowband sound attenuation. At "rst glance, the observed phenomenon looked like it could be explained in terms of the noise cancellation mechanism in Herschel}Quinke tubes, which can be described mathematically by a plane wave model. However, extensive experimental and numerical analyses by the present authors suggested that the narrowband noise attenuation due to sound path asymmetry involves signi"cant higher order mode contributions, hence cannot be described in terms of the plane wave model. Hence, an analytical model that can accurately describe the noise rejection phenomenon was developed. It was found that the higher modes although non-propagating are absolutely necessary to describe the narrowband noise attenuation phenomenon due to sound path asymmetry, which would not have been predicted by a plane wave model.
2000 Academic Press