Lined ducts in parallel for low-frequency noise reduction

A new method to enhance the low-frequency sound attenuation characteristics of a lined duct is presented. A rigid, longitudinal splitter plate is used to separate the lined duct section into two parallel branch ducts. The phase speed of the fundamental sound mode in each parallel branch is a function of the liner configuration and can differ substantially from the free space phase speed. By lining the parallel branches dissimilarly, destructive interference of the fundamental mode of the transmitted wave occurs downstream of the parallel absorbers at a selected set of frequencies. Strong reflections also occur at the silencer inlet for a larger set of frequencies due to an acoustical impedance mismatch at the junction of the main duct and the parallel branches. These interference effects, which result from placing the rigid splitter plate into the lined duct can increase the low-frequency transmission loss of the duct by more than two fold over a broad low-frequency band and by as much as 5-dB/unit duct width at selected frequencies.