Low frequency noise absorption by helium-filled duct lining

Low-frequency noise is hard to tackle by traditional sound absorption material because of its inherent high acoustic impedance and space limitation for installing such material. There is a significant sound reflection at the air-absorber interface due to the high impedance mismatch with ambient air and hence the sound wave does not have a chance to enter the absorber for dissipation. The performance of sound absorption material lining in the duct is even worse due to the grazing incidence on the air-absorber interface. The motivation of this study is twofold: (a) to extend the effective range of sound absorption to lower frequencies by filling helium gas in the porous material lining in the duct, and (b) to tackle low frequency noise with a thin layer of the porous material with near-grazing incidence. The work represents a further extension of our recent investigation of the sound propagation in helium-filled porous material. The acoustic impedance can be reduced by filling a low density gas such as helium in the porous material. This has been validated experimentally. Based on the acoustic properties of the helium-filled material found from the new function established in a recent publication by the authors of this paper, the sound absorption performance of helium-filled duct-lining (HDL) is investigated experimentally and theoretically in the current paper. This helps find out the optimal fibre diameter and the lowest frequency for achieving the 10 dB based on the expansion ratio of an empty expansion chamber of 6.5 which is high enough to assess the performance of silencer. There is good agreement between the theoretical prediction and experimental validation in terms of transmission loss, relative absorption coefficient and reflection coefficient. Helium-filled duct lining performs much better than air-filled duct lining in the thin and long configuration at low frequencies. It is because helium gas contributes large wave refraction that can enhance the sound absorption. This is demonstrated theoretically and experimentally with five different fibre diameter of porous materials.