Low frequency noise absorption by heliumfilled ductling

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. High magnitude of impedance results in a significant sound reflection at the air-absorber interface 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. 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 shallow thickness of the porous material. The work represents a further extension of our recently investigation of the sound propagation in helium-filled porous material [1]. 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 in [1], the sound absorption performance of helium-filled duct-lining (HDL) is investigated experimentally and theoretically. This helps to find out the optimal fibre diameter and the lowest frequency for achieving the 10dB criterion. 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 shallow and long configuration at low frequencies. It is because of the increased wave refraction attributed to helium gas enhances sound absorption. This is demonstrated theoretically and experimentally with five different fibre diameter of porous material.