Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong.
J Acoust Soc Am. 2009 Dec;126(6):3008-19. doi: 10.1121/1.3257182.
Low-frequency noise is difficult to deal with by traditional porous material due to its inherent high acoustic impedance. This study seeks to extend the effective range of sound absorption to lower frequencies by filling a low density gas, such as helium, in the porous material. Compared with conventional air-filled absorption material, the helium-filled porous material has a much reduced characteristic impedance; hence, a good impedance matching with pure air becomes more feasible at low frequencies. The acoustic properties of a series of helium-filled porous materials are investigated with a specially designed test rig. The characteristic of the sound propagation in a helium-filled porous material is established and validated experimentally. Based on the measured acoustic properties, the sound absorption performance of a helium-filled absorber (HA) of finite thickness is studied numerically as well as experimentally. For a random incidence field, the HA is found to perform much better than the air-filled absorber at low frequencies. The main advantage of HA lies in the middle range of oblique incidence angles where wave refraction in the absorber enhances sound absorption. The advantage of HA as duct lining is demonstrated both numerically and experimentally.
低频噪声由于其固有的高声阻抗，很难用传统的多孔材料来处理。本研究旨在通过在多孔材料中填充低密度气体（如氦气），将吸声的有效频率范围扩展到低频。与传统的充空气吸声材料相比，充氦多孔材料的特征阻抗大大降低；因此，在低频下，纯空气与多孔材料的阻抗匹配变得更加可行。利用专门设计的测试装置研究了一系列充氦多孔材料的声学性能。通过实验验证，建立了充氦多孔材料中声波传播的特性。基于测量的声学特性，对有限厚度充氦吸声体（HA）的吸声性能进行了数值和实验研究。对于任意入射场，在低频时，HA 的吸声性能明显优于充空气吸声体。HA 的主要优点在于斜入射角度的中间范围，在该范围内，吸声体中的波折射增强了吸声效果。通过数值和实验证明了 HA 作为管道衬里的优势。
