切向流条件下短舱单/双自由度声衬实验

使用声学流管实验台对一件双自由度（DDOF）声衬和一件单自由度（SDOF）声衬的声学特性进行对比测试。在最大0.26Ma切向流速和管道的截止频率之下，采用直接提取法SFM测得声衬的无量纲声阻抗，同时使用双传声器分解驻波法计算声衬安装段管道的传声损失（TL）和吸声系数等，基于声能量理论的传声损失可直观地展示两件被测声衬的吸声性能差异。结果表明在流管声学实验台上，相较于单自由度声衬，双自由度声衬能够有效拓宽声衬的吸声频带，同时共振频率处的传声损失不如单自由度声衬，切向流也会明显改变声衬的共振频率、弱化吸声能力。基于声能量的传声损失和吸声系数也为无等效阻抗的非均匀结构声衬提供了一种声学性能评估方法。

Single and double degree-of-freedom acoustic liners under grazing flow:Experiment

Two acoustic liners, one of Single Degree-Of-Freedom (SDOF) and the other of Double Degree-Of-Freedom (DDOF), are separately tested on a flow duct in the laboratory to compare their acoustic performance with grazing flow up to 0.26Ma. The normalized impedance of the liners is educed by Straight Forward Method (SFM), and the acoustic energy Transmission Loss (TL) and absorption coefficient of the liner-installed section are determined with the two-microphone method. The differences in acoustic performance between the two tested liners can be observed through TL as well as the impedance. Tested results show that the DDOF liner effectively broadens the sound absorption band compared with the SDOF liner on the flow duct. However, its maximum TL is less than that of SDOF. The grazing flow significantly changes the resonance frequency of the acoustic liner and weakens the sound absorption performance. Furthermore, the transmission loss and absorption coefficient based on sound energy also provide a new method for the assessment of the non-uniform structure acoustic liner which cannot be described with equivalent impedance.