The noise attenuation properties of an acoustic liner consisting of Helmholtz resonators with extended necks (HRENs) are investigated. An optimal liner constructed by 16 inhomogeneous HRENs is designed to be effective in sound absorption in a prescribed frequency range from 700 to 1000 Hz. Its quasi-perfect absorption capability (average absorption coefficient above 0.9) is validated by measurements and simulations. The resonance frequencies of the individual resonators in the designed liner are just located within the effective absorption bandwidth, indicating the overlapping phenomenon of absorption peaks. In addition, the liner maintains a thin thickness, about 1/25th with respect to the longest operating wavelengths. To assess the acoustic performance of the designed liner in the presence of mean flow, experimental investigations are performed in a flow tube. Results show a near flat transmission loss is attained in the target frequency range by the designed liner. Additionally, the impedance of the uniform HREN-based liner is extracted at flow condition. In all, the inhomogeneous HREN-based liner is featured by the thin thickness and the excellent wide-band noise attenuation property. These features make the designed liner an promising solution for noise attenuation in both static and flow conditions.

中文翻译：

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带有扩展颈部的亥姆霍兹共振器构造的声学衬里的噪声衰减研究

研究了由具有延伸颈部的亥姆霍兹共振器（HREN）组成的隔音衬垫的噪声衰减特性。设计了由16个不均匀的HREN构成的最佳衬垫，可在规定的700至1000 Hz频率范围内有效吸收声音。它的准完美吸收能力（平均吸收系数高于0.9）已通过测量和仿真得到验证。设计的衬管中各个谐振器的谐振频率仅位于有效吸收带宽内，表明吸收峰存在重叠现象。另外，衬套保持较薄的厚度，相对于最长的工作波长约为1/25倍。为了评估在存在平均流量的情况下设计的衬管的声学性能，实验研究是在流量管中进行的。结果表明，通过设计的衬管在目标频率范围内可获得接近平坦的传输损耗。另外，在流动条件下提取均匀的基于HREN的衬管的阻抗。总而言之，基于HREN的不均匀衬里具有薄的厚度和出色的宽带噪声衰减特性。这些功能使设计的衬管成为解决静态和流动条件下噪声衰减的有前途的解决方案。