彩虹捕获吸收器：通过带通风的亚波长板实现宽带、完美和非对称吸声
Rainbow-trapping absorbers: Broadband, perfect and asymmetric sound absorption by subwavelength panels with ventilation
摘要：
从理论、数值和实验三个方面研究了亚波长厚度板在透射问题中的完美、宽带和非对称吸声。该面板是由一个周期性阵列的不同截面的波导，他们中的每一个被加载的亥姆霍兹谐振器（HR）与梯度尺寸的阵列。吸收带的低截止频率由最深HR的谐振频率固定，这大大降低了透射率。前面的HR设计为具有稍高的谐振频率，其几何形状允许与周围介质进行阻抗匹配。因此，反射消失，结构临界耦合。这导致在单一频率下的完美吸声。我们报告完美的吸收在300赫兹的结构，其厚度是40倍小于波长。此外，通过将HR添加到波导来重复该过程，每个HR具有比前一个更高的谐振频率。使用这种频率级联效应，我们报告了由9个谐振器组成的面板在300至1000 Hz的几乎两个频率倍频程内的准完美吸声，总厚度为11 cm，即，比300 Hz时的波长小10倍。
英文摘要：
Perfect, broadband and asymmetric sound absorption is theoretically, numerically and experimentally reported by using subwavelength thickness panels in a transmission problem. The panels are composed of a periodic array of varying cross-section waveguides, each of them being loaded by an array of Helmholtz resonators (HRs) with graded dimensions. The low cut-off frequency of the absorption band is fixed by the resonance frequency of the deepest HR, that reduces drastically the transmission. The preceding HR is designed with a slightly higher resonance frequency with a geometry that allows the impedance matching to the surrounding medium. Therefore, reflection vanishes and the structure is critically coupled. This results in perfect sound absorption at a single frequency. We report perfect absorption at 300 Hz for a structure whose thickness is 40 times smaller than the wavelength. Moreover, this process is repeated by adding HRs to the waveguide, each of them with a higher resonance frequency than the preceding one. Using this frequency cascade effect, we report quasi-perfect sound absorption over almost two frequency octaves ranging from 300 to 1000 Hz for a panel composed of 9 resonators with a total thickness of 11 cm, i.e., 10 times smaller than the wavelength at 300 Hz.
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