The absorption of airborne noise at frequencies below 300 Hz is a particularly vexing problem due to the absence of natural sound-absorbing materials at these frequencies. The prevailing solution for low-frequency sound absorption is the use of passive narrow-band resonators, the absorption level and bandwidth of which can be further enhanced using nonlinear effects. However, these effects are typically triggered at high intensity levels, without much control over the form of the nonlinear absorption mechanism. In this study, we propose, implement, and experimentally demonstrate a nonlinear active control framework on an electroacoustic resonator prototype, allowing for unprecedented control over the form of nonlinearity and arbitrarily low intensity thresholds. More specifically, the proposed architecture combines linear feedforward control on the front pressure through a first microphone located at the front face of the loudspeaker and nonlinear feedback control on the membrane displacement estimated through the measurement of the pressure inside the back cavity with a second microphone located in the enclosure. It is experimentally shown that even at a weak excitation level, it is possible to observe and control the nonlinear behavior of the system. Taking the cubic nonlinearity as an example, we demonstrate numerically and experimentally that in the low-frequency range (50–500 Hz), the nonlinear control law allows improvement of the absorption performance, i.e., enlarging the bandwidth of optimal sound absorption while increasing the maximal absorption coefficient value and producing only a negligible amount of nonlinear distortion. The reported experimental methodology can be extended to implement various types of hybrid linear and/or nonlinear controls, thus opening new avenues for managing wave nonlinearity and achieving nontrivial wave phenomena.

中文翻译：

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通过非线性有源电声谐振器改善声音吸收

由于在这些频率下不存在自然的吸声材料，因此在低于300 Hz的频率下吸收空气传播的噪声特别麻烦。低频吸声的主要解决方案是使用无源窄带谐振器，利用非线性效应可以进一步提高其吸收水平和带宽。但是，这些效应通常是在高强度水平下触发的，而对非线性吸收机制的形式没有太多控制。在这项研究中，我们提出，实施并通过实验证明了电声谐振器原型上的非线性主动控制框架，从而可以对非线性形式和任意低强度阈值进行前所未有的控制。进一步来说，所提出的架构结合了通过位于扬声器正面的第一个麦克风对前压力的线性前馈控制和对通过测量后腔内部压力与外壳中的第二个麦克风估算的膜位移的非线性反馈控制的结合。实验表明，即使在弱激励水平下，也可以观察和控制系统的非线性行为。以立方非线性为例，我们通过数值和实验证明，在低频范围（50-500 Hz）中，非线性控制律可以改善吸收性能，即 扩大最佳声音吸收的带宽，同时增加最大吸收系数值，并且仅产生可忽略不计的非线性失真。所报道的实验方法可以扩展为实现各种类型的线性和/或非线性混合控制，从而为管理波浪非线性和实现非平凡的波浪现象开辟了新途径。