Abstract Impedance eduction experiments on acoustic liners with flow have systematically shown the educed impedance depending on the direction of the incident wave. Recent attempts to model this dependence include impedance boundary conditions with an additional degree of freedom. In this case, both upstream and downstream acoustic sources must be used to educe both the liner impedance and the extra degree of freedom of the model, which implies two different axial wavenumber conditions are used, and always result in a perfect collapse of the educed impedances; this would be true whether or not the model itself is correct. In this work, we describe a novel experimental setup that allows for four different axial wavenumbers per frequency to be measured: two upstream and two downstream. We use this experiment to investigate three different impedance boundary conditions, namely inviscid sheared, viscous, and momentum transfer conditions, in addition to the classical Ingard–Myers boundary condition, using an inverse eduction technique based on the mode matching method. The additional degree of freedom in each of the first three models is best fitted to experimental data, and then compared to the theoretically predicted values. Unphysical or unrealistic values are found at certain frequencies for each model, and therefore the validity of such models is questionable. The predictive capabilities of the models are tested and compared by means of the plane-wave scattering matrix, and no model is found to be truly predictive. Under certain conditions, educed impedances using the Ingard–Myers boundary condition show similar accuracy in terms of transmission coefficients when compared to the other models.

中文翻译：

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实验测试带流动的声学衬垫的阻抗边界条件：超出上游和下游

摘要 带有流动的声学衬垫的阻抗衰减实验系统地显示了依赖于入射波方向的衰减阻抗。最近对这种相关性建模的尝试包括具有额外自由度的阻抗边界条件。在这种情况下，必须同时使用上游和下游声源来推导线性阻抗和模型的额外自由度，这意味着使用两个不同的轴向波数条件，并且总是导致推导阻抗的完美崩溃; 无论模型本身是否正确，这都是正确的。在这项工作中，我们描述了一种新颖的实验装置，它允许每个频率测量四个不同的轴向波数：两个上游和两个下游。除了经典的 Ingard-Myers 边界条件外，我们还使用基于模式匹配方法的逆推导技术研究了三种不同的阻抗边界条件，即无粘性剪切、粘性和动量传递条件。前三个模型中每一个的额外自由度最适合实验数据，然后与理论预测值进行比较。对于每个模型，在某些频率处发现了非物理或不切实际的值，因此此类模型的有效性值得怀疑。通过平面波散射矩阵对模型的预测能力进行了测试和比较，没有发现任何模型是真正具有预测性的。在一定条件下，