No-Flow Impedance Models for Open-Variable-Depth Chambers

A surface impedance modeling approach for open-variable-depth chambers in a no-flow environment is described that takes into account the self and mutual radiation impedance of each chamber. This approach is compared in accuracy and efficiency to other established methods, using experimental data from a variety of samples tested in a no flow, normal-incidence impedance tube at relatively low sound pressure levels (<100 dB). It is found that the new approach described in this study is similar in accuracy to finite-element methods while operating at orders-of-magnitude lower computational time. A low-frequency approximation
to the proposed method further improves model efficiency while maintaining accuracy, at least for the samples and test conditions considered in this study. Finally, a prediction study is shown that assesses the need for modeling radiation impedance of open-variable-depth, narrow-width chambers, illustrating that radiation effects should also be modeled for these samples, at least for higher frequencies (3000–6000 Hz). This prediction technique lays the foundation for future models designed to capture impedance for realistic variable-depth liners in grazing-flow environments, such as perforated facesheet liners in an engine nacelle.