Optimal absorption associated with wall impedance in acoustic waveguides

2018 
In a previous talk at ASA it was shown that the double root for modal frequencies in a 2D waveguide can exhibit almost perfect absorption over a relative broad frequency range. The key to the phenomenon is that the wall impedance is such that modes coalesce at a complex-valued frequency. In this talk we consider the more realistic case of circularly symmetric waveguides. Numerical simulations are shown for reflection and transmission from a waveguide with impedance boundary conditions associated with double roots. The results again show large and almost perfect absorption at frequencies near the real part of the complex root. The model is linked to realistic passive impedance values based on various models for boundary impedance, including perforated and porous panels. These comparisons are discussed to illustrate the feasibility of optimized wall impedances in absorbing sound passing through ducts. Preliminary experimental data will be presented. Work supported by NSF.
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