Performance of Sharply Bent Acoustic Resonators at High Sound Levels

This study aims to understand how the presence of a single, sharp, 90-degree bend placed along the length of a quarter-wave resonator (QWR) affects its performance as a sound absorber. Several 3D-printed QWRs were tested in a two-microphone impedance tube with broadband noise and discrete tones (the latter at incident sound pressure levels as high as 148 dB re 20 μPa). At low sound levels, the QWRs’ resonance frequencies and absorption coefficients depended only subtly on the location of the bend, but some interesting trends were found nonetheless. For high-amplitude, discrete tones, the QWRs exhibited substantial nonlinearities tied to increasing acoustic resistance with increasing sound level. Most importantly, the degree of nonlinearity exhibited by the QWRs depended on the bend location. Direct numerical simulations of a two-dimensional model problem are presented to link some of the experimentally observed trends to acoustically induced vortex formation in the interior of the bent QWRs.