Acoustic impedance regulation of Helmholtz resonators for perfect sound absorption via roughened embedded necks

Acoustic impedance regulation of a neck embedded Helmholtz resonator is realized by introducing surface roughness to the neck so as to convert the initially non-perfect sound absorber to a perfect sound absorber. The proposed roughened-neck embedded Helmholtz resonator (R-NEHR) achieves perfect sound absorption ( alpha > 0.999) at 158Hz across a deep subwavelength thickness of lambda / 42. Theoretical predictions of the R-NEHR's performance are validated against experimental measurements. Physically, surface roughness triggers the periodic concentration effect of fluid vibration in the neck, thereby improving its acoustic mass and acoustic resistance and altering the resonant damping state of the absorber. As a result, the absorption peak position of the R-NEHR shifts by 16.0% to lower frequency, together with a peak value increase of 19.6%. This work provides an approach for perfect sound absorber design and impedance regulation of acoustic metamaterials.