An analysis and optimum design of the weak coupling between cavities for broadband sound absorption of parallel Helmholtz resonators

Abstract A structure of two parallel Helmholtz resonators coupled by a perforated baffle (PHRP) with extremely low perforation ratio between their separated cavities can achieve significant improvement in sound absorption performance. We show that optimizing the parameters of the orifice on the baffle can bridge the gap between the isolated resonance absorption peaks. Thus, the PHRP exhibits a continuously high sound absorption coefficient bandwidth. In this study, a sample of PHRP had sound absorption coefficient above 0.9 from 150 Hz to 230 Hz, under the 100 dB sound pressure level in the experiment, while the porosity of its baffle was only 0.013%. By establishing a theoretical model and analyzing the simplified reactance of the PHRP, we found that the optimum ratio of the cross-sectional area to the effective length of the orifice should be of the same order of magnitude as that of the resonator necks. The broadband absorption capacity of the PHRP is attributed to its optimized resistance and reactance, due to the weak coupling effect from the small orifice on the baffle. In the linear regime, they could sustain the condition that keeps the sound absorption coefficient close to 1 between the two absorption peaks (in other words, the dimensionless reactance approaches to 0, and the dimensionless resistance approaches 1). The influence from the nonlinear effect of orifices was presented through experiments. The restriction caused by the ratio of the resonance frequencies for two parallel resonators on the improvement effect was quantitatively illustrated.