Sound Absorption Characteristics of Back-perforated Honeycomb Panel with an Air-layer—A Study Based on a Two-degree-of-freedom Model

Abstract

A back-perforated honeycomb panel (BPHP) placed with an air-layer exhibits unique absorption characteristics composed of panel-type and resonator-type resonances. This paper presents theoretical and experimental considerations for the effective design of absorption performances of BPHPs. A BPHP is theoretically modelled as a lumped parameter system composed of panel and resonator’s neck masses, and multiple air stiffnesses. Resonance frequencies and mechanisms are clarified from the model. It is revealed that the panel-type resonance can arise from the panel mass and air stiffness corresponding to the total thickness of the panel and air-layer, even if a BPHP is placed considerably closely to the back wall. Furthermore, a supplementary numerical calculation confirms the validity of approximations involved in the lumped parameter system. Next, absorption coefficients under various incidence conditions are calculated with changing several parameters such as perforation diameters, panel thickness, back air-layer thickness and so on. Then, comparison with absorption coefficients measured by the reverberation room method shows that the proposed lumped parameter system is capable to capture the measured result fairly well. Finally, several charts are arranged for effective design of PBHPs as a sound absorber based on the proposed model.