A Comprehensive Study of Broadband Absorption Performance and Scattering Coefficient Evaluation of Spiral Resonators with Soundproof Sidewalls

Resonators designed in spiral and helical shapes exhibit sound absorption performance in the lower frequency band. The absorption characteristics including frequency range and acoustical impedance can be controlled by adjusting the permittivity and permeability are dependent on the length or dimensions of the resonator. As the helical structure is added, the absorption performance can deteriorate due to the change in the resonator geometry. The compliant wall trembles in response to changes in waveguide pressure, generating dissipation of sound energy. The resonator is surrounded by a compliant wall to compensate for the absorption effect of the resonator. Acoustic properties were obtained using the equivalent impedance concept consisting of the acoustic surface impedance of the cavity, the helical structure, and the soft wall. The theoretical scattering coefficients were compared with measurements obtained with the 4-microphone method using impedance tubes. In this study, the tunability of the length extension ratio of the helical structure was investigated by controlling geometric parameters (or boundary conditions of the outer wall of the resonator). Keywords: scattering coefficients; compliant wall; helical structure