Extension of the frequency range of resonant sound absorbers using two-degree-of-freedom Helmholtz-based resonators with a flexible panel

To enhance the effect of a sound absorber having Helmholtz resonators in a wide frequency range, a sound absorber with two-degree-of-freedom (two-DOF) Helmholtz-based resonators is proposed. The resonator has a flexible panel installed in its cavity so that the panel can be vibrated by incident sound. Thus, the resonator acts as a two-DOF system. The resonant frequency of the system consisting of the panel and back cavity is designed to tune to that of a conventional resonator system, splitting the absorption resonant peak into two. Moreover, by adjusting the dimensions, damping, and other characteristics of the proposed resonator, the absorption coefficient can be improved. However, the effect of these parameters is complicated; therefore, it is difficult to design a sound absorber with the appropriate characteristics. Hence, in this paper the fundamental characteristics of the proposed absorbers are investigated. The absorption coefficient and specific acoustic impedance are derived analytically using dimensionless parameters. The effects of the parameters are examined, and a practical method of broadening the frequency range of the effective sound absorption is discussed. Furthermore, a method in which the absorbent material is installed into the back cavity is proposed in order to extend the frequency range of the resonator-type sound absorber. Finally, experimental results are presented to confirm the validity of the proposed resonator.