Plate-type acoustic metamaterials with integrated Helmholtz resonators

Plate-type acoustic metamaterials (PAM) consist of a thin film with periodically added masses. These metamaterials can be designed to be very lightweight and exhibit narrow bands at low frequencies with high sound transmission loss values that can exceed the corresponding mass-law considerably. In this paper, a new approach for improving the bandwidth of PAM by using Helmholtz resonators which represent the added masses is investigated. The key principle of this design is that the Helmholtz resonance gives rise to an additional peak in the transmission loss spectrum which can be tuned to increase the bandwidth of the PAM. Sound transmission loss measurements of a large-scale test sample with 270 resonators are used to demonstrate the performance of the proposed metamaterial under diffuse field excitation. Then, numerical simulations based on the finite element method are used to further investigate the physical mechanisms of the PAM with Helmholtz resonators. It is shown that when the baseplates of the Helmholtz resonators are stiff enough, the Helmholtz resonance is decoupled from the vibro-acoustics of the PAM. This can be exploited to effectively increase the bandwidth of PAM without any significant reductions of the sound transmission loss due to coupling resonances.