Controlling the effective surface mass density of membrane-type acoustic metamaterials using dynamic actuators.

Membrane-type acoustic metamaterials (MAM) are thin and lightweight structures that offer exceptional low-frequency sound transmission loss (STL) values, which can exceed the corresponding mass-law significantly. Typically, the high STL of MAM is confined to a narrow frequency band, which is associated with the so-called anti-resonance. This narrow bandwidth reduces the range of potential noise control applications for MAM. To potentially overcome this challenge, this paper presents an investigation into actively controlling the effective surface mass density of MAM by actuating the MAM with a force that is correlated to the acoustic pressure difference acting on the MAM. In particular, it is shown using theoretical and numerical methods that the anti-resonance frequency of MAM can be adjusted over a wide frequency range by passing the incident sound pressure through an adjustable gain. A simple analytical model to predict the frequency shifting, depending on the gain value, is derived. A realization of this concept is further studied, consisting of a circular MAM with a small electrodynamic actuator (to apply a force to the MAM) and a microphone in front of the MAM (to estimate the pressure difference). Finally, experimental results from impedance tube measurements are used to validate the proposed analytical model.