A new membrane-type metamaterial for multiple peaks absorption at low frequencies

In the recent years, various membrane-type acoustic metamaterials were developed for low frequency sound absorption. However, a membrane absorber usually requires a large back cavity to achieve low frequency sound absorption and on the other hand, in order to guaranty a multiple peaks absorption decorated membrane resonators or membrane with multiple magnetic negative stiffness cell shall be considered. This research proposes a new concept of membrane-type metamaterial which can achieve multiple peaks and broadband absorption at low frequencies. The basic concept behind the design of the elementary cell is associated to the vibro-acoustic behavior of the structure. In fact, the maximum sound absorption is related to the symmetrical mode of the membrane, so playing with the geometry, the mass and the stiffness of the membrane the eigenfrequencies can be tuned easily in the prescribed frequency range. At same time local increase of strain energy around geometrical discontinuity or around discontinuity associated to the material properties may lead a gain in sound absorption. A mono-layer membrane structure is presented where the geometrical shape and material properties distribution in terms of density and stiffness in the elementary cell are optimized in order to manipulate the vibro-acoustic properties and maximize the absorption at required frequencies. To optimize the geometry and the vibro-acoustic properties of the proposed metamaterial, finite element simulation were carried out. The numerical model was then validated using experimental measurements. A preliminary prototype was tested into an impedance tube test ring and the normal sound absorption was measured following the transfer function approach and compared with the numerical results.