Characterization of the acoustic properties of complex shape metamaterials

The paper presents the design and the acoustic properties evaluation of complex shape metamaterials. Complex metamaterials are a better alternative to conventional acoustic materials to absorb acoustic energy at low frequencies when the available volume is limited. Moreover these metamaterials are well adapted for harsh environments (high temperature, high pressure). In addition, traditional methods of performance evaluation are generally developed and used for simple shape geometries with known acoustic properties. In the present study, the metamaterials were placed in the restricted available volume. Their design is based on periodic neck-cavity configurations. In order to broaden the sound absorption coefficient frequency range, different cells were connected in parallel and in series. Owing to the fact that the cavities are small and of complex shape, a numerical Thermo-Visco-Acoustic (TVA) model was used to simulate the acoustic behavior of each periodic cell. Finally, the TVA was combined with transfer matrix methods to deduce the acoustic absorption coefficient of the metamaterials. The numerically predicted results using the present approach are in good agreement with measurements on prototypes. It is shown that these relatively thin metamaterials allow for low frequency sound absorption with wide absorption coefficient peaks.