Periodicity-enhanced structures for efficient sound absorption

In the present work, an overview of experimental investigations of the two types of periodicity-enhanced acoustic absorbing structures is given. In the first type of structures, the performance of acoustic absorbing materials is improved by providing a smooth transition from the impedance of air to the impedance of the absorbing material in question. This smooth change in the impedance is materialised using gradient index metamaterial layers formed by quasi-periodic arrays of solid cylinders. In the second type of performance improving devices, the principle of acoustic black holes has been implemented. To achieve the required power-law decrease in sound velocity with propagation distance the cylindrical inhomogeneous acoustic waveguides enhanced by quasi-periodic systems of concentric rings have been used. Measurements of the reflection coefficients for both types of structures have been carried out. The results show the possibility of substantial reduction of the acoustic reflections in both cases.