Quasi-flat acoustic absorber enhanced by metamaterials

In this paper, the design of a new quasi-flat acoustic absorber (QFAA) enhanced by the presence of the impedance matching metamaterial layer is described, and the results of the experimental investigation into the reflection of sound from such an absorber are reported. The gradient metamaterial layer is formed by a quasi-periodic array of brass cylindrical tubes with the diameters gradually increasing from the external row of tubes facing open air towards the internal row facing the absorbing layer made of a porous material. The QFAA is placed in a wooden box with the dimensions of 569 x 250 x 305 mm. All brass tubes are of the same length (305 mm) and fixed between the opposite sides of the box. Measurements of the sound reflection coefficients from the empty wooden box, from the box with an inserted porous absorbing layer, and from the full QFAA containing both the porous layer and the array of brass tubes have been carried out in an anechoic chamber at the frequency range of 500-3000 Hz. The results show that the presence of the metamaterial layer brings a noticeable reduction in the sound reflection coefficients in comparison with the reflection from the porous layer alone.