Simplified calculus to estimate the acoustical absorption of nonplanar materials

The absorption curve, in function of the frequency, in hard‐backed layers of granular materials, shows undesirable series of maxima and minima related to the layer deep. In order to increase the efficiency of these absorbing materials, it is necessary to smooth their frequency response curve. It is obvious that one of the easiest solutions consists of modifying the layer surface with selected profiles (e.g., wedges). For these situations, a simplified method of analytical calculus for the valuation of the absorption, in function of the frequency, has been developed. The validity of the theoretical approximations has been checked against experimental measurements in different test samples by means of a standing wave tube. Additionally, the application of this study to the design of absorbent acoustic noise screens, against traffic noise, made with recycled rubber crumbs of tires, is also presented. [Work supported by a LIFE project.]