Modeling and optimizing a broadband sound absorber consisting of a granular activated carbon stack backed by a poro-elastic layer

Abstract

High surface area particles, like granular activated carbon (GAC), have shown advantages in absorbing low-frequency sound, both by themselves and when combined with other treatments. In this article, a novel sound absorption treatment is described: i.e., a GAC stack supported by a soft, porous layer. At first, it was experimentally observed from impedance tube measurements that the low-frequency absorption performance of GAC stacks can be significantly enhanced by inserting a melamine foam between the stack and a rigid backing. To enable an analytical model of this type of absorber, the interfaces between GAC stacks and other materials were first characterized. Further, both 1-D layered transfer-matrix and 2-D axi-symmetric finite-difference approaches were implemented to predict the measured absorption spectra. Because of the constraint of the stack at the impedance tube wall, the 2-D model leads to more accurate absorption spectrum predictions. However, the difference between the two model predictions becomes negligible for large-area treatments. Therefore, amore efficient 1-D theory was incorporated in the multi-objective optimization to design broadband GAC-foam absorbers. Two optimization objectives were defined to balance the overall absorption performance: i.e., one associated with absorption at low frequencies, and the other with absorption at higher frequencies. It was found that a minimum thickness broadband GAC-foam absorber favors a thin, soft foam. In conclusion, this article presented a framework for the development of general broadband sound absorbers, and by combining both granular and porous materials, it may be possible to design practical sound-absorbing metamaterials that perform well at low frequencies.