Modeling a low-frequency sound absorber consisting of a granular activated carbon stack backed by a poro-elastic layer

High surface area particles, like granular activated carbon (GAC), have shown advantages in absorbing low-frequency sounds by themselves and when combined with other treatments. In this article, a novel sound absorption treatment consisting of a high surface area particle stack supported by a soft, porous layer is described. It was first 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 analytically model this type of sound absorbing treatment, the interfaces between particle stacks and other materials were characterized. Further, a 1-D layered transfer-matrix approach and a 2-D axi-symmetric finite-difference approach were implemented to predict the measured absorption spectra. Because of the constraint of the stack at the impedance tube wall, the 2-D axi-symmetric model leads to more accurate absorption spectrum predictions, particularly at low frequencies. However, the difference between the two model predictions becomes negligible when applied to large-area treatments. Thus, the simpler and faster 1-D theory can be valuable for optimizing large-area sound packages. In addition, by combining both granular and porous materials, it may be possible to design sound-absorbing metamaterials that perform well at low frequencies.