Experimental study of granular particle stacks' circumferential edge-constraint effect and the level- and time-dependent acoustical behavior

Granular materials, such as activated carbon, have shown advantages in absorbing low-frequency sounds, but they also exhibit significant nonlinearities. In this article, three unique types of granular particle stacks' acoustical behavior are described and experimentally analyzed. First, when measuring the sound absorption spectra of particle stacks with varying depths and sample holder sizes, it was found that the frictional interaction with the tube circumferential wall can significantly affect the measurements, and a 1-D plane-wave acoustical model is only valid when the sample is thin. Secondly, when granular samples were tested with increasing excitation levels, the results indicated a decreased modulus and an increased damping. By further varying excitation's level and bandwidth, a fluid-displacement-related metric was identified to describe this behavior. Thirdly, when measuring particle stacks over a longer period, the stack's resonance peak gradually shifts to a higher frequency as the sample consolidates. This frequency shift was proportional to log(Time).