Effect of particle size on sound absorption behavior of granular aerogel agglomerates

We investigate the sound absorption characteristics of granular aerogel agglomerates of various particle sizes. The normal incidence sound absorption coefficients of six commercially available silica aerogels with different particle size distributions are measured using a two-microphone normal incidence impedance tube. The measurements show a strong correlation between the particle size and sound absorption behavior and reveal important differences between the sound absorption trends of granular aerogel particles larger than 100 μm and those smaller than 50 μm. These differences are further investigated using an inverse characterization method to extract the representative bulk properties necessary to model such agglomerates using the Johnson-Champoux-Allard formulation. Our results show that while agglomerates of larger aerogel granules can be modeled as limp porous media, smaller aerogel granules require a poro-elastic formulation. Further, we show that the inclusion of a logarithmically decreasing dynamic loss factor improves the low-frequency sound absorption predictions of the sub-50 μm aerogels.