Acoustic spectroscopy of aerogel precursors

Abstract

We investigate the acoustical properties of silica gels, which are precursors in the aerogel production process. These gels exhibit a strong “ringing gel” behavior, that is they emit a characteristic sound if one knocks against the container. We study this sound emission with a very simple spectroscopic technique and observe resonances which are characteristic for natural frequencies of a cylindrical body. From a fit of the experimental frequency positions to calculated values, we determine a sound velocity of c _T = 4 m/s for a gel sample with porosity φ = 97.5%. This low sound velocity can only be interpreted as the transverse sound mode predicted by Biot’s theory for sound propagation in porous media.