ACOUSTIC PROPERTIES AND POTENTIAL APPLICATIONS OF SILICA AEROGELS

Acoustic properties of cylindrical silica aerogels in both ultrasonic and audible range are presented. Velocity measurements for low ultraso nic frequencies show that the low-density aerogels can exhibit unexpec ted attenuation for well-defined frequency bands. Measurements of the acoustical impedance of samples in the audible range show that tile re sults depend dramatically on the geometry and/or the boundary conditio ns imposed to the samples. The 'attention' bands in which the samples present an unexpected high attenuation are related to the aerogel dens ity. These particular results are discussed in two ways, first for app lication purposes and second in terms of a possible theoretical explan ation. Neither the classical theory of propagation in a homogeneous ma terial nor the Blot theory for porous materials can explain the result s.