Microscale thermal relaxation during acoustic propagation in aerogel and other porous media

The longitudinal acoustic velocity in silica aerogel is presented as a func tion of the interstitial gas type and pressure. This was measured using air -coupled ultrasonic transducers configured for differential pulse transit t ime measurements. The results are interpreted in terms of the thermal relax ation of the acoustic pulse. The microscale temperature oscillations of the gas and solid phases of the aerogel due to the acoustic pulse are not iden tical if the rate of heat transfer between the two phases is slow compared to the period of the acoustic oscillation. The energy transferred from the gas to the solid phase is lost to the acoustic propagation and thus, reduce s the amplitude and velocity of the acoustic wave. The gas type and pressur e may preside independent variables for probing these effects in aerogel.