The acoustic transmission velocity in silica aerogel was measured as a function of the gas type and pressure in the aerogel pores. This was done with air-coupled ultrasonic transducers configured for differential pulse transit time measurements. At high pressures, the fractional reduction of acoustic velocity as a function of mean free path is identical in different gasses. This may indicate a thermal relaxation phenomenon, with the relaxation time dependent upon the rate of collision between gas molecules and the silica pore walls.

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