Impedance matching and acoustic absorption in granular layers of silica aerogels

Experimental results on the acoustic properties of silica aerogel granules of sizes 80 μm and 3.5 mm are presented. An acoustic absorber made of a two-layer granular aerogel structure, an impedance matching layer and an absorbing one, has been designed. The reflection coefficient, the attenuation, and the sound velocity of the granular aerogels have been measured as a function of frequency (between 20 and 2500 Hz) and compared with those of a glass wool sample. It has been found that the acoustic transmission losses in the aerogel absorber are at least 10 dB higher than in the glass wool sample of the same thickness (4 cm) for the frequency range 300–1700 Hz.