The acoustical absorption by air-saturated aerogel powders

The acoustical behavior of air-saturated aerogel powders in the audible frequency range is not well understood. It is not clear, for example, which physical processes control the acoustic absorption and/or attenuation in a very light, loose granular mix in which the grain diameter is on the order of a micron. The originality of this work is the use of a Biot-type poro-elastic model to fit accurately the measured absorption coefficients of two aerogel powders with particle diameters in the range 1-40 μm. It is shown that these materials behave like a viscoelastic layer and their absorption coefficient depends strongly on the root mean square sound pressure in the incident wave. Furthermore, it was found that the loss factor controlling the energy dissipation due to the vibration of the elastic frame is a key model parameter. The value of this parameter decreased progressively with the frequency and sound pressure. In contrast, other fitted parameters in the Biot-type poro-elastic model, e.g., the stiffness of the elastic frame and pore size, were found to be relatively independent of the frequency and amplitude of the incident wave. It is shown that these materials absorb acoustic waves very efficiently around the frequencies of the frame resonance.