EVALUATION OF THE VISCOUS CHARACTERISTIC LENGTH OF AIR-SATURATED POROUS MATERIALS FROM THE ULTRASONIC DISPERSION CURVE

This Note describes a method to determine the viscous characteristic l ength Lambda from an analysis and a linear regression of the dispersio n curve. This approach is valid in the high frequency domain, above 20 kHz, where some asymptotic expansions simplify the theoretical equati ons. It is shown that there is a linear relationship between the squar ed propagation index and the inverse of the square root of the frequen cy which has a slope proportional to Lambda, the thermal characteristi c length Lambda' and some physical constants. The value of Lambda' is determined using the Brunauer, Emmet and Teller (BET) method. Low freq uency ultrasound, in the 40-200 kHz range, is used to perform the expe riments. The computation of the dispersion curve from the experimental waveforms is done by using the phase spectrum method, as was outlined in a previous Note, i.e., Brown et al., C.R. Acad Sci. Paris, 319, Se ries II, 1994, p. 393.