Porous material characterization - ultrasonic method for estimation of tortuosity and characteristic length using a barometric chamber

An ultrasonic method of acoustic parameter evaluation for porous materials saturated by air (or any other gas) is discussed, The method is based on th e evolution of speed of sound and the attenuation inside the material when the static pressure of the gas saturating the material is changed. Asymptot ic development of the equivalent fluid model of Johnson-Allard is used for analytical description. The method allows an estimation of three essential parameters of the model: the tortuosity, and the viscous and thermal charac teristic lengths, Both characteristic lengths are estimated individually by assuming a given ratio between them. Tests are performed with industrial p lastic foams and granular substances (glass beads, sea sand) over a gas pre ssure range from 0.2 to 6 bars at the frequencies 30-600 kHz. The present t echnique has a number of distinct advantages over the conventional ultrason ic approach: operation at a single frequency, improved signal-to-noise rati o, possibility of saturation the porous media by different gases. In the ca se when scattering phenomena occur, the present method permits a separate a nalysis of scattering losses and viscothermal losses. An analytical descrip tion of the method is followed by a presentation of the set-up and the meas urement procedure. Experimental results and perspectives are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.