Ultrasonic measurements of the two characteristic lengths in fibrous materials

In a method for measuring the two characteristic lengths in rigid frame porous media proposed by Leclaire et al. [Ph. Leclaire, L. Keiders, W. Lauriks. Determination of the viscous and thermal characteristic lengths of plastic foams by ultrasonic measurements in helium and air. J Appl Phys 1996;80:2009–12] ultrasonic transmission measurements are made through a sample of material when it is saturated with two different gases. It is shown that the accuracy of this method can be limited when the porous medium in saturated with helium as proposed originally. A considerable improvement in the signal-to-noise ratio can be achieved if argon is used instead of helium because the characteristic impedance is higher and the viscous skin depth in argon is 3 times less than that in helium. This paper also discusses the measurement of the tortuosity and the sound velocity in glass wool.