N. R. Brown
Lab. d'Acoust. de l'Univ. du Maine, 72017 Le Mans, France
Low-frequency (20 kHz--180 kHz) ultrasonic measurements have been performed in numerous air-saturated acoustic materials, i.e., various foams and felts. By using a standard pulse-echo technique, very precise wave-speed measurements were achieved with either a cross-correlation procedure or by using the Hilbert transform of the transfer function between a reference signal (with no sample) and the signal transmitted through the sample. The amplitude measurements were done in the Fourier domain via the cross power. The signal processing routines were implemented in the LabVIEW 2.0 environment. Some significant physical properties (e.g., tortuosity) related to the propagation of acoustic waves in porous media [e.g., J.-F. Allard, Propagation of Sound in Porous Media (Elsevier, New York, (1993)] were obtained. Preliminary results made at 500 kHz, with the aim of measuring the viscous as well as thermal characteristic lengths, on water-saturated sandstone specimens will also be reported.