Abstract
A new experimental method for measuring the phase velocities of guided acoustic waves in soft poroelastic or poroviscoelastic plates is proposed. The method is based on the generation of standing waves in the material and on the spatial Fourier transform of the displacement profile of the upper surface. The plate is glued on a rigid substrate so that it has a free upper surface and a nonmoving lower surface. The displacement is measured with a laser Doppler vibrometer along a line corresponding to the direction of propagation of plane surface waves. A continuous sine with varying frequencies was chosen as excitation signal to maximize the precision of the measurements. The spatial Fourier transform provides the wave numbers, and the phase velocities are obtained from the relationship between wave number and frequency. The phase velocities of several guided modes could be measured in a highly porous foam saturated by air. The modes were also studied theoretically and, from the theoretical results, the experimental results, and a fitting procedure, it was possible to determine the frequency behavior of the complex shear modulus and of the complex Poisson ratio from 200 Hz to 1.4 kHz, in a frequency range higher than the traditional methods.
Highlights
The acoustical properties of porous materials can generally be well described in a wide frequency range with Biot’s theory,[1] which involves the determination of a number of physical parameters
A new method for measuring the shear modulus of air-filled porous materials based on the propagation of Rayleigh waves in thick layers was recently proposed by Allard et al.[11] and provided useful information above 3 kHz
We investigate the propagation of guided waves in a layer of porous material in a wide frequency range, from the typical frequencies of the classical vibrational methods to the high-frequency limit of the Rayleigh wave
Summary
The acoustical properties of porous materials can generally be well described in a wide frequency range with Biot’s theory,[1] which involves the determination of a number of physical parameters. First results of this work were presented in Kyoto.[21] Together with a complete model for guided waves in a soft porous material on a rigid substrate, a new experimental method is proposed in this article for the determination of the modes of propagation in a plate of finite sizes. This method, characterized by an increased signal-to-noise ratio and measurement accuracy, is based on the generation of standing waves in the layer of porous material. The phase velocities of two modes were measured and used as reference velocities for the fitting of the dispersion curves leading to the determination of the frequency variations of the complex shear modulus and Poisson ratio
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