Determination of the viscous and thermal characteristic lengths of plastic foams by ultrasonic measurements in helium and air

Abstract

International audienceThe acoustic wave most commonly transmitted and detected in the high-porosity absorbent materials used in noise control is generally the airborne slow compressional wave. In a new experiment, the air saturating the sample is replaced by helium and the transmission is studied at ultrasonic frequencies ͑70–600 kHz. The experiment is quite easily performed using standard ultrasonics and vacuum equipment. The main purpose of this work is to propose a method to determine simultaneously both the viscous and thermal characteristic lengths with the same precision. These two parameters characterize the viscous and the thermal interactions between the frame and the fluid at high frequencies. The characteristic lengths are deduced from the high-frequency asymptotic behavior of either the velocity or the attenuation curves obtained in the sample saturated by air and by helium. It also appears that due to the properties of helium, the discrepancy previously observed between predictions and measurements is shifted toward higher frequencies

Transient Acoustic Wave Propagation in Porous Media

“…The propagation equation can be easily obtained from (24,25). The propagation equation obtained has exactly the same form as the equation (13), the only difference appears at the…”

“…Note that the ratio of two lengths was set at 3 [20][21][22][23]. Currently the only method for the separate determination of the two lengths is based on the saturation of the porous by two different fluids [24]. The two characteristic lengths are acting the same way on the dispersion and attenuation of the transmitted signal.…”

“…Porosity cannot be determined from transmitted waves because of its weak sensitivity in this mode [4]. The advantage of the proposed method is that all the parameters can be determined at normal incidence (the ratio between viscous and thermal lengths is fixed as in classical methods [21][22][23][24] based on transmission measurement).…”

A time-domain model of transient acoustic wave propagation in double-layered porous media

“…Ultrasonic characterization of materials is often achieved by measuring the attenuation coefficient and phase velocity in the frequency domain, 3,4 or by solving the direct and inverse problems directly in the time domain. [5][6][7][8][9][10][11][12][13] In the frequency domain, measurements of the attenuation coefficient may be more robust than measurements of phase velocity.…”

“…3,4,[8][9][10][11] When a broadband ultrasound pulse passes through a layer of a medium, the pulse waveform changes as a result of attenuation and dispersion of the medium. The classic method for predicting a change in the waveform of a signal passing through a medium relies on the system's impulse response.…”