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

A modal model, initially developed to modelize diffraction of electromagnetic waves by rectangular-groove gratings, is used to describe ultrasonic surface waves above the same structures in air. A simple analytical formulation which provides results comparable to the modal method is presented. Measurements performed on ultrasonic surface waves are compared to predictions obtained with the simplified formulation and the modal model.

The acoustic properties of a porous sheet of medium static air flow resistivity (around 10,000 N m s(-4)), in which a periodic set of circular inclusions is embedded and which is backed by a rigid plate, are investigated. The inclusions and porous skeleton are assumed motionless. Such a structure behaves like a multi-component diffraction grating. Numerical results show that this structure presents a quasi-total (close to unity) absorption peak below the quarter-wavelength resonance of the porous sheet in absence of inclusions. This result is explained by the excitation of a complex trapped mode. When more than one inclusion per spatial period is considered, additional quasi-total absorption peaks are observed. The numerical results, as calculated with the help of the mode-matching method described in this paper, agree with those calculated using a finite element method.

The concept of viscous characteristic length is used to describe the acoustical behavior of fluid-saturated porous media in the high-frequency regime. A method to determine this parameter consists of measuring the wave attenuation in the high-frequency limit. This method has already been used for porous materials saturated by superfluid 2He. It is tested in the case of air-filled absorbent materials in a frequency range of [50–600 kHz]. The thermal characteristic length is assumed to be known or measured independently. Two examples are presented. In the first one the method is usable and the viscous characteristic length Λ is deduced from the high-frequency behavior of the attenuation per cycle. In the second example, an additional attenuation occurs at high frequencies and only an estimate of Λ can be given. Nevertheless, the estimation appears to be rather accurate. The values obtained by this method are compared to those determined by a nonlinear fit of the dispersion curves.

In the pore space of packed grain material, transport properties are characterized by macroscopic parameters. Some of them, tortuosity, characteristic dimensions, viscous permeability, and trapping constant, are measured for a random packing of glass beads and compared to evaluations performed in previous studies. These parameters are used to predict the surface impedance at normal incidence of a layer of glass beads. The predictions are compared to measurements performed at normal incidence in a Kundt tube.