Jean François Allard

Dynamic compressibility of air in porous structures at audible frequencies

Measurements of dynamic compressibility of air-filled porous sound-absorbing materials are compared with predictions involving two parameters, the static thermal permeability k0′ and the thermal characteristic dimension Λ′. Emphasis on the notion of dynamic and static thermal permeability—the latter being a geometrical parameter equal to the inverse trapping constant of the solid frame—is apparently new. The static thermal permeability plays, in the description of the thermal exchanges between frame and saturating fluid, a role similar to the viscous permeability in the description of the viscous forces. Using both parameters, a simple model is constructed for the dynamic thermal permeability k′(ω), which is completely analogous to the Johnson et al. [J. Fluid Mech. 176, 379 (1987)] model of dynamic viscous permeability k(ω). The resultant modeling of dynamic compressibility provides predictions which are closer to the experimental results than the previously used simpler model where the compressibility i...

Evaluation of tortuosity in acoustic porous materials saturated by air

Tortuosity is an important parameter for the prediction of the acoustical properties of porous sound absorbing materials. The evaluation of tortuosity by resistivity measurements is now used in several laboratories, although this method presents several drawbacks. In particular, the complete saturation by a conducting fluid of a porous foam having a high flow resistivity is difficult to obtain without partially damaging the structure of the cells. A simple technique based on ultrasonic wavespeed measurements in a material saturated by air is described. This method has been used previously only for water or superfluid helium saturated materials.Tortuosity is an important parameter for the prediction of the acoustical properties of porous sound absorbing materials. The evaluation of tortuosity by resistivity measurements is now used in several laboratories, although this method presents several drawbacks. In particular, the complete saturation by a conducting fluid of a porous foam having a high flow resistivity is difficult to obtain without partially damaging the structure of the cells. A simple technique based on ultrasonic wavespeed measurements in a material saturated by air is described. This method has been used previously only for water or superfluid helium saturated materials.

The acoustic transmission through layered systems

A new model for the transmission of sound through layered systems containing porous materials is presented. The sound propagation through the porous material is described using the Biot theory, and a matrix formalism for the plate/porous material/plate system is developed. A comparison is made with experimental results.

Inhomogeneous Biot waves in layered media

A description, using transfer matrices, is given of the propagation of the three inhomogeneous Biot waves in layered porous media. This is applied to predict the surface impedance of porous layered materials at oblique incidence, and an example is presented.

Sound propagation in air-saturated random packings of beads

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.

Measurements of acoustic impedance in a free field with two microphones and a spectrum analyzer

A technique is presented for evaluating the normal acoustic impedance at the surface of a panel of absorbing material in a free field. The evaluation is achieved by measuring the pressure and the acoustic velocity with two microphones set very close to the panel. This technique allows measurements of impedance for panels of area about 1 m2, the two microphones being located very close to the panel so that, contrary to other methods, the effect of the finite dimension of the panel is negligible. Because of the very small spacing between both microphones, this technique allows precise measurements only for frequencies higher than 500 Hz.

Modelization of layered sound absorbing materials with transfer matrices

A method is presented that allows the prediction of the acoustical properties of layered porous or fibrous materials in normal incidence from simple measurements. Transfer matrices obtained from the Biot theory are used to describe homogeneous layers and screens constituting layered materials. The description of an absorbing material with the method is given.

Ultrasonic surface waves above rectangular-groove gratings

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.

Spatial Fourier‐transform method for measuring reflection coefficients at oblique incidence. II. Experimental results

A method using spatial Fourier transforms for measuring the plane‐wave reflection coefficient at oblique angles of incidence has been proposed in an earlier paper [M. Tamura, J. Acoust. Soc. Am. 88, 2259–2264 (1990)]. The present paper gives experimental verification of the method. Experimental results are shown for two types of samples: a perfectly absorbing plane (an imaginary plane in the air) and a layer of a polyurethane foam backed by a hard plywood board. The measured results are generally in good agreement with the theoretical value (for the first sample) or with the results calculated from the acoustical parameters of the material (for the second sample). The agreement shows the usefulness of the method for measuring acoustic properties of absorbent materials.

Anisotropy effect in glass wool on normal impedance in oblique incidence

A new method has been used for measuring normal impedance for oblique incidence on a glass wool. The method allows measurements at low frequencies for large angles of incidence. A ratio close to two for the normal and the planar specific flow resistance gives a good agreement between theoretical predictions and measurements. This result confirms recent experiments carried out on glass wools.