Nathalie Geebelen

Near field Rayleigh wave on soft porous layers.

Simulations performed for a typical semi-infinite reticulated plastic foam saturated by air show that, at distances less than three Rayleigh wavelengths from the area of mechanical excitation by a circular source, the normal frame velocity is close to the Rayleigh pole contribution. Simulated measurements show that a good order of magnitude estimate of the phase speed and damping can be obtained at small distances from the source. Simulations are also performed for layers of finite thickness, where the phase velocity and damping depend on frequency. They indicate that the normal frame velocity at small distances from the source is always close to the Rayleigh pole contribution and that a good order of magnitude estimate of the phase speed of the Rayleigh wave can be obtained at small distances from the source. Furthermore, simulations show that precise measurements of the damping of the Rayleigh wave need larger distances. Measurements performed on a layer of finite thickness confirm these trends.

Anisotropy effects on the acoustical properties of porous materials

Porous materials are now widely used in noise control for their acoustic properties in sound absorption and transmission. These properties are function of the internal porous medium structure. Generally, most of the models assume that the porous medium is isotropic. Mineral wools (as well as some foams) clearly present an anisotropic structure. This communication is concerned with the adaptation of recent works on isotropic materials to the case of anisotropic porous materials. The general theory will be exposed for sound absorbing materials with anisotropic acoustical and mechanical parameters. The case of transverse isotropic materials (with fibers organized in planes running parallels one to each other) will then be detailed in particular in the case when the plane of fibers is not the same than the normal plane of the sample. The influence on the acoustical properties anisotropic porous materials will then be presented. Illustrations are then provided with measurements performed on wools and it will b...

Deduction of porous material properties using a point source

The acoustic field created by a point source above the plane boundary between two semi‐infinite fluid layers has been previously studied by Brekhovskikh. A straightforward generalization can be performed by replacing the fluid under the source by the fluid equivalent to a rigid‐framed porous medium. This leads to a method of evaluating, from pressure measurements, the surface impedance close to grazing incidence and the Brewster angle of total refraction of the porous medium. The model by Brekhovskikh can also be generalized when a thin porous layer is set under the point source. A pole of the reflection coefficient exists for an angle of incidence close to grazing incidence. This pole is related to an acoustic field similar to a surface wave above the layer. The measurement of this angle provides an evaluation of the surface impedance close to grazing incidence. The point source induces frame vibrations that can be predicted with the Biot theory. The rigidity coefficients of the frame at audible frequencies can be evaluated from measurements of the frame velocity with a laser velocimeter.