Absorption of a rigid frame porous layer with periodic circular inclusions backed by a periodic grating.

The acoustic properties of a periodic rigid frame porous layer with multiple irregularities in the rigid backing and embedded rigid circular inclusions are investigated theoretically and numerically. The theoretical representation of the sound field in the structure is obtained using a combination of multipole method that accounts for the periodic inclusions and multi-modal method that accounts for the multiple irregularities of the rigid backing. The theoretical model is validated against a finite element method. The predictions show that the acoustic response of this structure exhibits quasi-total, high absorption peaks at low frequencies which are below the frequency of the quarter-wavelength resonance typical for a flat homogeneous porous layer backed by a rigid plate. This result is explained by excitation of additional modes in the porous layer and by a complex interaction between various acoustic modes. These modes relate to the resonances associated with the presence of a profiled rigid backing and rigid inclusions in the porous layer.

Enhancing the absorption coefficient of a backed rigid frame porous layer by embedding circular periodic inclusions.

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.

A description of transversely isotropic sound absorbing porous materials by transfer matrices.

A description of wave propagation in transversely isotropic porous materials saturated by air with a recent reformulation of the Biot theory is carried out. The description is performed in terms of a transfer matrix method (TMM). The anisotropy is taken into account in the mechanical parameters (elastic constants) and in the acoustical parameters (flow resistivity, tortuosity, and characteristic lengths). As an illustration, the normal surface impedance at normal and oblique incidences of transversely isotropic porous layers is predicted. Comparisons are performed with experimental results.

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.

Investigation of the phase velocities of guided acoustic waves in soft porous layers.

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.

Comparison of technetium uptake in small joints with other indices of inflammation in rheumatoid arthritis.

The uptake of 99mTc in the small joints of the hand has been compared to various other mesures of inflammation in 22 patients with rheumatoid arthritis. The 99mTc uptake in the finger joints can reflect both disease activity at a given time and change in activity over a period of time. The place of Tc uptake amongst other measures of inflammation, from the point of view of the 'relative efficiency' of the assessment method, is rather inferior. In evaluating the effect of antiphlogistic drugs on local inflammation, techneticum uptake may, however, prove more discriminating than grip-strength or the number of painful joints, since the latter measures depend largely on the degree of destruction.

Critical appraisal of angioscintigraphy as a diagnostic procedure in cerebrovascular disease.

In order to assess the diagnostic usefulness of combined functional and morphological isotopic investigations of brain vascularization after intravenous injection of a bolus 99mTc pertechnetate, the authors explored, besides a control group, patients with acute cerebrovascular disease, angiomas, aneurysms and intracerebral hematomas. Combined flow evaluation and visualization of the intracerebral bolus transit was highly effective in patients with cerebrovascular disease. Angiomas with a diameter of 2 cm or more are always visible, the abnormal vascularization of the affected hemisphere being correctly evaluated. Cerebral aneurysms are frequently missed by angioscintigraphy. Intracerebral hematoma generally gives false negative results.