Sound absorption by clamped poroelastic plates

Measurements and predictions have been made of the absorption coefficient and the surface acoustic impedance of poroelastic plates clamped in a large impedance tube and separated from the rigid termination by an air gap. The measured and predicted absorption coefficient and surface impedance spectra exhibit low frequency peaks. The peak frequencies observed in the absorption coefficient are close to those predicted and measured in the deflection spectra of the clamped poroelastic plates. The influences of the rigidity of the clamping conditions and the width of the air gap have been investigated. Both influences are found to be important. Increasing the rigidity of clamping reduces the low frequency absorption peaks compared with those measured for simply supported plates or plates in an intermediate clamping condition. Results for a closed cell foam plate and for two open cell foam plates made from recycled materials are presented. For identical clamping conditions and width of air gap, the results for the different materials differ as a consequence mainly of their different elasticity, thickness, and cell structure

Predicted effects of fluid loading on the vibration of elastic porous plates

International audienceThe effects of fluid loading on the vibration of rectangular, clamped, porous elastic plates and on their radiated sound power are considered. This requires the inclusion of an extra term in the equations for the plate vibration, corresponding to the additional external force acting on the plate. As is the case for vibrating non-porous plates, fluid-structure coupling is a very complex phenomenon since the plate modes are coupled by the fluid. The radiation impedance matrix, including direct terms and cross-coupling terms, has been defined and computed. A Gaussian quadrature scheme including twenty terms of the Legendre polynomial has been used to compute the fluid loaded plate deflection for four types of porous and elastic plate and compare the effects of the loading by water and air. The corresponding vibroacoustic indicators including mean square velocity, radiated sound power and radiation efficiency, have been calculated also. For some plate and fluid parameters, the predicted effects of fluid loading are considerable

Padé approximants for the acoustical properties of rigid frame porous media with pore size distributions

Expressions for the viscosity correction function, and hence bulk complex impedance, density, compressibility, and propagation constant, are obtained for a rigid frame porous medium whose pores are prismatic with fixed cross-sectional shape, but of variable pore size distribution. The lowand high-frequency behavior of the viscosity correction function is derived for the particular case of a log-normal pore size distribution, in terms of coefficients which can, in general, be computed numerically, and are given here explicitly for the particular cases of pores of equilateral triangular, circular, and slitlike cross-section. Simple approximate formulae, based on two-point Pade´ approximants for the viscosity correction function are obtained, which avoid a requirement for numerical integration or evaluation of special functions, and their accuracy is illustrated and investigated for the three pore shapes already mentione

A Review of the State of Art in Applying Biot Theory to Acoustic Propagation through the Bone

Understanding the propagation of acoustic waves through a liquid-perfused porous solid framework such as cancellous bone is an important pre-requisite to improve the diagnosis of osteoporosis by ultrasound. In order to elucidate the propagation dependence upon the material and structural properties of cancellous bone, several theoretical models have been considered to date, with Biot-based models demonstrating the greatest potential. This paper describes the fundamental basis of these models and reviews their performance

Coherent and Incoherent Scattering Mechanisms in Air-Filled Permeable Materials

Ultrasonic evaluation of porous materials can take advantage of some very specific acoustic phenomena that occur only in fluid-saturated consolidated solids of continuously connected pore structure. The most interesting feature of acoustic wave propagation in such media is the appearance of a second compressional wave, the so-called slow wave [1,2]. The slow compressional wave represents a relative motion between the fluid and the solid frame. This motion is very sensitive to the kinematic viscosity of the fluid and the dynamic permeability of the porous formation. Certain material properties such as tortuosity, permeability, porosity, and pore size, shape and surface quality are inherently connected to the porous nature of the material and can be evaluated best from the propagation properties of the slow compressional wave.</p

Particle interactions in coupled phase theory for sound propagation in concentrated emulsions

Abstract: A thermal wave interaction model based on scattering theory is used to modify coupled phase theory to account for the effect of thermal wave interactions on sound propagation in concentrated emulsions. Sound propagation in emulsions is influenced by the compressibility of each phase and by inter-phase heat transfer. Three, related, theoretical approaches have been used to model the relation between these effects and the complex wavenumber. The most successful is scattering theory [I], which is applicable at arbitrarily high frequencies. Considerable simplifications can be made to scattering theory when the wavelength is much greater than the particle size. Multiple scattering theory takes into account that, when the volume fraction or the extinction cross-section is high, the wave incident on successive particles cannot be assumed to be the same. Coupled phase theory [2] uses a volume averaging procedure to account for the two phases. This gives a formulation that is self-consistent. Isakovich&apos;s approach [3] is scattering theory but neglecting pressure variations local to the particle. All the described methods assume that the particles are isolated and therefore the thermal waves of neighbouring particles do not interact. This is only strictly valid at low volume fractions or higher frequencies, where the boundary layer is thin. For this reason some researchers have attempted to model interactions between the individual particles&apos; thermal waves. Fukumoto and Izuyama [3] developed an accurate model for periodic emulsions, with involved computation. Hemar et a1 [4] used an approximate, effective medium, giving the particle boundary conditions. Both Fukumoto and Hemar calculated the complex wavenumber following Isakovich. Here the Hemar model is used to modify coupled phase theory. THEORY To account for the effect of its neighbouring particles, Hemar et nl Here the subscript f indicates the continuous phase liquid in the shell in the region n &lt; r &lt; b . The subscript e indicates the emulsion region r &gt; b . The emulsion density and thermal expansion coefficient are given by the volume average of the properties of the emulsion phases. The emulsion heat capacity is given by the mass average 195

Noise control by sonic crystal barriers made of recycled materials

A systematic study of noise barriers based on sonic crystals made of cylinders that use recycled materials like absorbing component is here reported. The barriers consist of only three rows of perforated metal shells filled with rubber crumb. Measurements of reflectance and transmittance by these barriers are reported. Their attenuation properties result from a combination of sound absorption by the rubber crumb and reflection by the periodic distribution of scatterers. It is concluded that porous cylinders can be used as building blocks whose physical parameters can be optimized in order to design efficient barriers adapted to different noisy environments

Limitations on validating slitted sound absorber designs through budget additive manufacturing

The potential usefulness of relatively simple pore microstructures such as parallel, identical, inclined slits for creating broadband sound absorption has been argued through analytical models. In principle, such microstructures could be realised through budget additive manufacturing. However, validation of the analytical predictions through normal incidence impedance tube measurements on finite layers is made difficult by the finite size of the tube. The tube walls curtail the lengths of inclined slits and, as a result, prevent penetration of sound through the layer. As well as demonstrating and modelling this effect, this paper explores two manufacturing solutions. While analytical and numerical predictions correspond well to absorption spectra measured on slits normal to the surface, discrepancies between measured and predicted sound absorption are noticed for perforated and zigzag slit configurations. For perforated microgeometries this is found to be the case with both numerical and analytical modelling based on variable length dead-end pores. Discrepancies are to be expected since the dead-end pore model does not allow for narrow pores in which viscous effects are important. For zigzag slits it is found possible to modify the permeability used in the inclined slit analytical model empirically to obtain reasonable agreement with data

Acoustic surface wave generation over rigid cylinder arrays on a rigid plane

Propagation of an airborne acoustic pulse from a point source above an array of regularly spaced rigid cylinders on a rigid plane has been investigated using a two-dimensional multiple scattering theory. Time domain simulations show a main arrival and a separate delayed “tail.” Fourier analysis of the tail shows that, for a sufficiently sparse array of cylinders, it is composed of a series of spectral peaks resulting from constructive interference consistent with Bragg diffraction theory and amplitudes depending on the spacing and size of the cylinders. For increasingly compact distributions of cylinders, the lowest frequency peak is dominated by a quarter wavelength “organ pipe” or “gap” resonance in the space between the cylinders. Simulated pressure maps show that there is a transition region in the acoustic field with an extent that depends on the spacing and size of the cylinders. Beyond this region, individual gap resonances combine to create a field that declines exponentially with height, consistent with the behaviour of a surface wave. Data from measurements of acoustic pulses above copper cylinders on rigid fibreboard under anechoic conditions demonstrate some of the predicted characteristics