Rodolfo Venegas

Acoustical properties of double porosity granular materials.

Granular materials have been conventionally used for acoustic treatment due to their sound absorptive and sound insulating properties. An emerging field is the study of the acoustical properties of multiscale porous materials. An example of these is a granular material in which the particles are porous. In this paper, analytical and hybrid analytical-numerical models describing the acoustical properties of these materials are introduced. Image processing techniques have been employed to estimate characteristic dimensions of the materials. The model predictions are compared with measurements on expanded perlite and activated carbon showing satisfactory agreement. It is concluded that a double porosity granular material exhibits greater low-frequency sound absorption at reduced weight compared to a solid-grain granular material with similar mesoscopic characteristics.

Effect of boundary slip on the acoustical properties of microfibrous materialsa)

A variety of new porous materials with unusually small pores have been manufactured in the past decades. To predict their acoustical properties, the conventional models need to be modified. When pore size becomes comparable to the molecular mean free path of a saturating fluid, the no-slip conditions on the pore surface are no longer accurate and hence the slip effects have to be taken into account. In this paper, sound propagation in microfibrous materials is modeled analytically, approximating the geometry by a regular array of rigid parallel cylinders. It has been shown that velocity and thermal slip on a cylinder surface significantly changes the model predictions leading to lower attenuation coefficient and higher sound speed values. The influence of material porosity, fiber orientation, and size on these effects is investigated. Finite element method is used to numerically solve the oscillatory flow and heat transfer problems in a square array of cylindrical fibres. Numerical results are compared with predictions of the analytical model and the range of its validity is identified.

On the influence of the micro‐geometry on sound propagation through periodic array of cylinders

Sound propagation in rigid porous media has been widely studied by using macroscopical models. These models make use of averaged quantities in which the microscopic details of the porous media geometry are represented by macroscopical parameters and, in a certain way, the influence of the microscopic geometry is not directly identified. In this paper, homogenization theory and finite element method are used for solving the full microscopic dynamic flow and dynamic heat problems for a porous medium modelled as an idealized geometry consisting of a periodic array of cylinders. Different cross‐section shapes of the cylinders (circular, ellipsoidal and square cross‐section shapes) and a wide range of porosity values are considered. The influence of the microscopic features of the porous media on dynamic permeability and dynamic compressibility is also studied.

Influence of sorption on sound propagation in granular activated carbon.

Granular activated carbon (GAC) has numerous applications due to its ability to adsorb and desorb gas molecules. Recently, it has been shown to exhibit unusually high low frequency sound absorption. This behavior is determined by both the multi-scale nature of the material, i.e., the existence of three scales of heterogeneities, and physical processes specific to micro- and nanometer-size pores, i.e., rarefaction and sorption effects. To account for these processes a model for sound propagation in GAC is developed in this work. A methodology for characterizing GAC which includes optical granulometry, flow resistivity measurements, and the derivation of the inner-particle model parameters from acoustical and non-acoustical measurements is also presented. The model agrees with measurements of normal incidence surface impedance and sound absorption coefficient on three different GAC samples.

Sound propagation in porous materials with annular pores

Long-wavelength sound propagation in porous materials with annular pores is investigated in this paper. Closed-form analytical expressions for the effective acoustical properties of this type of material were obtained. These are compared with both direct numerical calculations of the effective properties and their calculations obtained by using semi-phenomenological models. Analytical expressions for the input parameters of the latter, i.e., static viscous and thermal permeabilities, viscous and thermal characteristic lengths, and tortuosity, are also provided. The introduced model is successfully validated by comparing its predictions with measured data taken from literature. A parametric analysis that allows highlighting the influence of the different geometrical parameters of porous materials with annular pores on their sound absorptive properties is also presented.

Omnidirectional acoustic absorber with a porous core - theory and measurements

An omni-directional acoustic absorber consisting of a porous core and the impedance matching metamaterial layer has been designed and tested in the laboratory. Semi-analytical and numerical models have been developed and validated. The numerical model takes into account the viscous losses in the matching layer. A 1.5 m demonstrator has been built and tested under acoustic and weak shock excitation. Testing with acoustic excitation showed good agreement between measurement and model, with near perfect absorption between 400 and 1000 Hz. Testing against weak single-pulse shock in an anechoic chamber also confirmed a significant reduction in peak pressure levels when compared to a conventional porous absorber without matching layer. The findings suggest that structure is equally effective when wrapped around an object like a column, pipeline, or the underside of a vehicle, as it would be when entirely filled with an absorbing porous material.

Shape optimization of polygonal rooms for a correct modal distribution at low frequencies based on psychoacoustic criterion

Resonances in small rooms may lead to inadequate frequency responses. In rooms, where the exigencies on the listening conditions are important, these resonances may cause non wanted coloration effects, which implies a non desirable sound quality. By choosing the right shape and dimensions it is possible to reduce the audible effects of these resonances. The presented methodology aims to determine the shape and size of small and medium polygonal‐shaped rooms based on the finite element method for modeling the physical acoustic behavior of the room; a neural network for loudness estimation and genetic algorithm for estimating the optimal dimensions. A comparison with previous techniques used to choose the dimension of rectangular room is also presented.

Influence of Diffusion and Sorption on Sound Propagation in Multiscale Porous Materials

This paper investigates sound propagation in multiscale sorptive porous materials saturated with a pure gas. An example of this type of materials is a packing of porous grains where a mesoscopic, microscopic, and nanoscopic scale can be identified. These scales are associated with the grain size, and the pores inside the grains of micrometre and nanometre sizes, respectively. Considering that the inner-grain and inter-granular viscous permeabilities are highly contrasted, the macroscopic description of sound propagation through the material is established by upscaling a local mesoscopic description using the two-scale asymptotic method of homogenisation for periodic media. The local mesoscopic description is given by i) an homogenised description of the inner-grain physics that accounts for fluid flow and heat conduction in the micropores, inner-grain interscale mass difussion, and sorption occurring on the walls of the nanopores, ii) the equations describing fluid flow and heat conduction in the mesoscopic fluid network, and iii) the conditions of continuity of mass flux and pressure, and negligible temperature variations on the pore boundaries. It is concluded that the effective compressibility becomes significantly affected by sorption and interscale pressure and mass diffusion processes. This leads to a decrease in sound speed and an increase in sound attenuation, particularly around the characteristic frequencies associated to the diffusion processes.

Acoustical properties of nanoporous activated carbon fibres

This work continues a series of studies on the link between the microstructure of multiscale materials and their acoustical properties [1]-[3]. Granular activated carbons are excellent low frequency sound absorbers. Two factors contribute to this. (i) They have three scales of heterogeneities: millimetric grains and micrometric and nanometric inner-grain pores. (ii) The presence of sorption in nanometric pores leads to a decrease of static bulk modulus and, consequently, of the effective low-frequency sound speed. Activated carbon felts also show promising low-frequency sound absorption but have simpler microstructure. They do not contain inner-fibre micrometric pores, but still have inner-fibre nanometric pores. This, combined with a relatively regular fibre arrangement, makes them ideal for studying the effect of sorption on their acoustical properties. In this work, parameters describing the microstructure and sorption kinetics are measured independently for several types of felts with different levels...

Acoustical characterization of nano-porous carbons

The acoustical and related non-acoustical properties of activated carbon were studied to understand better the effect of added catalyst and acid treatment on the nano- and micro-structure of activated, porous nano-carbon. The acoustical impedance was measured in a 45 mm diameter impedance tube with a special adaptor to accommodate a very small material quantity available for this experiment. The presence of the adaptor was compensated using the procedure detailed in Dupont et al. [POMA 19, 065008 (2013); http://dx.doi.org/10.1121/1.4799701]. The acoustic impedance of activated carbon was predicted using the model proposed by Venegas [Section 6.1, Ph.D. thesis, University of Salford, 2011]. The micro- and nano-scale porosities and pore sizes were determined by fitting the model to the acoustic impedance data in the frequency range between 50 and 1000 Hz. It is shown that the acid treatment and addition of catalyst result in the reduced radius of nano-pores and reduced nano-porosity. These effects are small...