Acoustics modelling of open-cell foam materials from microstructure and constitutive properties

The dynamic relations for highly porous fibrous materials, having analytical expressions for dynamic viscous drag forces and oscillatory solid-to-fluid heat transfer, are now extended towards open-cell foam materials where the struts of the foam are considered to be primarily cylindrical except in the region of the joints. By also including analytical expressions for the stiffness of the foam cell, an entirely analytically-based model is presented for the acoustics of highly-porous, open-celled foam materials. This approach is extremely efficient, requiring only the mean cell size, mean strut diameter, and constitutive properties of the solid foam material and the surrounding viscous fluid as input. The acoustic performance prediction of not only isotropic foam cell designs, but also anisotropic ones may be performed rapidly and virtually, without the need for the determination of poroelastic material properties from existing material samples. The steps required for the development of the analytical foam-cell model are presented, along with the acoustic performance prediction of a typical Melamine foam cell, yielding very promising results in comparison against measurements. In order to understand the suitability of the cylindrical foam strut assumption, a viscous drag force comparison with foam struts having square and triangular cross-sectional profiles is also presented.

Noise measurements as a proxy to evaluating the response to recommendations in times of crisis: An update analysis of the transition to the second wave of the CoViD-19 pandemic in Central Stockholm, Sweden

Sweden stands out among the other European countries by the degree of restrictive measures taken towards handling the 2019 coronavirus outbreak, associated with the CoViD-19 pandemic. While several governments have imposed a nationwide total or partial lockdown to slow down the spread of the virus, the Swedish government has opted for a recommendation-based approach together with a few imposed restrictions. In a previous contribution by the authors, the impact of the Swedish strategy was observed through the monitored variation of the city noise levels during a period associated with the so-called "first wave" of the pandemic in Stockholm. A very strong impact of these recommendations was shown on the evolution of the noise levels in central Stockholm. This highlighted the potential of acoustic sensor networks both for enforcement of regulation and monitoring of the effectiveness of their implementation. The present contribution presents a follow-up to this urban noise monitoring in central Stockholm, Sweden, for the period leading to the so-called "second wave" of the pandemic in Europe. Both the evolution of adherence to the recommendations and the impact of the recurrence of cases combined with reinforced recommendations are observed through the evolution of the measured noise levels. While the measurements show a gradual lower level of compliance, in particular, past the summer break, these also show again a rapid response to the reinforced recommendations issued by the authorities in mid-fall of 2020. These observations thus confirm the potential associated with detailed urban noise monitoring, for instance here acting as a proxy to evaluating the response to recommendations or restrictions in times of crisis.

An observation of the impact of CoViD-19 recommendation measures monitored through urban noise levels in central Stockholm, Sweden

Sweden stands out among the other European countries by the degree of restrictive measures taken towards handling the 2019 coronavirus outbreak, associated with the CoViD-19 pandemic. While several governments have imposed a nationwide total or partial lockdown in order to slow down the spread of the virus, the Swedish government has opted for a recommendation-based approach together with a few imposed restrictions. In the present contribution, the impact of this strategy will be observed through the monitored variation of the city noise levels during the associated period. The data used are recorded during a campaign of over a full year of noise level measurements at a building façade situated in a busy urban intersection in central Stockholm, Sweden. The noise level reductions, observed during the period of restrictions, are shown to be comparable to those found for the two most popular public holidays in Sweden with a peak reduction occurring during the first half of April 2020. Contrary to what has been recently discussed in public media, the spread of the virus, the recommendations, and the restrictions imposed during the ongoing pandemic clearly have had a significant effect on the transport and other human-related activities in Stockholm. In this unique investigation, the use of distributed acoustic sensors has thus shown to be a viable solution not only to enforce regulations but also to monitor the effectiveness of their implementation.

Dynamic equations of a transversely isotropic, highly porous, fibrous material including oscillatory heat transfer effects

The dynamic equations of a transversely isotropic fibrous, highly porous material are presented in terms of microstructure-derived analytical expressions for viscous dissipation, and analytical expressions for the oscillatory heat transfer between the thermal fields of the solid cylindrical glassfibres and the surrounding viscous fluid. This represents the non-equilibrium thermal expansion of the fluid, occurring when waves propagate in the porous material, and results in a frequency-dependent scaling of the fluid dilatation term. A state-space transfer matrix solution of the governing equations has been introduced, allowing the numerical acoustical performance of the fibrous material to be investigated, including the acoustical effects of heat transfer. In order to understand the dissipation mechanisms of the viscous and thermal boundary layers on the surface of the fibres and the validity of the assumptions made in the current model, a thermoviscous acoustic fluid finite element procedure has also been introduced. The results from these simulations illustrate the frequency-dependent interaction of the boundary layers between neighbouring fibres in the porous material.

Erratum: A simplified model for thin acoustic screens [J. Acoust. Soc. Am. 144(1), EL76-EL81 (2018)]

The present erratum reports an error impacting the figures of a contribution published in 2018 about a simplified model for thin acoustic screens in a transfer matrix context [Gaborit, Dazel, and Göransson (2018). J. Acoust. Soc. Am. 144(1), EL76-EL81]. A mistake in the implementation of the rigid termination condition for the systems under study is identified and a correct version is proposed along with the corrected figures. It is shown that this error does not impact the conclusions of the original contribution and that the model proposed therein keeps its advantages as the approximation error remains very similar to the previously reported values.

A simplified model for thin acoustic screens

A generalization of the commonly used pressure jump modeling of thin porous layers is proposed. The starting point is a transfer matrix model of the layer derived using matrix exponentials. First order expansions of the propagating terms lead to a linear approximation of the associated phenomena and the resulting matrix is further simplified based on physical assumptions. As a consequence, the equivalent fluid parameters used in the model may be reduced to simpler expressions and the transfer matrix rendered sparser. The proposed model is validated for different backing conditions, from normal to grazing incidence and for a wide range of thin films. In the paper, the physical hypotheses are discussed, together with the origin of the field jumps.

Microstructure based estimation of the dynamic drag impedance of lightweight fibrous materials

This paper focusses on the prediction of one of the main mechanisms of acoustic attenuation, the dynamic drag impedance, of a bundle of fibres typical of lightweight fibrous porous materials. The methodology uses geometrical properties derived from microscopy, and is based on the assumption that the interaction between the shear stress fields of neighbouring fibres may be neglected in the predicted drag force of an individual fibre. An analytical procedure is discussed which provides an estimate of the drag forces acting on infinite longitudinal and transversely orientated cylinders oscillating sinusoidally in a viscous incompressible fluid of infinite extent, at rest. The frequency-dependent viscous drag forces are estimated from the shear stresses on the surface of the cylinders, and may be scaled in terms of fibre diameter distributions and orientation angles in order to predict the dynamic drag impedance of a real material. The range of validity for this modelling approach is assessed through finite element solutions of three different fibre arrangements.

Metal contaminant fluxes across the sediment water interface

To date, most estimates of contaminant fluxes across the sediment/water interface in risk assessments have been done using diffusive flux models. However, the reliability of these is limited as the overall flux from the sediment may have contributions caused by advection and bioturbation. We found through a comparison of modelled fluxes versus measured fluxes, that the methods Benthic Flux Chamber and surface leaching tests in a risk assessment context showed similar magnitude while calculated fluxes deviated at least by a factor of 100 from measured fluxes. This may be explained by the flux contribution in connection with bioturbation. The chamber-measured fluxes of copper were low compared to those of zinc and cobalt, but this is consistent with leaching tests that indicated copper to be more strongly bound. Risk assessments based on total concentrations may be misleading.

Derivation of the state matrix for dynamic analysis of linear homogeneous media

A method to obtain the state matrix of an arbitrary linear homogeneous medium excited by a plane wave is proposed. The approach is based on projections on the eigenspace of the governing equations matrix. It is an alternative to manually obtaining a linearly independent set of equations by combining the governing equations. The resulting matrix has been validated against previously published derivations for an anisotropic poroelastic medium.

Redescription of Tharyx killariensis (Southern) from Ireland and description of two new species of Tharyx from the Kattegat, Sweden (Polychaeta, Cirratulidae)

Two new species of the cirratulid genus Tharyx are reported from shallow waters in the Kattegat inshore Sweden. In addition, the lectotype of Tharyx killariensis (Southern, 1914) is redescribed resulting in a revised concept of the noto- and neuropodial acicular spines of posterior parapodia for that species. These spines were originally reported as bidentate crotchets with sharply pointed teeth; in reality the spines have blunt, knob-shaped tips, typical of several other species of Tharyx. Both of the new species are atypical for the genus Tharyx. T. maryae n. sp. has an expanded posterior end more typical of the genus Aphelochaeta, but otherwise shares characters of Tharyx. T. robustus n. sp. has a body shape that is consistently broad and dorsoventrally flattened throughout, rather than elongate and narrow as in other species of the genus. Both of the new species, however, have short, blunt-tipped spines in far posterior parapodia. With the addition of the two new species, the genus Tharyx now includes 11 species that are compared and contrasted. Morphology that defines and characterizes species of Tharyx is reviewed.

Identification of the full anisotropic flow resistivity tensor for multiple glass wool and melamine foam samples

The flow resistivity tensor, which is the inverse of the viscous permeability tensor, is one of the most important material properties for the acoustic performance of porous materials used in acoustic treatments. Due to the manufacturing processes involved, these porous materials are most often geometrically anisotropic on a microscopic scale, and for demanding applications, there is a need for improved characterization methods. This paper discusses recent refinements of a method for the identification of the anisotropic flow resistivity tensor. The inverse estimation is verified for three fictitious materials with different degrees of anisotropy. Measurements are performed on nine glass wool samples and seven melamine foam samples, and the anisotropic flow resistivity tensors obtained are validated by comparison to measurements performed on uni-directional cylindrical samples, extracted from the same, previously measured cubic samples. The variability of flow resistivity in the batch of material from which the glass wool is extracted is discussed. The results for the melamine foam suggest that there is a relation between the direction of highest flow resistivity, and the rise direction of the material.

A residue-based mode selection and sorting procedure for efficient poroelastic modeling in acoustic finite element applications

Analysis of three-dimensional sound propagation in porous elastic media with the Finite Element (FE) method is, in general, computationally costly. Although it is the most commonly used predictive tool in complex noise control applications, efficient FE solution strategies for large-size industrial problems are still lacking. In this work, an original procedure is proposed for the sorting and selection of the modes in the solution for the sound field in homogeneous porous domains. This procedure, validated on several 2D and 3D problems, enables to reduce the modal basis in the porous medium to its most physically significant components. It is shown that the size of the numerical problem can be reduced, together with matrix sparsity improvements, which lead to the reduction in computational time and enhancements in the efficacy of the acoustic response computation. The potential of this method for other industrial-based noise control problems is also discussed.

Vibroacoustic response sensitivity due to relative alignment of two anisotropic poro-elastic layers

The effects of relative alignment of two different types of anisotropic open cell porous materials are investigated in terms of the acoustic response of a multi-layered configuration. Numerical experiments, where gradient based optimization techniques were used, are conducted to find possible extremal values. It is shown that, depending on the degree of anisotropy of the porous material properties, their angular orientations have a significant and frequency dependent influence on the measured response. The results highlight the importance of further advancing the knowledge of anisotropic porous material behavior.

A modal-based reduction method for sound absorbing porous materials in poro-acoustic finite element models

Structural-acoustic finite element models including three-dimensional (3D) modeling of porous media are generally computationally costly. While being the most commonly used predictive tool in the context of noise reduction applications, efficient solution strategies are required. In this work, an original modal reduction technique, involving real-valued modes computed from a classical eigenvalue solver is proposed to reduce the size of the problem associated with the porous media. In the form presented in this contribution, the method is suited for homogeneous porous layers. It is validated on a 1D poro-acoustic academic problem and tested for its performance on a 3D application, using a subdomain decomposition strategy. The performance of the proposed method is estimated in terms of degrees of freedom downsizing, computational time enhancement, as well as matrix sparsity of the reduced system.

Inverse estimation of the elastic and anelastic properties of the porous frame of anisotropic open-cell foams

This paper presents a method for simultaneously identifying both the elastic and anelastic properties of the porous frame of anisotropic open-cell foams. The approach is based on an inverse estimation procedure of the complex stiffness matrix of the frame by performing a model fit of a set of transfer functions of a sample of material subjected to compression excitation in vacuo. The material elastic properties are assumed to have orthotropic symmetry and the anelastic properties are described using a fractional-derivative model within the framework of an augmented Hooke's law. The inverse estimation problem is formulated as a numerical optimization procedure and solved using the globally convergent method of moving asymptotes. To show the feasibility of the approach a numerically generated target material is used here as a benchmark. It is shown that the method provides the full frequency-dependent orthotropic complex stiffness matrix within a reasonable degree of accuracy.

Acoustic and vibrational damping in porous solids

A porous solid may be characterized as an elastic-viscoelastic and acoustic-viscoacoustic medium. For a flexible, open cell porous foam, the transport of energy is carried both through the sound pressure waves propagating through the fluid in the pores, and through the elastic stress waves carried through the solid frame of the material. For a given situation, the balance between energy dissipated through vibration of the solid frame, changes in the acoustic pressure and the coupling between the waves varies with the topological arrangement, choice of material properties, interfacial conditions, etc. Engineering of foams, i.e. designs built on systematic and continuous relationships between polymer chemistry, processing, micro-structure, is still a vision for the future. However, using state-of-the-art simulation techniques, multiple layer arrangements of foams may be tuned to provide acoustic and vibrational damping at a low-weight penalty. In this paper, Biot's modelling of porous foams is briefly reviewed from an acoustics and vibrations perspective with a focus on the energy dissipation mechanisms. Engineered foams will be discussed in terms of results from simulations performed using finite element solutions. A layered vehicle-type structure is used as an example.

Quantification of bacterial subgroups in soil: comparison of DNA extracted directly from soil or from cells previously released by density gradient centrifugation

All molecular analyses of soil bacterial diversity are based on the extraction of a representative fraction of cellular DNA. Methods of DNA extraction for this purpose are divided into two categories: those in which cells are lysed within the soil (direct extraction) and those in which cells are first removed from soil (cell extraction) and then lysed. The purpose of this study was to compare a method of direct extraction with a method in which cells were first separated from the soil matrix by Nycodenz gradient centrifugation in order to evaluate the effect of these different approaches on the analysis of the spectrum of diversity in a microbial community. We used a method based on polymerase chain reaction (PCR) amplification of a 16S rRNA gene fragment, followed by hybridization of the amplified fragments to a set of specific probes to assess the phylogenetic diversity of our samples. Control parameters, such as the relationship between amount of DNA template and amount of PCR product and the influence of competing DNA on PCR amplification, were first examined. Comparison between extraction methods showed that less DNA was extracted when cells were first separated from the soil matrix (0.4 microg g(-1) dry weight soil versus 38-93 microg g(-1) obtained by in situ lysis methods). However, with the exception of the gamma-subclass of Proteobacteria, there was no significant difference in the spectrum of diversity resulting from the two extraction strategies.