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Our recent investigation [1] showed that the absorption behavior of granular aerogel agglomerates with average particle diameters less than 50μm is dominated by periodically spaced, high absorption peaks and troughs. While the absorption peaks, especially at lower frequencies, are desirable from an engineering application perspective, overcoming the loss in high frequency performance remains a challenge. Furthermore, the practical application of granular aerogels is limited by the difficulty in handling them. In this work, we propose a new design for a broadband noise absorber achieved by layering the aerogel granules within a 3D printed porous network. The design provides a robust method of incorporating aerogels within noise reduction packages and can be tailored by altering the rigid porous network and the layering arrangement. In this presentation, we present our preliminary findings regarding the fabrication and testing of such hybrid sound absorbers. [1] Dasyam A, Xue Y, Bolton JS, Sharma B. Effect of particle size on sound absorption behavior of granular aerogel agglomerates. Journal of Non-Crystalline Solids. 2022 Dec 15;598:121942.

Here, it is shown that properly-designed limp porous materials such as fibrous layers can provide damping equivalent to conventional viscoelastic dampers while providing advantages such as light weight and effective sound absorption. This then allows porous layers to be used as multi-functional noise and vibration control solutions in automotive and aerospace applications. It has also been found that the addition of bulk elasticity to the solid phase of the porous medium is beneficial since it improves damping performance compared to equivalent limp treatments. In this study, porous media, such as fibers and foams, were designed to serve as treatments for various vibrating structures to examine their damping effectiveness. Both analytical modeling and numerical simulation based on finite element methods were involved depending on the complexity of the structure. Specifically, a Fourier transform-based computational method was introduced as the key step to realize the accurate prediction of a panel’s spatial response based on its wavenumber-frequency spectrum. Then, parametric studies were conducted on a porous layer to identify the optimal bulk properties that would allow the layer to provide the largest possible damping within the target frequency region. Finally, design concepts for achieving the maximum damping potential of porous layers are summarized.

Design of acoustic materials can be achieved through the connection between their geometry and acoustical performance. Here, we propose 3D-printing as an enabling technology that allows us to precisely control an acoustic material’s micro-geometry and orientation, which then eliminates microscopic geometry bias due to conventional manufacturing process, thus realizing precise material characterization at the 3D-printing CAD programming stage. This concept was practiced in the current study focusing on 3D-printing fibrous sound absorbing layers. A fused deposition modeling (FDM) method was applied to produce the fibers. A Tarnow-based airflow resistivity model was implemented together with Johnson-Champoux-Allard and Biot theories for modeling the geometry-performance connection for the fibers. The sound absorption prediction accuracy of the model was validated by E-1050 standing wave tube measurements on the printed sample.

The noise reduction of a (low) noise barrier can be enhanced by using an additional element with quarter-wavelength resonators with varying depths. The so-called WHISwall or WHIStop deflects sound upwards for specific frequencies creating an additional sound reduction. Different experiments on the WHISwall and WHIStop are performed as input for model validation. The development and validation of the model are described in a separate paper. In this paper the measurement campaign and its results are presented. We performed measurements on two setups. The first setup consists of a 1.1 meter high WHISwall, a 1.1m high noise barrier and a reference section (without noise measure). Measurements have been conducted with both an artificial sound source and pass by measurements with light and heavy motor vehicles. In a second test setup, the WHIStop was placed on top of a 4 meter high noise barrier and the diffraction was determined according the European standard EN 1793-4.

A key physical mechanism that controls the acoustic absorption and attenuation in a loose, air-satu-rated aerogel granular mix with the grain diameter being in the order of a few microns is not well understood. A particular challenge here is to understand sound propagation in an aerogel powder composed of highly porous, low-density particles with sub-micron pores. Experimental evidence suggest that a relatively thin layer of an aerogel powder can provide a very high narrow band acoustic absorption at relatively low frequencies. This study presents an attempt to explain this physical phenomenon with two well-known analytical models for the acoustical properties of porous media. The results of this study suggest that an aerogel powder behaves like a viscoelastic layer and its absorption coefficient depends strongly on the sound pressure level in the incident wave, i.e., this acoustic behaviour is non-linear. The loss factor seems to be a key parameter which predicts the observed acoustical behaviour. The loss factor is found to be higher than physically reasonable at low frequencies and decreases with the frequency exponentially. This behaviour is likely to relate to the frictional interaction between the particles in the powder and acoustic fluidisation effect.

There is a general lack of understanding of the acoustical behaviour of air-saturated aerogel powders in the audible frequency range. It is unclear what physical processes control the acoustic absorption and /or attenuation mechanisms in a very light, loose granular mix in which the grain diameter is comparable to a micron. This work attempts to fit a Biot-type viscoelastic model to predict the absorption coefficient of two aerogel powder mixes with the particle diameter in the range of 1 - 40 microns. it is shown that these materials behave like a highly flexible, viscoelastic layer. It is found that the absorption coefficient for these materials depends strongly on the root mean square pressure in the incident wave. It is also found that the loss coefficient which accounts for the energy dissipation due to vibration of the elastic frame is a key model parameter. The value of this parameter reduces progressively with the frequency and sound pressure. Other parameters in the adopted Biot-type viscoelastic model, e.g. the storage module of the elastic frame and pore size, are relatively independent of the frequency and amplitude of the incident wave. It is shown that this material can be a very efficient resonance absorber in the low frequency range.

Aerogels are defined as mesoporous materials obtained by replacing the liquid phase within a gel by a gaseous phase, typically air. This underlying mesoporous structure provides aerogels unique macrostructural properties such as ultralow density, high transparency, and low thermal conductivity. Given their ultralow density, aerogels are also an attractive, lightweight solution for noise control applications. Recent studies have shown that the acoustical properties of granular aerogels are very different from those observed for other granular media. In this presentation, we present results from our recent investigation of the acoustical behavior of Enova IC3100 aerogel granules. Manufactured and commercially sold by Cabot Corporation, IC3100 aerogels are characterized by their comparatively smaller granule dimensions. Our previous experimental measurements show that the acoustical performance of the IC 3100 aerogel granules differs from conventionally used sound absorption materials; multiple, lightly damped depth resonances with large peak values of absorption coefficients are observed low frequencies. Here, we present results from our attempt to model the acoustical behavior of IC3100 aerogels. The acoustical-related bulk properties required for the Johnson-Champoux-Allard (JCA) model are calculated using an inverse characterization approach. These properties are then used to model the acoustical behavior of the granular aerogel layer using the Biot theory for porous media.

For many years, industries have been searching for effective noise control solutions having cost effective features such as light weight and small volume. Conventional porous acoustical materials such as polymeric fibers, glass fibers and polyurethane foams fall into the category that meet most of these needs, but it is often said that these materials cannot provide effective absorption at low frequencies. Metamaterials or metasurfaces, on the other hand, have become popular recently due to their potential for nearly-perfect low frequency absorption performance. However, these materials still show obvious limitations regarding their implementation in industry due to their weight, volume, manufacturability and cost. So, to provide acoustical solutions that are applicable in automotive and aerospace industries, here we consider modification of sound packages that specifically enhance the low frequency acoustical performance of conventional porous media. It will be shown that by changing the surface treatment of layered sound packages, and by using a combination of limp fibrous and foam-type poro-elastic materials, excellent sound absorption can be achieved in a frequency range from a few hundred Hz to 10000 Hz without a significant weight or volume penalty. The various concepts will be illustrated through example calculations implemented by using a newly-developed transfer matrix method.

Presented here is the description of a software-based noise source detector that was developed to simplify the printing noise source identification process. The typical noise sources studied here were high frequency, stick-slip (squeaking) noises associated with rollers along the paper path. The detector was built by combining several digital signal processing procedures to first create a tone detector, and then to calculate the modulation spectrum of the tone. The latter is particularly important because it has been found that features appearing in the modulation spectrum can be directly associated with the rotational speed of various components in the printer; this allows the faulty part to be identified. By passing different printing noise samples through the detector, it was found that the detector accurately returned the acoustical characteristics of each noise sample, and then by matching those acoustical characteristics to the rotational speed of different parts, the detector was proven capable of providing robust and precise fault identification results.

Originally inspired by a classical modeling methodology in electrical engineering, the transfer matrix method (TMM) has proven to be an accurate and efficient way to model layered acoustic media. In the case of fluid, or effective fluid, media, the acoustic TMM elements are conventionally modeled as two-by-two matrices. In contrast, a six-by-six matrix is required to model a poro-elastic layer because of the multiple types of waves that can propagate within it. Introduced here is a modified TMM that draws on various matrix operations to couple the six-by-six poro-elastic layer matrix with the two-by-two matrices of other acoustic elements. The matrix operations mainly include two steps: singular value decomposition and QR decomposition, which allows the order of a poro-elastic layer matrix to be reduced from six-by-six to two-by-two, so that a layered system can be modeled by multiplying together a sequence of two-by-two matrices for all the layered acoustic elements, thus finally creating a “global” two-by-two matrix. In this article, the proposed method was applied to several different layered or multi-panel structures, and the predicted acoustical properties were compared to results obtained by using other existing methods to validate the modified TMM.

In previous studies, theories have been developed and simulations performed to show that properly designed fibrous media can be very effective at reducing structural vibration as well as absorbing airborne sound. Therefore, the potential range of noise control applications for fibrous media such as glass and polymeric fiber has been broadened due to this multi-functionality. Since the acoustical, sound absorption properties of these two kinds of fibers are well-known, the current study focused on a comparison of their damping performance when they are used as layered dampers for a fuselage-like structure: i.e., a panel with frames. A layer of each kind of fiber was modeled as resting on the stiffened panel, which was then driven by a convective pressure. The solution for the panel motion was first found in the wavenumber/frequency domain, and an inverse transform was then applied to obtain the spatial results. Through model calculations, the structural damping characteristics of these two fibrous dampers were analyzed and quantified in terms of an equivalent loss factor. In addition, microstructures were designed for each fiber type that results in optimal damping performance over a frequency range of interest. Guidelines were also established for the effective use of these media in structural damping applications.

It is shown here that properly designed fibrous media (e.g., glass or polymeric fibers) can be used to damp structural vibration as well as to absorb sound. The materials can then be multi-functional, reducing the number of elements required to achieve a specified level of noise and vibration performance. Past work demonstrated that layers of fibrous media placed on panels can damp the panel motion by removing energy from the nearfield acoustical motion generated by the panel vibration. The current study focused on designing the fibrous medium to ensure optimal vibration damping in a particular application. First, a method of calculating the response of a panel with an attached fibrous layer is recalled and updated to allow layers of limp or elastic porous media to be modeled. Example results will be presented, and then it will be shown that an optimal flow resistivity exists for a given frequency range and configuration of interest. Finally, based on a recently developed theory for the flow resistivity of fibrous media, the optimal flow resistivity identified in that way can be translated into a particular fiber size, given properties such as density of the fiber material and the desired superficial density.

The focus of the present work was on a low density, polymeric, fibrous medium, comprising a mix of blown micro-fibers, having a relatively broad fiber size distribution, and a second, so-called staple fiber component having a very narrow fiber size distribution. The airflow resistivity of such a material is usually considered to be its most important macroscopic property when it comes to defining the material’s acoustical properties (since the tortuosity and the porosity are very close to unity, for example). Thus, in the first instance, it is of interest to be able to calculate the flow resistivity of such a material on the basis of the distributions of the various fiber sizes, the densities of the fibers and the bulk density of the material. A recent survey of methods for predicting the flow resistivity of fibrous media has revealed a wide variety of approaches, largely based on a knowledge of a material’s solidity (1 minus the porosity), and mean fiber spacing, but in all cases it is assumed that the fiber radius is uniform. An example of such an approach is the work of Tarnow who has developed a model based on the viscous drag experienced by uniform-sized fibers positioned within randomly-spaced Voronoi cells. Recent measurements have shown that Tarnow’s “perpendicular random” model allows accurate predictions of flow resistivity for fibrous media comprising a single fiber component having a very narrow fiber size distribution. It has also been shown that Tarnow’s theory can be modified to account for multiple fiber components having different fiber size distributions. It is then assumed that the flow resistivity calculated in that way can be used to predict the acoustic properties of the medium, although the latter approach takes no specific account of the range of fiber sizes existing within the fibrous medium. Thus, in the present work, two specific issues are addressed: first, how accurate is the new method of calculating the flow resistivity, and secondly, can the effect of fiber size distributions be neglected when predicting sound propagation through such media. The execution of this work has primarily involved the use of numerical tools, GeoDict and Comsol. In particular, Fiber Geo was used to generate a variety of fiber arrays having different orientations and fiber size distributions, and FlowDict was used to compute the pressure drop resulting from low speed viscous flow through a cell of fibers, from which the flow resistivity can be calculated, for example. In the first stage of the work, fibers were modeled as occupying a fluid volume. It is possible to specify fiber sizes and orientations in order to represent materials consisting of fibers having a specified distribution of fiber radii, based for example of micro CT scans of real materials. Flow resistivity results for sets of fibrous media having differing solidities were computed in this way. These results are for practical purposes “exact”, and so provide benchmarks against which parameterized predictions such as presented in reference can be compared. Close agreement with the latter results has been found. Further, the effect of fiber orientation has been studied, which has allowed the prediction of direction dependent flow resistivity in non-isotropic fiber arrays. In the second phase of the work, the fiber geometries considered above were imported into Comsol. By using that software, all of the JCA parameters can be calculated, hence making it possible to calculate the acoustical properties of the fibrous media: e.g., complex densities and sound speeds. At the same time, finite element models of the fiber arrays can be generated, and then linearized visco-thermal models may be solved to yield the “exact” wave propagation characteristics of the fiber arrays. Initially, rigid models were studied: i.e., no motion of the fibers was allowed. Subsequently, full fluid-structure interaction models were implemented to allow for fiber motion in response to oscillatory acoustic flows. In this way, the propagation properties of limp porous materials may be predicted, as can the properties of elastic fiber networks when the fibers are connected. By using the models described here, wave propagation in fiber arrays having realistic bi-modal fiber size distributions has been predicted and compared with corresponding predictions made using conventional Biot-based models. As a result, it has been possible to draw conclusions regarding the ability of those models to accurately represent wave propagation in fibrous media having relatively broad fiber size distributions.

Overview of James B. Moreland’s 1976 paper on: Controlling Industrial Noise by Means of Room Boundary Absorption

Science, Technology, Engineering, Mathematics in education is commonly referred to as STEM. The last decades illustrate that our society is transferring into an ever accelerating technological environment. In parallel, the general public became an important driving force in collecting citizen science data to trigger legislative pressure and impact on policy makers to accelerate the improvement of their quality of life. That practice is currently extending into the environmental impact of noise related quality of life. This publication suggests to merge those educational STEM goals, citizen science monitoring and the need for population based noise monitoring data for efficient policy support. The presented educational project can be regarded as a proof-of-concept and can be repeated in different schools and classes every year. This approach has the potential to acquire abundant noise monitoring data and provides an unbiased population sampling dataset by design. This population driven involvement allows to assess real-life and long-term noise policy impact and could become a fundamental pillar in achieving the overall societal goal of improving noise related environmental quality of life.

Excessive road traffic noise exposure in (sub)urban parks hinders its restorative function and will negatively impact the number of visitors. Especially in such green environments, noise abatements by natural means, well integrated in the landscape, are the most desired solutions. Although dense vegetation bordering the park or raised berms could come first in mind, local landscape depressions are typically underused. In this work, a case-study of a small suburban park, squeezed in between two major arterial roads, is analyzed. The spatially dependent road traffic noise exposure in the park is assessed in detail by mobile sound pressure level measurements. Local reductions of up to 6-7 dBA are found at landscape depressions of only a few meters deep. It can therefore be concluded that this is an efficient measure and should be added to the environmental noise control toolbox for noise polluted parks.

[ES] Desde el establecimiento de las relaciones diplomáticas entre China y España en 1973 y la Reforma y apertura, los intercambios económicos y culturales son cada vez más frecuentes. Muchas obras literarias chinas se han traducido gracias a los continuos esfuerzos de sinólogos y traductores de ambos países. El objetivo de este estudio es analizar la recepción de la narrativa contemporánea china en España desde la perspectiva editorial y crítica, revelar su estado actual y sus tendencias, con el fin de que la narrativa contemporánea china tenga más difusión y mejor aceptación en España. A partir de las teorías de la literatura comparada y de la estética de la recepción, intentamos examinar la evaluación de la narrativa contemporánea china en España mediante los puntos de vista de los editores, las instituciones literarias, los medios de comunicación y los lectores, respectivamente. Comprobamos que, el número de traducciones de la literatura contemporánea china aún está en un nivel bajo en comparación con la literatura anglosajona, y está lejos de integrarse en el sistema literario español. Los estudios de la narrativa contemporánea china en los círculos académicos españoles se centran en las obras de Mo Yan, así como en las traducciones y los fenómenos culturales y sociales chinos; la cobertura de las novelas chinas en los principales medios de comunicación españoles está impregnada con matices ideológicos, y la mayoría de los lectores españoles carecen de interés y tienen un conocimiento muy limitado de los escritores contemporáneos chinos y sus obras. Sin embargo, al juzgar por sus tendencias, el estatus marginal de estas novelas contemporáneas chinas en España ha cambiado en la última década, y poco a poco se están convirtiendo en una parte cada vez más importante de la literatura extranjera pluralista.