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

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

Fibrous Material Microstructure Design for Optimal Structural Damping

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

Fibrous Material Microstructure Design for Optimal Damping Performance

Properly designed and manufactured fibrous media can be very effective at absorbing airborne sound, and as a result, glass fiber and polymeric fiber media are used in a wide variety of noise control applications. The value of these treatments could be increased if they could also be used to damp the vibration of structures to which they are attached. The materials could then serve a multi-functional role, thus reducing the number of elements required to achieve a specified level of noise and vibration performance, and also reducing the total weight of a sound package, an outcome of particular interest at the moment. In past work, it has been demonstrated that layers of fibrous media placed on panels can effectively dissipate the panel motion by removing energy from the nearfield acoustical motion generated by the panel vibration. That is, subsonic panel vibration results in an oscillatory and essentially incompressible flow of fluid parallel to the panel surface. The magnitude of that motion decays exponentially with distance from the surface, but it has been shown that a fibrous medium placed close to the panel can dissipate significant amounts of energy by viscous interaction with the nearfield, which thus has the consequence of damping the panel vibration to which it is coupled. The question then is: How can a fibrous medium be designed to ensure optimal vibration damping in a particular application? The object of the present work was to address that question. First, a method of calculating the response of a panel with an attached fibrous layer is recalled and updated to allow layers of limp, fibrous media to be modeled. Examples of the results that can be obtained by using such a model are presented, and then it is 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 the density of the fiber material and the desired superficial density

Low Frequency Absorption Enhancement by Modification of Poro-Elastic Layered Sound Package

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

Numerical Modelling of the Acoustics of Low Density Fibrous Media having a Distribution of Fiber Sizes

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

Optimal Design of Multi-Layer Microperforated Sound Absorbers

Microperforated polymer films can offer an effective solution when it is desired to design fiber-free sound absorption systems. The acoustic performance of the film is determined by hole size and shape, by the surface porosity, by the mass per unit area of the film, and by the depth of the backing air layer. Single sheets can provide good absorption over a one of two octave range, but if absorption over a broader range is desired, it is necessary to use multilayer treatments. Here the design of a multilayer sound absorption system is described, where the film is considered to have a finite mass per unit area and also to have conical perforations. It will be shown that it is possible to design compact absorbers that yield good performance over the whole speech interference range. In the course of the optimization it has been found that there is a tradeoff between cone angle and surface porosity. The design of lightweight, multilayer functional absorbers will also be described, and it will be shown, for example, that it is possible to design systems that simultaneously possess good sound absorption and barrier characteristics

Design of Lightweight Fibrous Vibration Damping Treatments to Achieve Optimal Performance in Realistic Applications

I n recent work, it has been shown that conventional sound absorbing materials (e.g., lightweight fibrous media) can provide structural damping when placed adjacent to vibrating structures, including infinite panels, partiallyconstrained panels and periodically-supported panels typical of aircraft structures. Thus, a fibrous layer may serve two functions at once: absorption of airborne sound and the reduction of structure-borne vibration. It has also been found that the damping is primarily effective below the critical frequency of the structure, and that the damping results from viscous interaction between the fibrous layer and the evanescent near-field of the panel, in the region where incompressible flow caused by the panel vibration oscillates primarily parallel with the panel surface. By using a near-field damping (NFD) model based on the Biot model for acoustical porous media, it has been shown that a properly-optimized fibrous layer can provide levels of damping comparable with those provided by conventional, constrained-layer, visco-elastic, damping treatments. Based on the idea that vibrating structures exhibit a certain wavenumber/frequency response spectrum, the focus of the current study has been on evaluating the power dissipated by a fibrous treatment as a function of wavenumber and frequency, and on identifying the material microstructure (i.e., fiber size) required to maximize the power dissipation, and hence damping, in a specific wavenumber/frequency range. To demonstrate the wavenumber/frequency-matching procedure, an example involving a simplified model of a vehicle component will be considered here, and it will be shown how a fibrous layer can be designed to maximize its damping effectiveness when applied to a realistic base structure, such as an automotive floor pan

Acoustical Investigation of Aerogel Granules Modeled as a Layer of Poroelastic Material

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

Programable Sound Absorption Performance Enabled by 3D Printing

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 Acoustical Properties of Air Saturated Aerogel Powders

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