Future Trends in Noise Control Technology

Although acoustics plays an important role in noise control, noise control is arguably more difficult than acoustics owing to the constraints that are usually imposed on noise control solutions by manufacturers. That is, ideally, noise control solutions should occupy no space, weigh nothing, cost nothing, not impact the operation of the device being treated, and remain effective for 20 years without maintenance. Hence, identifying effective noise control solutions is a challenging, multidimensional problem. In this presentation, future trends and opportunities in noise control will be highlighted in four categories: targets, measurement techniques, predictive tools, and noise control methods and materials

Normal Incidence Absorption Properties of Single Layers of Elastic Porous Materials

Recently, a theory has been developed which describes wave propagation in relatively stiff, partially reticulated polyurethane foams, the type most commonly used in noise control applications [J.S. Bolton and E. Gold, J. Acoust. Soc. Am. Suppl. 1, vol. 77, S59 (1985)]. A high impedance wave associated with the bulk mechanical properties of the foam matrix is usually significantly excited in these materials. As a consequence, the acoustical performance of finite depth layers of foam of this type is very sensitive to the boundary conditions which apply at the front and rear layer surfaces. Specifically, it will be shown in this paper that the action of a film facing is dependent on how it is attached to the foam layer. In addition it will be demonstrated that a small gap, e.g., 1 mm, separating a foam layer from a hard backing can increase the low-frequency absorption dramatically. A similar effect occurs when a film facing is not bonded directly to the surface of a foam layer but is separated from it by a thin air gap. This work has suggested an arrangement for enhancing the low frequency absorption of thin foam layers

Poro-elastic Materials and the Control of Low Frequency Sound

In the introductory sections of active noise control and metamaterial articles, it is often said that “conventional”, i.e., poro-elastic materials such as foams and fibrous media, do not work well at low frequencies. While that observation may be true for the simplest treatments, e.g., a single layer of a homogeneous, limp fibrous layer, there are many cases in which excellent weight and cost-effective acoustical treatments can be realized by using poro-elastic media. The first example involves the serendipitous discovery of a configuration that allows a 25 mm thick foam layer to provide effective absorption at 300 Hz, at a surface density substantially less than 1 kg/m2. In the context of sound transmission, it will be shown that cells of edge-constrained fibrous media can yield astonishingly high transmission losses at low frequencies, say below 100 Hz, owing to a mechanism similar to that exploited in cellular membrane metamaterials. However, in both cases, a fair comparison with the performance of “conventional” barrier materials, i.e., simple impermeable mass layers, can only be drawn when the weight required to achieve the edge-constraint effect is accounted for

The Reduction of Tire/Road Interaction Noise

In this presentation, a new conceptual approach to the reduction of tire/road interaction noise will be described. The approach is based primarily on the use of wave number transform techniques to characterize tire vibration Measurements of the wave number spectrum of tire vibration have allowed the identification of the particular wave types that contribute to tire vibration and to radiated noise. These findings have important implications regarding tire modeling, and it will be shown that a composite, hybrid two-dimensional finite element model that includes effects of curvature and inflation may be used to predict tire vibration efficiently. Further, it will be shown that the vibration predictions may be used in combination with a boundary element code to predict the total sound power radiated by tire vibration. It has been found, for example, that the horn effect has a significant impact on the sound power radiated by tire vibration. It will be shown that below the horn effect cut-on frequency, almost all sound is radiated by fast, low-order circumferential components of tire vibration, but that above that frequency, many vibration components radiate efficiently. It will be suggested that tire/road interaction passby noise, in particular, can be reduced by a combination of tire structural modifications and tread design changes

Poro-Elastic Materials and the Control of Low Frequency Sound

In the introductory sections of active noise control and metamaterial articles, it is often said that “conventional”, i.e., poro-elastic materials such as foams and fibrous media, do not work well at low frequencies. While that observation may be true for the simplest treatments, e.g., a single layer of a homogeneous, limp fibrous layer, there are many cases in which excellent weight and cost-effective acoustical treatments can be realized by using poro-elastic media. The first example involves the serendipitous discovery of a configuration that allows a 25 mm thick foam layer to provide effective absorption at 300 Hz, at a surface density substantially less than 1 kg/m2 . In the context of sound transmission, it will be shown that cells of edge-constrained fibrous media can yield astonishingly high transmission losses at low frequencies, say below 100 Hz, owing to a mechanism similar to that exploited in cellular membrane metamaterials. However, in both cases, a fair comparison with the performance of “conventional” barrier materials, i.e., simple impermeable mass layers, can only be drawn when the weight required to achieve the edge-constraint effect is accounted fo

The plastic limit of clays

The plastic limit of soils was first described by Atterberg in 1911. The thread-rolling test was standardised at the US Public Roads Bureau in the 1920s and 1930s, and has subsequently become one of the standard tests of soil mechanics. This paper reviews the original definitions of plastic limit as proposed by Atterberg, and proposes that the brittle failure observed in the plastic limit test is caused by either air entry or cavitation in the clay. Critical state soil mechanics is used to show that the observed range of undrained shear strengths of soils at plastic limit is consistent with this hypothesis. The fallacy that strength at plastic limit is a constant is highlighted, and the implications for geotechnical practice are discussed. </jats:p

Cosmic Variance In the Transparency of the Intergalactic Medium After Reionization

Following the completion of cosmic reionization, the mean-free-path of ionizing photons was set by a population of Ly-limit absorbers. As the mean-free-path steadily grew, the intensity of the ionizing background also grew, thus lowering the residual neutral fraction of hydrogen in ionization equilibrium throughout the diffuse intergalactic medium (IGM). Ly-alpha photons provide a sensitive probe for tracing the distribution of this residual hydrogen at the end of reionization. Here we calculate the cosmic variance among different lines-of-sight in the distribution of the mean Ly-alpha optical depths. We find fractional variations in the effective post-reionization optical depth that are of order unity on a scale of ~100 co-moving Mpc, in agreement with observations towards high-redshift quasars. Significant contributions to these variations are provided by the cosmic variance in the density contrast on the scale of the mean-free-path for ionizing photons, and by fluctuations in the ionizing background induced by delayed or enhanced structure formation. Cosmic variance results in a highly asymmetric distribution of transmission through the IGM, with fractional fluctuations in Ly-alpha transmission that ar larger than in Ly-beta transmission.Comment: 7 pages 3 figures. Replaced with version accepted for publication in Ap

The Effect of Sample Edge Conditions on Standing Wave Tube Measurements of Absorption and Transmission Loss

The acoustical properties of isotropic, elastic porous materials are now conventionally specified by a set of nine, frequency-independent macroscopic parameters: e.g., flow resistivity, tortuosity, viscous and thermal characteristic lengths, etc. When these properties are known, it is possible to predict the absorption performance of the material, for example, in arbitrary geometries. Conversely, it has become popular to infer the macroscopic parameters of porous materials by finding the set of parameters that results in an optimal match of measurements and prediction. The software packages FOAM-X and COMET/Trim, for example, offer inverse characterization features of this type. However, here it is shown that that it may not be possible to represent large and small diameter samples of the same material as measured in standing wave tubes by using a single set of parameters. In practice, measurements in standing wave tubes can be significantly affected by sample edge effects, particularly gaps around the sample resulting from minor damage of the sample during cutting. Here it will be illustrated that it is necessary to model the sample inhomogeneity resulting from edge damage if both large and small tube results are to be modelled by using a single, consistent set of parameters

Effect of Thermal Losses and Fluid-Structure Interaction on the Transfer Impedance of Microperforated Films

It has been shown previously that incompressible computational fluid dynamics (CFD) models can be solved in the time domain to calculate the transfer impedances of microperforated panels. However, these models require relatively lengthy run times, do not allow for thermal losses due to irreversible heat transfer to the panels, and rely on the assumption that the solid parts of the panels are rigid. In the present work, compressible, thermo-acoustic models, solved in the frequency domain, have been used to compute thermal losses in addition to viscous losses; these calculations enable the visualization and spatial localization of both loss mechanisms. Thermal losses prove to be relatively small compared to viscous losses in typical geometries, but they become progressively more important as the frequency increases. Additionally, the fully-coupled fluid-structure interaction (FSI) problem has been solved to determine the range of parameters within which the transfer impedance of a rigid microperforated panel can be added in parallel to the impedance of a limp panel ( ) to account for panel flexibility. In particular it will be shown under what conditions the relative motion between the fluid velocity through the perforations and the velocity of the panel, including its phase, must be explicitly considered