Prediction Of Transmission Loss Research Articles

A point collocation approach to modelling large dissipative silencers

A numerical matching technique known as point collocation is used to model mathematically large dissipative splitter silencers of a type commonly found in HVAC ducts. Transmission loss predictions obtained using point collocation are compared with exact analytic mode matching predictions in the absence of mean flow. Over the frequency range in which analytic mode matching predictions are available, excellent agreement with point collocation transmission loss predictions is observed for a range of large splitter silencers. The validity of using point collocation to tackle large dissipative silencers is established, as is the computational efficiency of the method and its suitability for tackling dissipative silencers of arbitrary, but axially uniform, cross-sections.

Vibration attenuation at asymmetric cross‐form joints of buildings

The aim of the work is to estimate sound energy transmission through asymmetrical cross‐junctions in buildings. Several kinds of these joints have been investigated. Theoretical calculation model to obtain transmission loss was developed and a system of equations established. The equations were solved and expressions were obtained for predicting transmission loss. Using this procedure for different structural connections, the simplified calculation formulas were received for different cases of asymmetrical cross‐joint. The influence of geometrical parameters and material properties were determined for the sound energy attenuation in the joints of buildings. The obtained results may be applied for evaluation of influence flanking noise transmission through different asymmetric cross‐junctions in buildings. Vibracijų silpnumas asimetrinėse kryžiaus formos pastatų jungtyse Santrauka Pagrindinis darbo tikslas yra nustatyti garso energijos slopinimą asimetrinėse kryžiaus formos pastatų konstrukcijų jungtyse. Analizuoti keli asimetrinių kryžiaus formos mazgų tipai. Sudarytas teorinis vibracijų perdavimo koeficientų skaičiavimo modelis ir gauta lygčių sistema. Ją išsprendus, nustatytos vibracijų perdavimo koeficiento (perdavimo nuostolių) supaprastintos skaičiuojamosios išraiškos. Kartojant šią procedūrą, gautos supaprastintos lygtys įvairiems asimetrinių mazgų atvejams. Naudojantis gautomis išraiškomis, nustatyti pagrindiniai parametrai, turintys įtakos garso energijos slopinimui pastato jungtyse. Gauti rezultatai gali būti panaudoti netiesioginio triukšmo sklidimo įvairiomis asimetrinėmis kryžiaus formos jungtimis įtakai įvertinti. Raktiniai žodžiai: struktūrinė akustika, vibracijos, perdavimo nuostoliai, asimetriniai mazgai. First Published Online: 14 Oct 2010

A Practical Radiosity Method for Predicting Transmission Loss in Urban Environments

The ability to predict transmission loss or field strength distribution is crucial for determining coverage in planning personal communication systems. This paper presents a practical method to accurately predict entire average transmission loss distribution in complicated urban environments. The method uses a 3D propagation model based on radiosity and a simplified city information database including surfaces of roads and building groups. Narrowband validation measurements with line-of-sight (LOS) and non-line-of-sight (NLOS) cases at 1800 MHz give excellent agreement in urban environments.

Assessment of sandwich models for the prediction of sound transmission loss in unidirectional sandwich panels

The consistent higher-order approach and the two-parameter foundation formulation are used for the derivation of sound transmission loss in symmetric unidirectional (infinitely wide) sandwich panels with isotropic face sheets. In both models, transmission loss is calculated using decoupled equations representing symmetric and anti-symmetric motions of a sandwich panel. The closed-form expressions for impedances and transmission coefficient of a symmetric sandwich panel with an isotropic core are derived for the two-parameter foundation model. A comparison between the numerical predictions based on the two sandwich models and available experimental data shows that the consistent higher-order formulation can be used to predict the transmission loss in symmetric sandwich panels with both honeycomb and isotropic cores. For prediction of transmission loss of symmetric sandwich panels with an isotropic core, the two-parameter foundation model is more convenient, while the consistent higher-order approach is more accurate.

Prediction of sound transmission loss of honeycomb sandwich panel by higher order approach

People have studied the sound transmission loss (STL) of sandwich panels since the 1970s. However, most of the existing prediction methods have been based on single-layer dynamical models, neglecting the symmetric (dilatational) movements of the skins. Consequently, the symmetric coincident frequency of the sandwich panel cannot be predicted using those approaches. To account for this dilatational motion of the sandwich structures, different methods were utilized. However, most of them were based on one dimensional sandwich beams theories. The authors have also applied the consistent higher order beam approach to calculate the sound transmission loss of a unidirectional sandwich panel. Although the one dimensional approximation is good in predicting STL, the effects of some factors, such as the anisotropy and orientation of the principle axis of the panel, cannot be estimated. In the current work, the authors extended that one dimensional beam model into two dimensions, which allows us to calculate the STL of sandwich plates with composite laminate face sheets and honeycomb core. Both flexural (antisymmetric) and dilatational (symmetric) motions of the sandwich panel were considered in the study. The predictions were finally compared with our experimental data.

Improvement of the prediction of transmission loss of double partitions with cavity absorption by minimization techniques

In this paper, a method based on the minimization of a quadratic error function is used as a tool to improve the predictions of noise transmission loss, and it is applied to the case of a double partition filled with an absorbent material. When experimental results are available, the method can be used to determine the value of the parameters of the materials composing the structure.

Geoacoustic inversion of a shallow fresh-water environment

A recent experiment was conducted at The University of Texas/Applied Research Laboratories test station located at Lake Travis, Austin, TX. Implosive (light bulb), explosive (firecracker), and tonal sources were recorded on a dual receiver system located on the bottom next to a range-independent underwater river channel. Inversion results of the broadband time series obtained over ranges less than 1.5 km were used to predict measured transmission loss at several tonal frequencies in the band from 250–1000 Hz. The average water depth was approximately 38 m along the channel during the experiment. Sound speed profiles were calculated from recorded temperature readings measured as a function of depth. Implosive source spectrums were measured and used to evaluate a model/data correlation cost function in a simulated annealing algorithm. Comparisons of inversion results using both a normal mode and a ray-based plane wave reflection coefficient forward model [Stotts et al., J. Acoust. Soc. Am. (submitted)] are discussed. Predicted transmission loss based on the inversion results are compared to the measured transmission loss. Differences between fluid and elastic layer bottom models will also be presented.

Transmission loss predictions for dissipative silencers of arbitrary cross section in the presence of mean flow.

A numerical technique is developed for the analysis of dissipative silencers of arbitrary, but axially uniform, cross section. Mean gas flow is included in a central airway that is separated from a bulk reacting porous material by a concentric perforate screen. The analysis begins by employing the finite element method to extract the eigenvalues and associated eigenvectors for a silencer of infinite length. Point collocation is then used to match the expanded acoustic pressure and velocity fields in the silencer chamber to those in the inlet and outlet pipes. Transmission loss predictions are compared with experimental measurements taken for two automotive dissipative silencers with elliptical cross sections. Good agreement between prediction and experiment is observed both without mean flow and for a mean flow Mach number of 0.15. It is demonstrated also that the technique presented offers a considerable reduction in the computational expenditure when compared to a three-dimensional finite element analysis.

Errors in transmission loss prediction—the bispectrum and kurtosis approaches

The measurements of the acoustic performance of automotive mufflers are influenced by the wave propagation with reflection and absorption. This way, the measured signal can have linear and non-linear interactions of the wave components. The bispectrum, which is the measure of the phase relationship between three spectral components, has been shown to be a useful tool in the study of linear and non-linear wave interactions. The bicoherence spectrum may be used to discriminate between non-linearly coupled waves and spontaneously excited waves. At the same time, the kurtosis parameter is used as an indicative of the signals of microphones. The performances of two different physical models (bipartite chamber and a chamber with concentric perforated tube) are analysed considering the transmission loss. The experimental data are obtained through a two microphone method and the numerical values through the finite element method.

Helmholtz resonator with extended neck.

Acoustic performance of a concentric circular Helmholtz resonator with an extended neck is investigated theoretically, numerically, and experimentally. The effect of length and shape of, and the perforations on the neck extension is examined on the resonance frequency and the transmission loss. A two-dimensional analytical method is developed for an extended neck with constant cross-sectional area, while a three-dimensional boundary element method is applied for the variable area and perforated extension. Lumped and one-dimensional approaches are also included to illustrate the effect of the higher order modes. For a piston-driven model, predicted resonance frequencies using lumped, one-dimensional, and two-dimensional analytical methods are compared with those from multidimensional boundary element method. Analytical and computational transmission loss predictions for pipe-mounted model are compared to the experimental data obtained from an impedance tube setup. It is shown that the resonance frequency may be controlled by the length, shape, and perforation porosity of the extended neck without changing the cavity volume.

Acoustic characteristics of a three-chamber hybrid silencer

The acoustic characteristics of a hybrid silencer consisting of two identical single-pass, concentric, perforated dissipative chambers combined with a reactive Helmholtz resonator in between are investigated computationally and experimentally. Transmission loss predictions from a three-dimensional boundary element method are compared with experimental results obtained from an impedance tube setup in the absence of mean flow. In addition to the overall design, the effects of filling material density in dissipative chambers and the neck geometry of the reactive resonator are examined. The dissipative chambers are found to be effective at high frequencies, while the reactive resonator is shown, in general, to improve the acoustic attenuation at low frequencies typical of airborne noise in internal combustion engines running at low- to mid-speed range. The acoustic behavior near the resonance frequency is determined, however, to be sensitive to the duct length connecting dissipative and reactive chambers. Potential merits of the hybrid concept are assessed.

A direct mixed-body boundary element method for packed silencers.

Bulk-reacting sound absorbing materials are often used in packed silencers to reduce broadband noise. A bulk-reacting material is characterized by a complex mean density and a complex speed of sound. These two material properties can be measured by the two-cavity method or calculated by empirical formulas. Modeling the entire silencer domain with a bulk-reacting lining will involve two different acoustic media, air and the bulk-reacting material. Traditionally, the interior silencer domain is divided into different zones and a multi-domain boundary element method (BEM) may be applied to solve the problem. However, defining different zones and matching the elements along each interface is tedious, especially when the zones are intricately connected. In this paper, a direct mixed-body boundary element method is used to model a packed silencer without subdividing it into different zones. This is achieved by summing up all the integral equations in different zones and then adding the hypersingular integral equations at interfaces. Several test cases, including a packed expansion chamber with and without an absorbing center bullet, and a parallel baffle silencer, are studied. Numerical results for the prediction of transmission loss (TL) are compared to experimental data.

An experimental investigation on the directional distribution of incident energy for the prediction of sound transmission loss

It is suggested from a previous theoretical study that an angular distribution function can be successfully used for describing the directional distribution of incident energy on the wall, instead of the field incidence assumption being used in the prediction of sound transmission loss. This study is devoted to an experimental investigation on the directionality of incidence sound field for validating the underlying assumption in the theoretical work. To this end, the directional magnitudes of incident energy on a wall in a reverberation chamber are measured by employing the sound intensimetry. The experimental results suggest that the transmission coefficient formula should be revised by introducing a proper angular distribution function as a weighting function for describing the directional energy density distribution, which replaces the conventional field incidence method. The revised formula is applied to the prediction of transmission loss for a double-leaf panel with an air cavity, in which its effectiveness is strongly supported by good agreements between measured and predicted values.

Measurement of acoustic impedance and prediction of transmission loss of the porous woven hose in engine intake systems

For the prediction of the acoustic performance of an engine intake system with a porous woven hose, the acoustic wall impedance of the hose should be known. In this paper, a measurement technique is proposed that is valid over the low frequency range in the absence of mean flow. The impedance measurement is performed in two stages. The first stage of impedance measurement is performed in a cylindrical apparatus, within which a short specimen of given ‘porous frequency’ is laid. Because the predicted transmission loss (TL) does not agree with the measured one, due to error in the measured resistance, the second stage of impedance measurement is attempted to estimate the resistance from the measured reactance in the first stage and the measured TL data. Using such new impedance data measured in the second stage for the prediction of TL for samples of different lengths, the agreement between measured and predicted TL is generally improved, as compared to the initial comparison in the first stage. When the curve-fitted impedance for an arbitrary porous frequency is used, it is shown that the measured TL for samples with both arbitrary length and porous frequency generally agree reasonably well with the predicted ones. Exceptions are samples which require the impedance to be extrapolated from measured results, or samples with highly inhomogeneous weaving and coating conditions, and in particular samples with low porous frequency.

SIMPLIFIED TECHNIQUES FOR PREDICTING THE TRANSMISSION LOSS OF A CIRCULAR DISSIPATIVE SILENCER

A closed-form analytical solution for the transmission loss of a dissipative silencer with a circular cross-section is described. The silencer contains a bulk reacting acoustic absorbent which is separated from a mean gas flow by a perforated screen. Theoretical predictions of the silencer transmission loss for three different dissipative silencers are compared both with experimental data and with another more complex modelling technique. Good agreement is noted between the analytical theory and experimental data in the low-to-medium frequency range. Below a defined upper frequency limit the analytical technique is also shown to provide good agreement with the finite element method. In addition it is observed that, even for relatively high open area porosities, the perforate screen has a significant effect on dissipative silencer performance.

Modeling of perforated absorbing silencers

The acoustic characteristics of single-pass perforated ducts filled with absorbing material are studied experimentally and analytically. The transmission loss predictions from three models [lumped, one-dimensional decoupled, and three-dimensional boundary element method (BEM)] are compared with experimental data from an extended impedance tube setup in the absence of mean flow. Experimentally determined complex characteristic impedance and propagation constant are used to account for the wave propagation through the absorbing material. The perforation impedance suggested by Sullivan [J. Acoust. Soc. Am. 64, 207–215 (1978)] is modified to incorporate the effect of absorbing material. The results show that: (1) the effect of absorbing material on the perforation impedance needs to be taken into account for accurate predictions; (2) silencers filled with absorbing material exhibit significantly higher attenuation at higher frequencies than those without the filling material; (3) the three-dimensional BEM shows good agreement for the overall frequency range of interest (0–3200 Hz), while the one-dimensional approach is reasonable at relatively low frequencies; and (4) the lumped model, which treats the filled silencer as a Helmholtz resonator, may provide an approximate frequency for the location of peak attenuation.

Sound Transmission Loss of Thick Perforated Panels

A study is reported of the sound transmission loss of perforated panels. The study includes a theoretical analysis and measurement by means of sound intensity. The predicted transmission loss is similar to that measured above 630 Hz. The maximum discrepancy is less than 2 dB. The perforation in a thick panel is found to reduce the coincidence effect at the critical frequency.

A numerical model for the low frequency diffuse field sound transmission loss of double-wall sound barriers with elastic porous linings

This paper discusses the prediction of the low frequency diffuse field transmission loss through double-wall sound barriers with elastic porous linings. The studied sound barriers are made up from a porous-elastic decoupling material sandwiched between an elastic skin and a septum. The prediction approach is based on a finite element model for the different layers of the sound barrier coupled to a variational boundary element method to account for fluid loading. The diffuse field is modeled as a combination of uncorrelated freely propagating plane waves with equal amplitude, no two of which are traveling in the same direction. The corresponding vibroacoustic indicators are calculated efficiently using a Gauss integration scheme. Also, a power balance is presented to explain the dissipation mechanisms in the different layers. Typical results showing the effects on the transmission loss of several parameters such as the septum mass, the decoupling porous layer properties and the multi-layer mounting conditions are presented.

Circular asymmetric Helmholtz resonators

A three-dimensional (3D) analytical approach is developed to account for the nonplanar wave propagation in the cavity and neck of "piston-driven" circular asymmetric Helmholtz resonators. The present 3D analytical results are compared with (1) the numerical predictions from the boundary element method (BEM) to evaluate the analytical approach; and (2) the one-dimensional (1D) solution to examine the effect of nonplanar waves at area discontinuity between the neck and the cavity. In order to improve the 1D solution, the end correction is also determined by using the 3D analytical approach. The effect of neck offset on the resonance frequency of circular asymmetric Helmholtz resonators is investigated. Predictions of resonance frequency and transmission loss from the present 3D and corrected 1D analytical approaches are, respectively, identical and close to the BEM results, while the corrected 1D approach provides a better accuracy compared to the 1D solutions with Ingard's correction. Finally, the boundary element method is employed to determine the wave attenuation performance of the "pipe-mounted" Helmholtz resonators to examine the effect of multidimensional waves in the vicinity of the main duct and neck junction.

Prediction of sound transmission loss through multilayered panels by using Gaussian distribution of directional incident energy

In this study, a new prediction method is suggested for sound transmission loss (STL) of multilayered panels of infinite extent. Conventional methods such as random or field incidence approach often given significant discrepancies in predicting STL of multilayered panels when compared with the experiments. In this paper, appropriate directional distributions of incident energy to predict the STL of multilayered panels are proposed. In order to find a weighting function to represent the directional distribution of incident energy on the wall in a reverberation chamber, numerical simulations by using a ray-tracing technique are carried out. Simulation results reveal that the directional distribution can be approximately expressed by the Gaussian distribution function in terms of the angle of incidence. The Gaussian function is applied to predict the STL of various multilayered panel configurations as well as single panels. The compared results between the measurement and the prediction show good agreements, which validate the proposed Gaussian function approach.