Acoustic Transmission Loss Research Articles

Sound transmission loss of laminated composite sandwich structures with pyramidal truss cores

This paper presents a theoretical model that allows to calculate the acoustic transmission loss through laminated composite pyramidal sandwich structure consisting of two parallel plates connected by trusses. First-order shear deformation theory is adopted to model the vibration of such a structure, accounting for in-plane motion and coupling between extension and flexure of the different components, and taking into account elastic anisotropy. The interaction between the structure and the surrounding fluid is taken into account by imposing a velocity continuity condition at the interfaces. The displacement and stress fields are calculated in Fourier domain by solving the set of boundary condition equations at the connecting points. The theoretical predictions for the acoustic transmission through the structure show satisfactory agreement with experimental results of a standing wave tube experiment on specimens that were fabricated by cutting trusses from carbon fiber reinforced composite plates and snap-fitting them to plates made of the same material. Numerical simulations are used to verify, the influence of the stacking geometry and material parameters on the acoustic transmission for different frequencies and angles of incidence. Conclusions are presented that are helpful for practical design.

The Analysis of Acoustic Propagation Characteristic Affected by Mesoscale Cold-Core Vortex Based on the UMPE Model

The ocean mesoscale vortex can make uneven distribution of sound energy, which has a significant impact on underwater acoustic equipment, weapons and submarine warfare. Based on the observed data of 2014 West Pacific sea oceanographic survey, the influence of the properties, strength, position and depth of the mesoscale vortex on the acoustic propagation characteristics is simulated by the parabolic equation. Our observed vortex is a typical cyclone-type cold vortex in the Kuroshio extension body, and the cold water turns on intensely at the center of vortex. The effect depth of vortex is 500 m, and then, the parabolic equation of acoustic model is used to analyze and simulate the impact of mesoscale cold-core vortex on acoustic transmission loss. Results show that the horizontal perturbation of the surface velocity caused by mesoscale vortex has a great influence on the spatial distribution of water sound field. The cold vortex makes the position of the convergence zone move forward, with the width reduced, and the gain efficiency enhanced. When sound propagates outside of the cold-core vortex, compared with inside of the cold-core vortex, the assembled areas are backward, with the width increased, and gain effect weakened. The non-uniform hydrological environment caused by the ocean mesoscale vortex is a key modulating factor of the abnormal variation of acoustic energy in spatial distribution.

Model approximation for sound transmission from underwater structures in high-frequency range

Sound-insulation model provides a straightforward way to describe sound transmission behaviours of the thin-walled structures in engineering applications. The sound transmission characteristics depend on the parameters of incident wave, such as incident wave amplitude and incident angles. However, this model is limited when the sound source is located in an enclosed space (e.g., noise source in underwater cabins), because it is difficult to obtain incident angles especially in the high-frequency range. In this paper, we develop a simply analytical model that can effectively study the sound transmission from an enclosed shell with internal acoustic excitation. In order to extend the application of the sound-insulation model to a submerged shell, the structural vibration equation is firstly simplified to the plate vibration equation. Then, the sound pressure near the inner surface of the shell is decomposed into an expansion of orthogonal cavity eigenmodes, and each cavity mode is replaced by two pairs of incident plane waves. Finally, the acoustic transmission loss can be obtained by substituting the parameters of incident waves into the sound-insulation model. Numerical results show that the sound transmission for the fundamental cavity mode (0, 0, 0) can be explained by the normal incidence in the sound-insulation model, while every other modes corresponds to a group of oblique incident plane waves whose incident angles decrease monotonically with the increase of frequency. In addition, it can be observed that the total reflection phenomenon in the sound-insulation model is consistent with the low radiation efficiency of the high order modes in the shell model.

Improvement of acoustic transmission loss of double wall acoustic metamaterial by thin membrane bottom resonator

Improvement of acoustic transmission loss of double wall acoustic metamaterial by thin membrane bottom resonator

Study on the influence of stagger joint structure on acoustic transmission loss of partition wall

The existence of gap has always restricted the improvement of sound insulation performance of traditional partition wall structure. The sound transmission loss of partition wall under the slit existence condition was studied. The effects of the staggered slit depth and the staggered slit distance on the sound transmission loss of partition wall were analyzed. The experimental results showed that the sound insulation property of the plasterboard whole component with the staggered slit was better than one with the straight slit. The effect of the staggered slit depth depended on the variation of surface density of partition wall structure and sound wave propagation path. The When the staggered slit distance was equal to the partition wall thickness, the sound insulation property of the partition wall is best. The reason can be explained is that the transmission level of sound energy through the wall is equivalent to that through staggered slit. It makes up the “short board” of slit transmission in partition wall structure.

Modeling of the Acoustic Transmission Loss of Turbochargers

Modeling of the Acoustic Transmission Loss of Turbochargers

Adaptive Helmholtz resonator based on electroactive polymers: modeling, characterization, and control

This paper presents a new concept and strategy allowing adaptive control of a membraned Helmholtz resonator (HR) embedded in a melamine foam. The designed system aims to adapt the acoustic absorption performances and transmission loss in low frequencies (<500 Hz). The proposed concept consists in replacing the resonator front wall by an electroactive polymer membrane. The stiffness of the membrane can be controlled by an electric field, resulting in a resonance frequency shift. A 2D axisymmetric numerical model based on the finite elements method is developed to characterize the complex structure-acoustic coupling between the membrane, the HR and the host foam to determine the potential of the concept. Experimental measurements are then performed in an impedance tube and compared to numerical results. A feedforward algorithm based on neural networks allows the adaptivity of the membraned HR to the acoustic excitation variation inside the impedance tube.

Measured variability of acoustic propagation loss in the Salish Sea, British Columbia, Canada

The Salish Sea encompasses the waters between southern British Columbia mainland coast, Vancouver Island, and Washington State. Its waters are used by commercial and recreational fishers, host shipping traffic to the major ports of Vancouver and Seattle, and is travelled frequently by local ferries. It is also an important habitat for many marine mammal species, including the endangered Southern Resident Killer Whale (Orcinus orca). Southern Resident Killer Whales use calls, whistles, and clicks to communicate and find prey. Human activities on the water may affect the marine acoustic environment by masking whale vocalizations, which in turn will negatively impact the ability of the whales to forage and socialize. A field study was conducted to measure acoustic transmission loss at various places in the Salish Sea. Sound signals simulating killer whale call elements were broadcast from different angles to a calibrated hydrophone. Results from the study showed that acoustic transmission loss is highly spatially variable in this area. We suggest that these observations are due to changes in oceanography conditions and ambient sound levels, which are important factors to consider when assessing impact on Southern Resident Killer Whales. [Work supported by the Oceans Protection Plan of the Government of Canada.]

Are aerogels an emerging noise control solution or it is just beefed up fiber glass?

Recently, there has been a number of publications which suggested that aerogels can offer high acoustical performance in terms of their ability to absorb or to insulate against sound. It seems that a bulk of these publications report laboratory experiments on the acoustical absorption or transmission loss performance for samples of particular thickness and lateral dimensions. None of these results appear to be rigorously linked to the aerogel's chemistry or micro-structure. This paper attempts to fill in this gap in information by explaining what these materials actually are, why they work acoustically and what is the underpinning physical phenomenon of aerogel. It is shown that the main reason that these materials are used in applications is that they are fire retardant or their elastic modulus is relatively independent of frequency and that they highly hydrophobic. It is also shown that the absorption performance of fibre glass impregnated by aerogel is generally controlled by the fibre diameter of fibreglass and by the density of the composite structure.

Optimized vibro-acoustic design of suspended glass panels

Suspended glass panels are monolithic or laminated frameless windows sustained by a number of holders, typically located in the vicinity of the edges. These panels can be used, among other purposes, as noise barriers. The vibro-acoustic behaviour of glass windows is critical at low frequencies, where the problem is often tackled by increasing the thickness, thus the mass, of the panels. As a consequence, solutions which preserve low mass are greatly sought by industries. In this study, the vibro-acoustic behaviour of different suspended glass panels is addressed. An optimization procedure is implemented, aiming at finding the position of the holders which maximizes the acoustic transmission loss (TL) averaged at low- and very low- frequency ranges. First, an iterative procedure, based on comparison of experimental and numerical modal data, has been implemented to extract the material properties (Young’s modulus and Poisson’s ratio) of the panels. Second, these properties have been used in an optimization procedure based on finite difference approximation of the objective function, the averaged transmission loss. The vibro-acoustic analyses, required by the optimization procedure, have performed by means of hybrid finite element method/statistical energy analysis (FEM/ SEA). 16 different design cases have been considered in the optimizations, i.e. 2 different frequency ranges (20-300 Hz and 20-1000 Hz), 2 panel geometries (square 1m x 1m and rectangular 2.5m x 0.8m), 2 constitutive material properties (monolithic tempered glass and laminated tempered glass) and 2 mounting solutions (4 and 6 holders). The transmission losses of the optimized and the standard configurations, where the holders are placed close to the edges, are compared.

Numerical studies on the acoustic characteristics of the U-shaped corrugated pipes and application investigations

This paper presents a numerical study on the effect of structural parameters on acoustical transmission loss of U-shaped corrugated pipe using the control variable method. Then, the interactions between the cutoff frequencies and structural parameters are modeled based on the dimension analysis method using the numerical results. An investigation into the application of the U-shaped corrugated pipe into a simple expansion chamber muffler is carried out using both numerical and experimental methods. The results show that the U-shaped corrugated pipes have unique acoustic characteristics in noise attenuation, which resembles a resonant muffler. Furthermore, the numerical results of the transmission loss of U-shaped corrugated pipes are in good agreement with experimental results, which indicates that the transmission loss results obtained by acoustic finite element method is accurate. In addition, the corrugation height, corrugation length, and corrugation internal diameter are the factors that affect the acoustical behavior of the corrugated pipe most significantly. Moreover, the proposed empirical formulas for predicting the cutoff frequencies of U-shaped corrugated pipes are verified with high accuracy and with a prediction error within ±3.0%. What is more, the application of U-shaped corrugated pipes in expansion chamber muffler can effectively reduce the noise in the mid- to high-frequency band, which is useful to reduce the muffler volume and the amount of sound-absorbing material. Hence, automobile mufflers can be designed in a much more environmentally friendly way. In conclusion, this work could provide a theoretical basis for the acoustical design of U-shaped corrugated pipes, and it initiates a new research and application field for corrugated pipes as well.

Vibro-acoustic response and sound transmission loss characteristics of truss core sandwich panel filled with foam

This paper presents the studies carried out for improving the acoustic behavior of truss core sandwich panel, which is mostly used in aerospace structural applications. The empty space of the truss core is filled with polyurethane foam (PUF) to achieve better vibro-acoustic and sound transmission loss characteristics. Initially equivalent elastic properties of the foam filled truss core sandwich panel are calculated. Then, the vibration response of the panel under a harmonic excitation is obtained based on the equivalent 2D finite element model. Finally, the vibration response is given as an input to the Rayleigh integral code built in-house to obtain the acoustic and sound transmission loss characteristics. The results revealed that PUF filling of the empty space of the truss core, significantly reduces resonant amplitudes of both vibration and acoustic responses. It is also observed that foam filling reduces the overall sound power level significantly. Similarly, sound transmission loss studies revealed that, sudden dips at resonance frequencies are significantly reduced. Also an experiment is conducted on forced vibration response of honeycomb core sandwich panel to show that equivalent 2D model can be used for predicting sound power level and transmission loss behavior.

Porous material effect on gearbox vibration and acoustic behavior

In this paper, we define a resolution method to study the effect of a porous material on vibro-acoustic behavior of a geared transmission. A porous plate is coupled with the gearbox housing cover. The developed model depends on the gearbox characteristic and poroelastic parameters of the porous material. To study the acoustic effect of the housing cover, the acoustic transmission loss is computed by simulating numerically the elastic-porous coupled plate model, and the numerical implementation is performed by directly programming the mixed displacement-pressure formulation. To study the vibration effect, the bearing displacement is computed using a two-stage gear system dynamical model and used as the gearbox cover excitation. Numerical implementation is performed by direct programming of the Leclaire formulation.

Numerical analysis on the effect of the sea surface roughness on the acoustic transmission loss in the atmosphere

Atmospheric sound propagation over the sea surface is affected by several factors including wind, temperature profile, and sea state. Preliminary numerical studies using a Crank-Nicolson parabolic equation solution approach have demonstrated that sea roughness introduces excess transmission loss. This excess loss induced by the sea roughness can be represented instead by a flat surface with a compensatory effective impedance. Calculation of transmission loss for the flat surface is computationally more efficient but requires the evaluation of the effective impedances for any given sea state. In this study, a 2-D finite-difference time-domain solver is used to determine the effective impedances for several sea states. The effect of sea roughness on transmission loss is then quantified by using the Crank-Nicolson parabolic equation.

Estimation of the probability density function of transmission loss in the ocean using area statistics

Predictions of acoustic transmission loss (TL) in the ocean and its uncertainty are useful for a variety of ocean and naval engineering applications. Previously, an ad-hoc technique, area statistics (AS), was developed as a computationally efficient method to estimate the probability density function (PDF) of TL in any uncertain ocean environment from a single range-depth TL-field calculation in that environment. Here, the performance of AS is reported for ten ocean environments having uncertain sound speed profile, bathymetry, and seabed properties. In each environment, a parabolic-equation-based prediction of TL from a point source was generated as a function of range and depth at frequencies of 100, 200, and 300 Hz using the most probable values of the uncertain environmental parameters. From these calculations, AS was used to estimate the PDF of TL at more than 3000 locations within the ten environments. These estimated PDFs are then quantitatively compared, via an L1-error norm, to PDFs generated from traditional Monte Carlo techniques for the same locations and frequencies. Current results show that the L1-error of the AS PDF is less than 0.5, indicating 75% or greater overlap between the AS and Monte Carlo PDFs, at 90% of the test locations. [Sponsored by ONR.]

Facile dip-coating process towards multifunctional nonwovens: Robust noise reduction, abrasion resistance and antistatic electricity

In this study, a multifunctional hybrid coating consisting of waterborne polyurethane (WPU), graphite nanoplatelets (GNPs) and sodium dodecyl sulfate (SDS) was deposited onto the surface of nonwoven fabrics via a dip-coating approach. The results of noise reduction indicated that the acoustic transmission loss of control nonwoven fabrics increased from 3.87 dB to higher than 18 dB after WPU and graphite modification. Moreover, the prepared nonwoven fabrics could withstand more than 2000 times the abrasion against grinding wheels, which significantly improved in comparison with control nonwoven fabrics. The surface resistance of modified nonwoven fabrics decreased approximately seven orders of magnitudes, which exhibited robust antistatic properties. Therefore, it can be concluded that the facile dip-coating method has potential for noise reduction simultaneously with abrasion resistance and antistatic electricity applications.

Numerically estimating acoustic transmission loss of a reactive muffler with and without mean flow

The present work numerically mimic the measurement techniques used to measure the acoustic transmission loss (TL) of a muffler without and with mean flow. First, the three-pole measurement method has been simulated in frequency domain for estimating acoustic TL of a muffler without mean flow. Along with advantages and disadvantages, the inability of such method in estimating acoustic TL in presence of mean flow is highlighted. Such restriction has been resolved by establishing the time domain simulation of three-pole and four-pole measurement methods. The requirement of anechoic termination for three-pole method is well known. Therefore, the issues involved in simulating such boundary condition numerically in frequency and time domain analyses have been addressed. The necessities of boundary layer meshing, selection of appropriate solver, impulse simulation, requirement of non-reflecting velocity inlet, simulation of two-load boundary condition, and role of microphone spacing have been discussed in the context of TL evaluation in the presence of mean flow. Additionally, some recommendations have been made to reduce the computational overhead in numerical analysis which is another key issue. Robustness of the proposed methods have been demonstrated by comparing estimated acoustic TL with experimental measurements.

Design and closed-loop control of an electroactive polymer-based tunable Helmholtz resonator

A Helmholtz resonator is a passive acoustic resonator used to control a single frequency resulting from the cavity volume and the resonator neck size. The aim of the proposed study is to present a new concept and strategy of tunable Helmholtz resonator in order to enhance acoustic performances in lower frequencies (&amp;lt;500 Hz). The proposed concept consists in replacing the resonator rigid front plate by an electroactive polymer (EAP) membrane. When an electric field is applied, a change is made in the mechanical properties of the EAP membrane which induce a resonance frequency shift. A closed-loop control algorithm is developed to allow a real-time adaptability. Experimental measurements are performed on the developed prototype to determine the potential of this concept in term of acoustic absorption and Transmission Loss for low-frequency.

Spectral stochastic finite element method in vibroacoustic analysis of fiber-reinforced composites

The attractive high strength and low weight of fiber-reinforced composites (FRC) make them ideal in many applications as defense and automotive industry. Such materials, however, possess some level of uncertainties which have to be taking in account for a reliable performance and quality control. In this paper, non-sampling based spectral stochastic finite element method is applied to vibroacoustic analysis of FRC plates with uncertain elastic and damping parameters. The impact of parameter uncertainties on the acoustical transmission loss is investigated. The results denote various impacts of uncertainties in different frequency ranges. Further, the calculated random transmission loss with few realizations on collocation points show a reasonable accuracy compared to results obtained from large number of realizations by sampling-based stochastic Monte Carlo simulations, and consequently, very efficient in terms of computational time.

Quieting a rib-framed honeycomb core sandwich panel for a rotorcraft roof.

A rotorcraft roof composite sandwich panel has been redesigned to optimize sound power transmission loss (TL) and minimize structure-borne sound for frequencies between 1 and 4 kHz where gear meshing noise from the transmission has the most impact on speech intelligibility. The roof section, framed by a grid of ribs, was originally constructed of a single honeycomb core/composite facesheet sandwich panel. The original panel has acoustic coincidence frequencies near 600 Hz, leading to poor TL across the frequency range of 1 to 4 kHz. To quiet the panel, the cross section was split into two thinner sandwich subpanels separated by an air gap. The air gap was sized to target the fundamental mass-spring-mass resonance of the panel system to less than 500 Hz, well below the frequency range of interest. The panels were designed to withstand structural loading from normal rotorcraft operation, as well as 'man-on-the-roof' static loads experienced during maintenance operations. Thin layers of viscoelastomer were included in the facesheet ply layups, increasing panel damping loss factors from about 0.01 to 0.05. Transmission loss measurements show the optimized panel provides 6-11 dB of acoustic transmission loss improvement, and 6-15 dB of structure-borne sound reduction at critical rotorcraft transmission tonal frequencies. Analytic panel TL theory simulates the measured performance within 3 dB over most frequencies. Detailed finite element (FE)/boundary element (BE) modeling simulates TL slightly more accurately, within 2 dB for frequencies up to 4 kHz, and also simulates structure-borne sound well, generally within 3 dB.