Experimental Transmission Loss Research Articles

Effects of urea–formaldehyde and polyvinyl acetate adhesive on sound absorption coefficient and sound transmission loss of palmyra palm fruit fiber composites

Composite materials made from palmyra palm fruit fiber (PPFF) were formed using urea–formaldehyde (UF) and polyvinyl acetate (PVAc). We prepared two sets of five different cylindrical samples with varying PPFF contents. The PPFF composites’ normal-incident sound absorption coefficient (SAC) and transmission loss (TL) were measured by using the impedance tube method. The sample with higher PPFF content shows a lower SAC spectrum. It is the opposite for the TL, where a sample with high PPFF content demonstrates a higher TL spectrum. We conducted the least-square fitting method on the experimental SAC and TL spectra utilizing the Johnson-Champoux-Allard (JCA) equivalent fluid model. Non-acoustic parameters were acquired from the fitting. The optimized porosity (ϕ), viscous characteristic length (Λ), and thermal characteristic length (Λ′) are inversely proportional to the PPFF content. The airflow resistivity (σ) and tortuosity (α∞), on the other hand, demonstrate a direct correlation with the PPFF content. Even though the UF samples have an average density of 14.7 % higher than the PVAc samples, their σ,Λ′, and α∞ are just 7.7 %, 4.5 %, and 0.39 % higher than PVAc samples. On the other hand, PVAc samples show higher average Λ and ϕ of 1.4 % and 0.73 %, respectively. The optimized porosity values obtained from the JCA model (ϕJCA) are coherent with ones from the direct estimation method assuming adhesive-coated fiber (ϕfa). It can be concluded that the adhesive’s quantity and density contribute to the composites’ porosity value, ultimately affecting material acoustic properties. Researchers can control and predict how the SAC and TL of fibrous sound absorbers would behave by varying the quantity and density of an adhesive.

Dynamic behavior of soft, resonant metamaterials: Experiments and simulations

Though additive manufacturing and novel optimization techniques have led to many recent advances in elastic metamaterials, difficult fundamental challenges (e.g., narrow bandgaps) and practical challenges (e.g., dissipation and friction) remain. This work introduces simple and hierarchical resonant metamaterials made of soft polydimethylsiloxane rubber and removable steel insets. The additively manufactured samples are able to produce bandgaps with a gap–midgap ratio of 81.8%, which surpasses the majority of resonant, metamaterials of the same class and greatly outperforms analogous resonant structures with a stiff epoxy matrix. The role of several physical features on the transmission loss (TL) curve is assessed in detail numerically and compared to the experimental TL data. Matrix compliance is found to be a rich mechanism for bandgap widening with a dual effect: it deepens the traditional resonant bandgaps, and it can selectively shift certain vibrational modes to lower frequencies and aid in the merging of multiple bandgaps. This can lead to an overall increase of the bandgap width of over an order of magnitude. Viscous dissipation, friction, and the stochastic nature of geometrical inaccuracies common in additive manufacturing were also found to shape the TL curve and associated bandgaps to various degrees. Some of these mechanisms, combined with a soft frame, can further help merge bandgaps in rainbow or hierarchical designs and form ultrabroad, subwavelength bandgaps.

Sequential Geoacoustic Inversion Using an Improved Kalman Particle Filter

Geoacoustic inversion is an efficient method to study the physical properties and structure of ocean bottom while sequential geoacoustic inversion is a challenging task due to the complexity and non-linearity of the underwater environment. In this paper, an ensemble Kalman Particle filter is described to address the sequential geoacoustic inversion problem of range-dependent environment in shallow water. This filter combines the advantages of Particle filter and ensemble Kalman filter so its ability of tracking dynamical geoacoustic parameters is improved. The proposed filtering method is demonstrated with simulated data in a changing oceanic environment and outperforms Particle filter and ensemble Kalman filter. This method is also tested in sea-trial data collected from a shallow-water experiment in the East China Sea. The inverted sound speed in sediment is consistent with in situ measurement and the error between transmission loss predicted by inverted parameters, and the experimental transmission loss is small.

Acoustic performance of composite sandwich panels for building floors: Experimental tests and numerical-analytical simulation

Composite sandwich panels have been increasingly considered in structural applications due to their lightweight, high strength, environmental resistance, and ease of assembly. Yet, since transmission loss phenomena are usually governed by mass, their low weight renders composite sandwich panels potentially inefficient in terms of acoustic performance, a subject that has received little attention in the past. This paper presents experimental, numerical and analytical investigations about the acoustic performance of composite sandwich panels used in building construction. Standard airborne and impact sound transmission tests are performed on a full-scale composite sandwich panel, designed for structural application in building floors, and their acoustic performance is compared with the applicable code requirements. In the numerical study, a finite element model is developed to simulate the airborne sound transmission. The analytical study assesses the applicability of the reciprocity theorem to such panels. The obtained results confirmed the overall low acoustic performance of lightweight composite sandwich panel floors and pointed out the need for additional sound insulation measures. The numerical simulations presented good agreement with the experimental data, following the overall development of the experimental transmission loss curve, and predicting the dip associated with the coincidence effect. The application of the reciprocity theorem to the sandwich panel floors was validated with the experimental results and allowed for an accurate prediction of the impact sound pressure level based on the numerical results of the airborne sound reduction.

Band Structure of Phononic Crystal Consist of Hollow Aluminum Cylinders in Different Media; Finite Element Analysis

In this study, we investigated band structure of a 2D Phononic Crystal (PnC) which was consist of hollow aluminum cylinders in different media. To validate the finite element predictions, we made a square lattice PnC which has lattice constant a = 14mm, outer radius r1 = 5mm and inner radius r2 = 4mm of cylinders. In our experimental measurement, by using a signal generator software, we sent 100 Hz-20kHz sine signal for 5 seconds duration from speaker to the PnC where placed 1m away from. With the help of a microphone, we recorded the sound in front of the PnC (Pin ) and backside the PnC (Pout ). Then we were converted our recording into frequency domain by standard FFT algorithm, we calculated the Transmission Loss value according to the formula TL = 20*log10 (Pout / Pin ) and obtained the experimental Transmission Loss. After experimental validation, we conducted band structure analysis of different combination of the 2D PnC by finite element method.

Reduction in edge effects for small panels characterized by a parametric array source.

This study sought to explore effects of panel size on underwater acoustic transmission and reflection measurements made with a truncated parametric array source. The transmission loss (TL) and reflection loss spectra of aluminum panels of several different sizes were determined experimentally, and compared to predictions from plane wave theory in the 5-100 kHz frequency range. For the smallest panel size, there were significant discrepancies between experimental data and plane wave theory, which were attributed to contributions from edge-diffracted waves and interferences from the mounting fixture for the panel. For reflection measurements, the latter interference could be corrected for in part by measurement of the reflected signal from the sample holder (with no panel present), and subtracting this signal from reference and test panel waveforms. In order to reduce the contributions from edge diffracted waves in TL measurements, an alternative panel mounting system was investigated, which involved surrounding the panel in a reflective baffle. There was a significant improvement in agreement of experimental TL spectra with plane wave theory for the smallest aluminum panel when the baffle mounting arrangement was used.

Vibro-acoustic optimisation of Wood Plastic Composite systems

“Wood Plastic Composite” or WPC is becoming increasingly popular in outdoor applications because of the advantage of a better durability in wet environments compared to natural wood. The possibility of using WPC as a sound barrier, or as façade cladding, is investigated in this paper. The sound transmission loss (TL) of an orthotropic WPC panel, obtained by coupling together several boards, is computed by means of the transfer matrix method. The plate is modelled as a thin orthotropic layer, described by frequency dependent elastic properties. A numerical procedure, based on a finite element simulation, is proposed in order to determine the stiffness properties along the principle direction of the panel. The reliability of this approach is verified by comparing the numerical results with the experimental stiffness measured on a WPC beam. The orthotropic behaviour is approximated by an elliptic interpolation of the flexural stiffness along the two principle directions, based on a simplified assumption which considers the in-plane shear modulus proportional to the orthotropic elastic moduli. The model based within the transfer matrix method framework is validated with the experimental transmission loss measured on a WPC panel in a reverberant room. Finally, the possibility of increasing the acoustic performance of WPC structures by optimising their cross-section is investigated.

Horizontal refraction effects of seamounts on sound propagation in deep water

The seamounts usually have important effects on sound propagation in deep water. A sound propagation experiment was conducted in the South China Sea in 2016. One of the experimental goals is to investigate the three-dimensional(3D) effects of seamounts on sound propagation. Phenomena about horizontal refraction of acoustic waves are observed in different propagation tracks which go through the seamount along different directions when the source depth is 200 m. Ray methods (BELLHOP N×2D and 3D models) which can calculate sound field efficiently and show clear physical images, are used to analyze and explain the causes of the phenomena. The experimental and numerical results show that the convergent zone structures are destroyed by the direct blockage of seamount due to the multiple reflection of acoustic waves, which leads to the increase of transmission loss (TL), and horizontal-refraction zone with obvious boundaries appears behind the seamount. Some experiment phenomena cannot be explained by BELLHOP N×2D model in which the horizontal refraction effects are not taken into consideration. The experimental sound field structure behind the seamount is obviously different from N×2D model numerical result, i.e.the width of shadow zone based on the experimental data is wider than that calculated by N×2D model, and the width of strong horizontal-refraction zone from the experiment is narrower than the N×2D model result. Moreover, the TLs calculated by N×2D model is about 10 dB less than the experimental result in horizontal refraction zone. After analyzing the difference between experimental data and N×2D model numerical results by BELLHOP 3D model which contains the azimuth-coupling capability, it can be concluded that sound waves reach the receiver through horizontal refraction after the interaction with seamount when the source is located behind the seamount. The eigenrays obtained from 3D model are less than N×2D model numerical result because some of sound beams cannot reach the receiver as a result of the horizontal refraction effects, which leads to the experimental TLs larger than the numerical results calculated by N×2D model. Therefore, 3D effect of seamount has an obvious influence on sound field within a certain angle range behind the seamount, and the investigation of 3D effects of seamounts is meaningful for the sound propagation and target detection in deep water.

Sound insulation properties in low-density, mechanically strong and ductile nanoporous polyurea aerogels

Aerogels are quasi-stable, nanoporous, low-density, three-dimensional assemblies of nanoparticles. In this paper, an extremely high sound transmission loss for a family of ductile polyurea aerogels (e.g., over 30dB within 1 to 4kHz at bulk density 0.25g/cm3 and 5mm thickness) is reported. The fundamental mechanisms behind the aerogel acoustic attenuations are investigated. Sharing striking similarities with acoustic metamaterials, initially, aerogels are studied via a one-dimensional multi degree-of-freedom mass-spring system. Different effects such as spring constant disparity are investigated in regards to the structural vibration wave transmission loss. Results are given for different configurations consistent with the aerogel nano/microstructures. A significant wave attenuation is observed by considering a random spring distribution. In the next step towards modeling such a complex hierarchical and random structural material, the continuum Biot's dynamic theory of poroelasticity is implemented to analyze the experimental sound transmission loss results. In this framework, a two-dimensional plane strain analysis is considered for the interaction of a steady state time-harmonic plane acoustic wave with an infinite aerogel layer immersed in and saturated with air. The effects of bulk density and thickness on the aerogel sound transmission loss are elucidated. By comparing the theoretical results with the experimental observations, this study develops a qualitative/quantitative basis for the dynamics of the aerogel nanoparticle network as well as the air flow and solid vibroacoustic interactions. This basis provides a better understanding on the overall acoustic properties of the aerogels that might also be helpful in the design of the future hierarchical materials.

Tunable Acoustic Wave Propagation Through Planar Auxetic Metamaterial

AbstractThis paper presents a tunable planar auxetic metamaterial (PAM) for controlling and filtering acoustic waves and provides guidelines for bandgap design of the proposed PAMs. Numerical results for deformed and undeformed PAMs were obtained from several finite element analyses based on Bloch–Floquet theory. The acoustic band structures of the PAMs were calculated with periodic boundaries. Tunable bandgaps in certain frequency ranges were generated by various deformations applied to the PAMs. Wave attenuation in experimental transmission loss at specific frequencies was demonstrated, showing favorable agreement with the bandgaps obtained from numerical calculations. Both the numerical and experimental results indicate that the proposed structure demonstrates great tunability and offers significant advantages over the regular materials for controlling sound wave propagation and filtering sound waves within specific frequency ranges.

Geoacoustic Inversion for Bottom Parameters via Bayesian Theory in Deep Ocean**Supported by the National Natural Science Foundation of China under Grant No 11174235.

We develop a new approach to estimating bottom parameters based on the Bayesian theory in deep ocean. The solution in a Bayesian inversion is characterized by its posterior probability density (PPD), which combines prior information about the model with information from an observed data set. Bottom parameters are sensitive to the transmission loss (TL) data in shadow zones of deep ocean. In this study, TLs of different frequencies from the South China Sea in the summer of 2014 are used as the observed data sets. The interpretation of the multidimensional PPD requires the calculation of its moments, such as the mean, covariance, and marginal distributions, which provide parameter estimates and uncertainties. Considering that the sensitivities of shallow-zone TLs vary for different frequencies of the bottom parameters in the deep ocean, this research obtains bottom parameters at varying frequencies. Then, the inversion results are compared with the sampling data and the correlations between bottom parameters are determined. Furthermore, we show the inversion results for multi-frequency combined inversion. The inversion results are verified by the experimental TLs and the numerical results, which are calculated using the inverted bottom parameters for different source depths and receiver depths at the corresponding frequency.

Acoustic Optimization of a High-speed Train Composite Sandwich Panel Based on Analytical and Experimental Transmission Loss Evaluation Integrated by FE/Test Correlation Analysis

Present work purpose is to optimize the acoustic attenuation properties of a composite sandwich panel used for a high-speed train structure, choosing the best panel configuration which allows to improve the performances. Firstly, Nilsson's analytical formulation for Transmission Loss (TL) evaluation has been implemented and experimentally validated on a typical material used for high-speed railway applications, highlighting the opportunity to use a different material to satisfy the new required design specifications. Different materials and stratifications have been then considered and TL parameter of each configuration have been calculated using Nilsson's formulation, characterizing acoustic behavior in the frequency domain. Once found the composition which ensures the best compromise between high acoustic insulation and low weight, the panel has been physically realized. Finally, an experimental and a numerical modal analysis have been performed on it. Starting from both FE simulation and impact testing outcomes, a correlation study through the computation of the Modal Assurance Criterion (MAC), has been carried out. A good agreement between numerical and experimental analyses has been found, obtaining a reliable FE model for future improvements.

The Effects of Seamounts on Sound Propagation in Deep Water**Supported by the National Nature Science Foundation of China under Grant Nos 11434012 and 11174312.

A propagation experiment was conducted in the South China Sea in 2014 with a flat bottom and seamounts respectively by using explosive sources. The effects of seamounts on sound propagation are analyzed by using the broadband signals. It is observed that the transmission loss (TL) decreases up to 7 dB for the signals in the first shadow zone due to the seamount reflection. Moreover, the TL might increase more than 30 dB in the converge zone due to the shadowing by seamounts. Abnormal TLs and pulse arrival structures at different ranges are explained by using the ray and wave theory. The experimental TLs and arrival pulses are compared with the numerical results and found to be in good agreement.

Sound Transmission Properties of Mineral-filled High-Density Polyethylene (HDPE) and Wood-HDPE Composites

Wood plastic composites (WPCs) offer various advantages and potential as a competitive alternative to conventional noise barriers. For this purpose, the influence of composite formulation on the sound transmission loss (TL) of WPCs needs to be fully understood. In TL testing, stiffness and surface density are major factors influencing the sound insulation property of filled plastics and WPCs. Experimental TL values decreased as sound frequency increased; and the TL values increased after passing a certain frequency level. The comparison of experimental TL curves among filled composites showed that the addition of fillers led to an increase in resonance frequency and TL values. However, at high filling levels, the stiffness decrease led to TL reductions. The experimental TL curves of filled composites, composed of mass law and stiffness law predictions, were well approximated with their combined TL predictions.

Determination of the characteristic impedance and the complex wavenumber of an absorptive material used in dissipative silencer

The silencers are acoustic filters that have the purpose of reducing unwanted noise emitted by engines or equipment to acceptable levels. The vehicular silencers have large volume and dissipative properties. Dissipative silencers have absorptive material inside. These materials are typically fibrous and have good acoustic dissipation. However, few works depict the acoustic behavior of silencers with absorptive materials. The difficulty in evaluating this type of silencer is determining the acoustic properties of the absorptive material: the characteristic impedance and the complex wavenumber. This work shows an inverse methodology for determining the acoustic properties of the absorptive material used in silencers. First, it is found the silencer's acoustic efficiency in terms of the experimental sound transmission loss. Second, the absorptive material properties are determined with a parameters adjustment through a direct search optimization algorithm. In this step, the adjustment is done by applying The Finite Element Method in the search for the silencer's computational efficiency. The final step is to verify the difference between the experimental and computational results. For this work is used the acoustic efficiency of a silencer that has already been published in the literature. The results show good agreement.

Model and analysis of a cylindrical in-line hydraulic suppressor with a solid compressible liner

An in-line hydraulic noise suppressor with a lossy, compressible liner made of foamed polyurethane liner is introduced which is intended to provide an alternative to current in-line silencing devices using compressed nitrogen gas volumes. The liner is engineered to be compressible at elevated pressures, such that it can provide effective noise abatement for practical hydraulic systems. In support of such work, a multimodal model is developed to characterize the device and the liner material. Because the hydraulic system is pressurized after insertion of the liner, the model must address liner compression and the corresponding small gaps introduced in the expansion volume; additionally, both compression and shear wave propagation must be considered in the liner. Several mode matching solutions are investigated, and a pseudoinverse mode matching method is found to provide good convergence characteristics. The multimodal model is validated against a finite element model, and also used in an optimization algorithm to estimate the material properties of a prototype liner using experimental transmission loss data. Experimental results show broadband transmission loss performance at 2.8MPa system pressure; transmission loss decreases with increasing system pressure, and data at 4.1MPa system pressure produces about 4dB less transmission loss than a similarly sized commercial device. The multimodal model with estimated material properties at 2.8MPa achieves a root mean squared error of 1.7dB or less for two different length devices over a frequency range of 50–2000Hz.

Noise reduction in a flow duct: Implementation of a hybrid passive/active solution

This paper deals with the design of a hybrid acoustic treatment combining porous material properties and active control techniques. Such an acoustic system was studied with a view to reducing broadband noise spectra in flow duct applications. Special attention was paid to the selection of the passive layer. The main objective was to achieve target impedance at the front absorber face, so as to attain maximum sound-attenuation over a wide frequency bandwidth. This investigation was carried out for a specific laboratory flow duct. Different porous layers were studied to reproduce optimum impedance at the hybrid liner surface. Results showed the difficulty of simultaneously achieving optimum resistance and reactance. Thus, a compromise was struck by applying a criterion of maximum attenuation. A wire mesh with a resistance close to a third of the characteristic impedance of air was selected as the optimal passive layer. Experiments were carried out in the flow duct under grazing acoustic incidence and with flow velocities up to 50 m/s. The experimental transmission loss was in agreement with predictions. Significant noise reduction levels were achieved throughout a large frequency range from 0.7 to 2.5 kHz, with a cut-off frequency between active and passive mode set at 1.8 kHz.

Boundary element analysis of packed silencers with protective cloth and embedded thin surfaces

Bulk-reacting porous materials are often used as absorptive lining in packed silencers to reduce broadband noise. Modelling the entire silencer domain with a bulk-reacting material will inevitably involve two different acoustic media, air and the bulk-reacting material. A so-called direct mixed-body boundary element method (BEM) has recently been developed to model the two-medium problem in a single-domain fashion. The present paper is an extension of the direct mixed-body BEM to include protective cloth and embedded rigid surfaces. Protective cloth, an absorptive material itself with a higher flow resistivity than the primary lining material, is usually sandwiched between a perforated metal surface and the lining to protect the lining material from any abrasive effect of the grazing flow. Two different approaches are taken to model the protective cloth. One is to approximate sound pressure as a linear function across the cloth thickness and then use the bulk-reacting material properties of the cloth to obtain the transfer impedance. The other is to measure the transfer impedance of the cloth directly by an experimental set-up similar to the two-cavity method. As for an embedded thin surface, it is a rigid thin surface sandwiched between two bulk-reacting linings. Numerical modelling of an embedded thin surface is similar to the modelling of a rigid thin surface in air. Several test cases are given and the BEM results for transmission loss (TL) are verified by experimental TL measurements.

Sensitivity analysis of the acoustic field in shallow water to environmental parameters

Environmental parameters, such as sound-speed profile and bottom structure, have very important effects on the acoustic propagation in shallow water. In this paper, some detailed simulations are presented for a typical shallow water with a thermocline. The simulated properties include the coherent and incoherent transmission loss, averaged energy loss, group velocity, phase of the acoustic field, and the multipath arrival structure. The concerned environmental parameters include surface speed, thermocline depth and thickness, water depth, bottom structure, and its geoacoustic parameters. The results show that the coherent transmission loss and the phase of the acoustic field are sensitive to the environmental parameters, while the other properties are relatively stable. Theoretical explanations of these results are also given. Both the sensibility and the relative stability can be utilized. Through reasonable cost function design, the former can be used to invert the geoacoustic parameters of the bottom, while the latter can be used to localize distant targets. Finally, the geoacoustic parameter inversion for a complicated bottom is given by using the real data from a vertical line array. With the obtained bottom parameters, experimental transmission loss is well explained.

The role of grazing waves in sound transmission through a finite plate

Air sound transmission loss (TL) of a finite elastic plate, placed in an infinite rigid baffle, is computed with regard to, first, a lack of near-grazing waves in incident sound field and, second, diffractive three-dimensional expansion of the sound field radiated by forced bending waves of the finite plate. It is shown that both effects lead to the lack of near-grazing sound waves in transmitted sound field what in turn appreciably increases TL compared with diffuse field and infinite plate. Below coincidence, especially at low frequences, the second effect usually predominates over the first. At coincidence, in contrast, the first effect always prevails and results in an increase of both TL and limiting frequency. Above coincidence, both effects are always negligible. The computations are compared with numerous measurements of various authors for glass panels of different dimensions and satisfactorily explain the well-known excess of experimental TL values over infinite-plate theory at and below coincidence. Nevertheless the familiar integration formula for infinite plate may be successfully used for finite panels with the only difference that the limiting integration angle of incidence becomes depending on acoustic wave number.