Incident Sound Wave Research Articles

Application of a porous media model for the acoustic damping of perforated plate absorbers

Perforated panel, or plate, absorbers are commonly employed to reduce sound pressure levels across a broad range of applications including the built environment, industrial installations and propulsion devices. The acoustic performance of a perforated plate absorber depends upon a number of parameters such as physical geometry of the absorber, acoustic spectrum and sound pressure level of the acoustic source. As a consequence, experimental determination of acoustic properties is often required on an individual basis in order to optimise performance.Computational simulation of a perforated plate absorber would alleviate the necessity for experimental characterisation. Fundamentally this can be achieved by the direct numerical solution of the underlying governing equations, the compressible form of the Navier-Stokes equations. The numerical methodology is available and routinely implemented as a Computational Fluid Dynamics solver. However, the numerical simulation of flow through a perforated plate with a large number of very small orifices would require significant computational resource, not routinely available for engineering design simulations.In this paper, a porous media model, implemented as a sub-model within a CFD solver, is investigated and validated against a number of well-acknowledged acoustic experiments undertaken in an impedance tube, for a sound pressure wave incident normal to a perforated plate. The model expresses the underlying governing equations within the perforated plates in terms of a pseudo-physical velocity representation. Comparison between three dimensional, compressible, laminar flow CFD simulations and experimental data, demonstrate that the porous model is able to represent acoustic properties of perforated plate absorbers in linear and non-linear absorption regimes and also the inertial effect in the presence of a mean bias flow.The model significantly reduces the computational resource required in comparison to full geometric resolution and is thus a promising tool for the engineering design of perforated plate absorbers.

Static and Monoharmonic Acoustic Impact on a Laminated Plate

A discrete layered damping model of a multilayer plate at small displacements and deformations, with account of the internal damping of layers according to the Thompson–Kelvin–Voight model, is presented. Based on the equations derived, an analytical solution to the static deformation problem for single-layer rectangular plate hinge-supported along its contour and subjected of a uniformly distributed pressure applied to one of its boundary planes is obtained. Its convergence to the three-dimensional solution is analyzed in relation to the dimension of mesh in the thickness direction of the plate. It is found that, for thin plates, the dimension of the problem formulated can be reduced on the basis of simplified hypotheses applied to each layer. Аn analytical solutions is also constructed for the forced vibrations of two- and three-layer rectangular plates hinged in the opening of an absolutely stiff dividing wall upon transmission of a monoharmonic sound wave through them. It was assumed that the dividing wall is situated between two absolutely stiff barriers; one of them, owing to the harmonic vibration with a given displacement amplitude of the plate, forms an incident sound wave, and the other is stationary and is coated by a energy-absorbing material with high damping properties. Behavior of the acoustic media in spaces between the deformable plate and the barriers is described by the classical wave equations based on the model of an ideal compressible fluid. To describe the process of dynamic deformation of the energy-absorbing coating of the fixed barrier, two-dimensional equations of motion are derived based on the model of a transversely soft layer, a linear approximation of displacement fields in the thickness direction of the coating, and the account of damping properties of its material by using the hysteresis model. The effect of physical and mechanical parameters of the mechanical system considered and of frequency of the incident sound wave on the parameter of its sound insulation, and the characteristics of stress-strain state of the plate is investigated

DOA (direction of arrival) estimation of incident sound waves on a spherical microphone array: Comparison of some correction methods proposed to solve the DOA bias

A spherical 16-microphones array has been designed and built for room acoustics applications (measurement of directional room impulse responses or DRIR). The identification of strong specular reflections in a DRIR requires an accurate determination of their direction of incidence on the sphere. Beamforming methods are therefore applied for their DOA (direction of arrival) estimation. However, the crude applications of these methods revealed significant deviations between the measured and expected DOA. In-depth experiments in anechoic conditions have been carried out to analyze this problem and they revealed that the origin of these deviations could be related to the non-rigidity of the spherical surface of the array. In these experiments, incident waves were generated by a pseudo—point source at several frequencies in the anechoic chamber. The microphone array is rotated and oriented at several azimuthal positions. The sound pressures measured on the sphere by the 16 microphones are then compared with their theoretical values obtained with a rigid sphere assumption, which revealed some differences. Some methods are then proposed to correct this problem. These methods are finally presented and their results in DOA estimation tasks are compared.

Computational study of head geometry effects on sound pressure gradient with applications to head-related transfer function

Effects of object geometry on sound scattering of far-field incident sound waves for two different head models (spherical and ellipsoidal) have been studied using an optimized Finite Element Method (FEM) based on frequency-dependent adaptive dimensions. This optimized FEM technique is proven both efficient and accurate when compared with analytical results for the spherical model, with maximum deviation of 0.6 dB. Comparisons between models have been made on the equivalent Head-Related Transfer Functions (HRTFs) for acoustic pressure, and for the first and second order pressure gradients on the surface. It is shown that while directionality cannot be achieved at lower frequencies using only pressure, pressure gradients provide sound cancellation for certain source orientations. Hence, it is possible to cancel incoming sound from the front (or behind) or sides depending on the direction (radial, azimuthal) and order of pressure gradients. While the pattern of pressure gradient directionality remains similar between the spherical and ellipsoidal models, the difference in dimensions affects the amplitude of the equivalent HRTFs for these parameters. This study provides insight into the placement of directional microphones on the head for hearing aids.

Ultrathin lightweight plate-type acoustic metamaterials with positive lumped coupling resonant

The experimental realization and theoretical understanding of a two-dimensional multiple cells lumped ultrathin lightweight plate-type acoustic metamaterials structures have been presented, wherein broadband excellent sound attenuation ability at low frequencies is realized by employing a lumped element coupling resonant effect. The basic unit cell of the metamaterials consists of an ultrathin stiff nylon plate clamped by two elastic ethylene-vinyl acetate copolymer or acrylonitrile butadiene styrene frames. The strong sound attenuation (up to nearly 99%) at low frequencies is experimentally revealed by the precisely designed metamaterials, for which the physical mechanism of the sound attenuation could be explicitly understood using the finite element simulations. As to the designed samples, the lumped effect from the frame compliance leads to a coupling flexural resonance at designable low frequencies. As a result, the whole composite structure become strongly anti-resonant with the incident sound waves, followed by a higher sound attenuation, i.e., the lumped resonant effect has been effectively reversed to be positive from negative for sound attenuation, and the acoustic metamaterial design could be extended to the lumped element containing multiple cells, rather than confined to a single cell.

Reflecting and scattering of acoustic wave from sea ices

In order to build an efficient underwater acoustic sensor network in the Arctic Ocean environment, transmission characteristics of under-ice acoustic channels need comprehensive understanding. The reflecting and scattering of acoustic waves from sea ices have great influences on under-ice acoustic channels. Both topology and structure of sea surface ices are very complex and variable. The physical dimension, acoustic property and interface roughness of sea ices depend not only on local environment, but also on climate and formation time. Therefore, it is of great significance to develop a model of reflecting and scattering of acoustic waves from sea ices for investigating the sound propagation in the under-ice environment. Assuming that sea ices are a multi-layered elastic solid medium and the ice-water interface is rough and satisfies the boundary condition of perturbation, we develop a system of linear equations to solve the coherent reflection coefficient of the incident sound wave from water to sea ice. The coherent reflection coefficient is a function of the frequency of sound wave and incident grazing angle, and is numerically evaluated. The influences of ice thickness and ice-water interface roughness on the coherent reflection coefficient are analyzed. Furthermore, the method of calculating scattering coefficient by using the power spectrum density of the scattering field is introduced. The scattering coefficient as a function of the scattering grazing angle is numerically evaluated. The influences of ice thickness and ice-water interface roughness on scattering coefficient are analyzed. The results show that both the coherent reflection coefficient and the scattering coefficient are dependent on the frequency of acoustic wave, ice thickness and grazing angle. The coherent reflection coefficient is close to 1.0 and the scattering coefficient is less than 0.01 when incident grazing angle is less than 15°. In addition, the frequency of acoustic wave and ice thickness have weak influences on them. However, the frequency of acoustic wave and ice thickness have significant influences on the coherent reflection coefficient and the scattering coefficient when the incident grazing angle is big, say, greater than 30°. In general, the thicker the ice is, the smaller the coherent reflection coefficient and the scattering coefficient are. The coherent reflection coefficient is less than 0.18 when the ice thickness is greater than 10.0 m and the frequency of acoustic waves is greater than 2 kHz. The ice-water interface roughness has great influences on both the coherent reflection coefficient and the scattering coefficient. The rougher of the ice-water interface is, the smaller the coherent reflection coefficient is, and the bigger the scattering coefficient is.

On the role of corotation radius in the lowT/Wdynamical instability of differentially rotating stars

We investigate the nature of so-called low $T/W$ dynamical instability in a differentially rotating star by focusing on the role played by the corotation radius of the unstable oscillation modes. An one dimensional model of linear perturbation, which neglects dependence of variables on the coordinate along the rotational axis of the star, is solved to obtain stable and unstable eigenmodes. A linear eigenmode having a corotation radius, at which azimuthal pattern speed of the mode coincides with the stellar angular velocity, is categorized to either a complex (growing or damping) mode or a purely real mode belonging to a continuous spectrum of frequency. We compute canonical angular momentum and its flux to study eigenmodes with corotation radius. In a dynamically unstable mode, sound wave transports its angular momentum in such a way that the absolute value of the angular momentum is increased on both sides of the corotation radius. We further evaluate growth of amplitude of reflected sound wave incident to a corotation point and find that the over-reflection of the wave and the trapping of it between the corotation radius and the surface of the star may qualitatively explain dependences of eigenfrequencies on the stellar differential rotation. The results suggest that the low $T/W$ instability may be caused by over-reflection of sound waves trapped mainly between the surface of the star and a corotation radius.

Acoustic impact on the laminated plates placed between barriers

On the basis of previously derived equations, analytical solutions are established on the forced vibrations of two-layer and three-layers rectangular plates hinged in an opening of absolutely rigid walls during the transmission of monoharmonic sound waves. It is assumed that the partition wall is situated between two absolutely rigid barriers, one of them by harmonic oscillation with a given displacements amplitude on the plate forms the incident sound wave, and the other is stationary and has a coating of deformable energy absorbing material with high damping properties. The behavior of acoustic environments in the spaces between the deformable plate and the barriers described by classical wave equation based on the ideal compressible fluid model. To describe the process of dynamic deformation of the energy absorbing coating of fixed barrier, two-dimensional equations of motion based on the use of models transversely soft layer are derived with a linear approximation of the displacement field in the thickness direction of the coating and taking into account the damping properties of the material and the hysteresis model for it. The influence of the physical and mechanical properties of the concerned mechanical system and the frequency of the incident sound wave on the parameters of its insulation properties of the plate, as well as on the parameters of the stress-strain state of the plate has been analyzed.

Acoustic properties of multiple cavity resonance liner for absorbing higher-order duct modes.

This paper describes analytical and experimental studies conducted to investigate the acoustic properties of axially non-uniform multiple cavity resonance liner for absorbing higher-order duct modes. A three-dimensional analytical model is proposed based upon transfer element method. The model is assessed by making a comparison with results of a liner performance experiment concerning higher-order modes propagation, and the agreement is good. According to the present results, it is found that the performance of multiple cavity resonance liner is related to the incident sound waves. Moreover, an analysis of the corresponding response of liner perforated panel-cavity system is performed, in which the features of resonance frequency and dissipation of the system under grazing or oblique incidence condition are revealed. The conclusions can be extended to typical non-locally reacting liners with single large back-cavity, and it would be beneficial for future non-locally reacting liner design to some extent.

Enhanced Soundproof Performance of Construction Coatings Using Polyvinyl Acetate Resin and Calcium Sulfate Whiskers

AbstractThis study reports an enhanced soundproof performance of polyvinyl acetate (PVA)-based coating by adding high-aspect-ratio calcium sulfate whiskers. One efficient microwave-assisted (MA) method is adopted to synthesize CaSO4 whiskers with a high aspect ratio of 40–80. The two-step MA method is capable of forming highly crystalline CaSO4·2H2O crystals at 85°C, demonstrated by X-ray diffraction analysis. This work uses an anechoic termination method to analyze the sound transmission loss (TL) of PVA-based coatings. The TL of CaSO4/PVA composite coating shows an increased trend with the amount of CaSO4 whiskers within the frequency region of 400–3,600 Hz. The improved TL can be attributed to the fact that an air pocket is contained in the CaSO4 stacking layer, imparting the reflection and refraction during the sound wave transmission in the composite coating. The energy of incident sound waves is weakened by consecutive reflection, scattering, refraction, and diffraction of sound waves in the CaSO4/P...

A method for controlled target strength measurements of pelagic fish, with application to European anchovy (Engraulis encrasicolus)

Abstract Measuring fish target strength (TS) in the wild is challenging because: (i) TS varies versus physical (orientation relative to the incident sound wave, size, and depth) and physiological fish attributes (maturity and condition), (ii) the target species and its aforementioned attributes are difficult to assess in near real time, and (iii) in the case of packed fish schools, accepted echoes may originate from multiple unresolved targets. We propose a method for controlled TS measurements of densely packed small pelagic fish during daytime, based on the joint use of a Remotely Operated Towed Vehicle, “EROC”, with a pelagic trawl fitted with a codend opening system, “ENROL”. EROC, equipped with a 70-kHz split-beam echosounder (Simrad EK60) and a low-light black and white camera, can be moved inside the fishing trawl. Pelagic fish are funnelled into the open trawl and their TS is measured in the middle of the net, where small groups actively swim towards the trawl mouth. The swimming behaviour allows for near-dorsal TS to be measured, minimizing the large effect of incidence angle on TS variability. The EROC camera, located near the open codend, provides optical identification of the species. This method was used to measure the TS of European Anchovy, Engraulis encrasicolus in the Bay of Biscay during 2014. The mean, near dorsal TS was −43.3 dB, for a mean fork length of 12.5 cm. This value is compared to published values of clupeiforms mean TS obtained for a range of natural incidence angles and discussed in the light of TS modelling results obtained for E. encrasicolus.

Sound Transmission Through Double Leaf Partitions: A Criterion for Quick Convergence Using Space Harmonic Analysis

Space harmonic expansion has been used successfully to model sound transmission through infinite, periodically rib-stiffened double leaf partitions. Since the solution to this method is obtained in a series form, computational accuracy needs to be balanced with computational cost as calculation time increases with the number of space harmonic terms. The aim of this article is to provide a criterion to decrease the computational time when using space harmonic analysis. The new criterion helps to select the appropriate space harmonics to be included in the solution based on frequency and the angle of incidence of sound waves. The results are verified by comparing with experimental data available in the literature. For the partition investigated, the computational time is reduced by a factor of ten without compromising the accuracy of the result.

Verification of mathematical formulae applied to overhead stage canopy design

Abstract This paper presents a comparison of the experimental research concerning overhead stage canopies with a numerical approach based on selected mathematical models. The numerical predictions are made using the simplified asymptotic curves suggested by Rindel and modified by Skalevik. For singular cases a prediction with detailed calculations based on the Fresnel–Kirchhoff approximation is also given. The aim of the work is to verify proposed algorithms for designing reflective panels as well as to determine the conditions of conducting such procedures. It is shown that based on Rindel’s approximation one may determine some substantial information about sound reflection from the panels i.e. the value of upper limit frequency as well as the relative sound reflection level. On the other hand, the lower cut-off frequency should be calculated using Skalevik’s model as the value obtained from Rindel’s formula is undervalued. Such an approach could be applied to design reflective structures. However, it has some limitations for example for arrays of perturbed symmetry or sparse arrays as well as in the case of non-perpendicular angles of sound wave incidence. Then it may be necessary to apply more accurate numerical models.

Numerical investigation of a honeycomb liner grazed by laminar and turbulent boundary layers

Direct numerical simulations are used to study the interaction of a cavity-backed circular orifice with grazing laminar and turbulent boundary layers and incident sound waves. The flow conditions and geometry are representative of single degree-of-freedom acoustic liners applied in the inlet and exhaust ducts of aircraft engines and are the same as those from experiments conducted at NASA Langley. The simulations identify the fluid mechanics of how the sound field and state of the grazing boundary layer impact the in-orifice flow and suggest a simple flow analogy that enables scaling estimates. From the scaling estimates the simulations are then used to develop reduced-order models for the in-orifice flow and a time-domain impedance model is constructed. The liner is found to increase drag at all conditions studied by an amount that increases with the incident sound pressure amplitude.

An investigation into sound transmission loss by polypropylene needle-punched nonwovens

In this work, the effects of variables such as initial carded web mass, needle penetration depth, punch density, and the frequency of incident sound wave on transmission of sound through polypropylene needle-punched nonwovens were investigated. Fibrous carded webs using commercially available 17dtex, 90mm staple length polypropylene fibers were prepared with different mass per unit area using carding machine. Samples were needled at various punch-densities and needle penetration depths were produced. Design points of experiments were set up using Taguchi experimental design method. Sound transmission loss (STL) of needled samples was measured using an impedance tube equipped with four microphones. Minitab software was used to analyze the sound transmission ability of the samples. Results indicated that all of the considered controllable factor have significant effects on STL values determined for the needled nonwovens. Also, initial carded web mass was found to be the most influential factor affecting sound transmission through the samples. It was concluded that an increase in thickness of the samples as well as mass per unit area of nonwovens results in higher sound transmission loss by the samples.

The emergence of resonance within acoustic fields of the float gyroscope suspension

The formation nature of resonance phenomena, which give rise to the rapid growth of the gyro errors in hypersonic flight conditions in the suspension of a two-stage float gyroscope is revealed. On the example of the industrial design of the float gyroscope, computational models are built. They allowed determining the influence of the antisymmetric and symmetric impedance of the housing on the emergence of resonance conditions in the suspension. The influence of the propagation direction of acoustic radiation on the occurrence of the features, leading to the rapid growth of additional autonomous positioning errors is determined. The resonance content at high (above the cutoff) and low (below the cutoff) frequencies is revealed. It is shown that at high frequencies the resonance is formed by only bending vibration of the housing in the sound field provided that the antisymmetric impedance is much lower than the symmetrical impedance. It is found that the low-frequency resonance can be formed only by the circumferential vibration of the housing depending on the sound wave incidence angles. It appears in the form of the wave coincidence of the trace of the incident and circumferential waves, or coincidence of the circumference of the frame and the circumferential wave on the plane of the incident wavefront. The resonance methods of dealing with the influence of the sound fields as the only effective ones in the resonance conditions of hypersonic flight are analyzed.

Modelling the inhomogeneous coating of an elastic plate with optimum sound-reflecting properties

The problem of determining the laws governing the inhomogeneity of the coating of a plane elastic plate that ensure the least reflection with a given angle of incidence of a plane sound wave is considered. On the basis of the direct problem solution, a functional representing the intensity of reflection is constructed, and an algorithm for its minimisation is proposed. Analytical expressions describing the mechanical parameters of the inhomogeneous coating are obtained.

Acoustically induced transparency using Fano resonant periodic arrays

A three-dimensional acoustic device, which supports Fano resonance and induced transparency in its response to an incident sound wave, is designed and fabricated. These effects are generated from the destructive interference of closely coupled one broad- and one narrow-band acoustic modes. The proposed design ensures excitation and interference of two spectrally close modes by locating a small pipe inside a wider and longer one. Indeed, numerical simulations and experiments demonstrate that this simple-to-fabricate structure can be used to generate Fano resonance as well as acoustically induced transparency with promising applications in sensing, cloaking, and imaging.

English

A method for the minimization of the ratio of the sound pressure level at the exit to that at the entrance is presented for a rectangular 3D straight lined duct carrying a uniform gas flow. The duct dimensions, the incident sound wave as well as the frequency of the sound wave propagation are assumed to be known. It is assumed also that the liner impedance model is the mass-spring-damper model. The idea is to compute the considered ratio for a large number of combinations of the values of the inertance, resistance and stiffness of the liner and, by fitting the obtained data, to establish a formula for the dependence of the ratio in discussion on these three parameters of the liner. Using the obtained formula, the minimum value of the ratio, as well as the values of the inertance, resistance and stiffness, for which the minimum is obtained, are found.

Interference of sound waves in a moving fluid

We investigate sound propagation in a moving fluid confined in a randomly corrugated tube. For weak randomness and small fluid velocities $v^{(0)}$, the localization length $\xi$ shows extreme sensitivity to the variation of $v^{(0)}$. In the opposite limit of large fluid velocities, $\xi$ acquires a constant value which is independent of the frequency of the incident sound wave, the degree of randomness and $v^{(0)}$ itself. Finally, we find that the standard deviation $\sigma_{\ln T}$ of the logarithm of transmittance $\ln(T)$ is a universal function of the ensemble average $\langle \ln T\rangle $, which is not affected by the fluid velocity.