Incident Sound Field Research Articles

On the use of ground-board mounted microphones for outdoor noise measurements

Outdoor noise measurements made by a microphone mounted into or on an acoustically rigid thin board lying on the ground can be affected to some degree by diffraction from the edges of the board due to the thickness of the board or the finite impedance of the surrounding ground. This can result in the sound pressure level varying over the top surface of the board and deviating from the 6 dB increase (relative to the incident sound field) expected if the board was very thin and the surrounding ground was also acoustically rigid. In this paper, an experimental and numerical investigation is carried out to assess the variation in the sound pressure level over the top surface of the ground-board from sound incident at several different angles. The experiments utilise a probe microphone flush-mounted into a 0.4 m diameter circular thin metal ground-board slightly protruding from a simulated grass-covered field in an anechoic chamber. The experiments are repeated on an outdoor grass-covered field and compared. The effect of different installation methods such as the flush-mounted probe microphone, inverted microphone, and a side-set microphone are also investigated. The variation of the sound pressure level on the top surface of the ground-board due to diffraction from the edges of the board is shown. It is also shown that the presence of the microphone body begins to affect the noise measurements from a frequency of 3 kHz. The results presented in this paper suggest that narrow-band noise measurements using a microphone mounted on a circular ground-board installed on a grass-covered surface can deviate by more than 3 dB from the nominal 6 dB increase above the incident field level.

Method of acoustic separation on the surface of cylindrical baffle

This paper suggests a statistically optimal near-field acoustic holography method based on the combination of plane waves and cylindrical waves, which is used for underwater cylindrical baffles. The method addresses the issue that the extraction of scattered acoustic waves from the surface of underwater cylindrical baffles requires too many measurement points in practical applications: separation of incident and dispersed sound fields and near-field measurements of sound fields at baffle surfaces.The underwater detection sound source is situated in the scatterer's far-field region, so in the actual application scenario for the surface acoustic scattering separation of the cylindrical baffle, the plane wave expansion is used to represent the surface incident acoustic wave and the cylindrical wave expansion is used to describe the scattered acoustic wave on the baffle surface. As a result, fewer expansion orders are required to describe the entire sound field on the baffle's surface. It is verified by simulation that the method is valid and efficient for near-field total sound field measurement and dispersed sound field separation of roughly cylindrical targets under underwater far-field incidence situations because the measurement holographic surface of the surface permits unlimited geometries.

Design of acoustic coatings with symmetric and asymmetric resonant inclusions

An acoustic coating for underwater applications, designed using periodically distributed cavities and hard spherical particles embedded in a soft matrix, is herein modelled as monopolar and dipolar scatterers positioned along the centre of a sound hard duct. Modified equations of motion for the scatterers account for resonance phenomena as well as multiple scattering of waves by a lattice. Combinations of monopolar and dipolar scatterers in the direction of the incident sound field are considered. The acoustic performance of the various coating designs composed of symmetric and asymmetric resonant inclusions are compared. The influence of asymmetry in coating designs, arising from variation in scatterer size and material distribution, on effective material properties and Willing coupling are reported.

Computational approach for the acoustic modelling of large aftertreatment devices with multimodal incident sound fields

The influence of multimodal incident sound fields on the acoustic behaviour of large aftertreatment devices incorporating a monolith is modelled and analysed in detail. The analytical mode matching method is applied to the compatibility conditions of the three-dimensional acoustic fields at the device geometric discontinuities, leading to the computation of the complex wave amplitudes in all the subdomains involved and the corresponding device transmission loss. To have a realistic model, three-dimensional propagation must be considered in the inlet/outlet ducts and chambers, while one-dimensional wave propagation has to be assumed along the small capillaries of the aftertreatment device monolith (such catalytic converters and particulate filters); therefore, the monolith can be replaced by a plane wave four-pole transfer matrix from an acoustical point of view. On the other hand, for large aftertreatment device inlet ducts such as those found in heavy-duty and off-road engines, the usual models with plane incident wave excitation are not accurate since the onset of higher order incident modes in the inlet duct is expected for the frequency range of interest. Therefore, a variation of the acoustic attenuation performance is likely to occur depending on these modes, similar to the results previously found in the case of large dissipative silencers. Results are presented for three different multimodal incident sound field hypotheses: equal modal amplitude, equal modal power and equal modal energy density. A relevant influence on the sound attenuation is found for the test problems considered in the current investigation.

Theoretical investigation on the underwater sound absorption of a functionally graded anechoic coating under the phased arc array incidence

Laying anechoic coatings on the surface of underwater vehicles is an effective technology to improve the acoustic stealth performance. Meanwhile, the detection waves may have complex directivity, and it is important to study their influence on the sound absorption of the coating. This article introduces a functionally graded coating under a phased arc array incidence, which simulates a novel anechoic coating on an underwater vehicle to absorb sound waves emitted by active sonars. The incident field of the phased array is constructed by spherical harmonic expansion and the superposition principle. The wave propagation of the functionally graded coating is analyzed through an equivalent laminated model and the reverberation matrix method. Utilizing the angular spectrum method, the reflected sound field and absorption coefficient of the coating under the phased arc array incidence are predicted and the theoretical method is formed. Then the selections of parameters in Fourier transforms of the angular spectrum method are discussed for achieving converged results. A finite element analysis is applied to verify the effectiveness and calculation speed of the proposed method through the establishment of detailed quarter or half model. The effect of phased arc array parameters and the coating on the absorption and reflection performances are further studied. The angular spectrum decomposition of the incident sound field explains the changing trends of the sound absorption curves. The present work provides an important way to deal with this complex underwater sound propagation problem.

Low frequency sound absorption performance of large sized active micro-perforated panel absorber in free field

The active micro-perforated panel absorber has excellent low frequency sound absorption performance, which is expected to realize low-frequency noise reduction in large space of the cabin. Since its active sound absorption performance depends on the incident sound field environment, on the basis of the existing research conclusions in the duct, the active sound absorption performance of the large-sized active micro-perforated panel absorber under the excitation of a normal incident plane wave in typical free field environment is in depth investigated. First, the theoretical model of the active micro-perforated panel absorber is established by using the modal analysis approach, in which a reflection coefficient varying with position is introduced to represent the reflected sound wave on the surface of the active micro-perforated panel absorber in free field. Then, the physical mechanism of active control is thoroughly analyzed and the error sensing strategy is established. Finally, an experiment is carried out to validate the theoretical model and findings. Results obtained demonstrate that the greater the amplitude of the (0, 0, 0) cavity mode excited by the incident plane wave, the stronger the reflection effect on the incident sound wave is, and vice versa. The control source suppresses the (0, 0, 0) mode so that this mode will not reflect and absorb the incident plane wave substantially when its amplitude is reduced to an optimal value. This is main mechanism of the sound absorption improvement in the low frequency range. However, the excited high order cavity modes (except for (0, 0, 0) mode) greatly reflect the incident sound energy in free field and exert a negative effect on sound absorption improvement. Thus, the control performance of the active micro-perforated panel absorber weakens in free field in comparison with that in the duct. The pressure-release and impedance-matching strategies are still applicable to free field as long as such a situation holds, i.e. the (0, 0, 0) cavity mode can be substantially suppressed by the control source and at the same time the high order cavity modes cannot be highly excited.

Sound absorption characteristics of thin parallel microperforated panel (MPP) for random incidence field

In this study, the absorption coefficient of a thin parallel MPP for random incidence field is investigated. A coiled backing air cavity realizes the thin structure, and the parallel configuration is applied to have a wider sound absorption bandwidth than a single MPP. The effects of the oblique sound incidence on the thin parallel MPP are investigated by numerical studies for both single oblique incidence and random incidence. These effects are significant from a practical standpoint, whereas the result of a coiled backing air cavity must be carefully evaluated. This paper focuses on developing, modeling, and evaluating parallel MPP with a coiled backing cavity. It is found that the oblique incidence can result in lower sound absorptions and shift in resonance absorptions compared to the normal incidence angle, especially for high incidence angles (>70°). In addition, the performance of the thin MPP at a normal incidence angle can be maintained up to the incident angle of 70°, where the sound absorption coefficients of 0.8 and above are present. Compared with a straight backing cavity, the use of coiled one does not introduce deviation in sound absorption characteristics except for the cavity modes slightly higher resonance frequency under the same axial length. Moreover, the proposed structure behaves as a locally reacting material as long as the cavity’s longest dimension is smaller than half an operating wavelength for the targeted frequency range. This allows the sound absorption characteristics of normal incidence and diffuse sound field excitation is insignificantly different. Hence, this characteristic can deal with two folded problems in terms of bulky absorber issue and sensitive performance on oblique incidence.

An experimental study on the influence of incident characteristics on sound insulation between rooms: Study on the incident sound field in the insulation performance measurement for partition walls. Part 2

AbstractThe sound field characteristics due to room modes are considered to have strong influences on the low‐frequency sound insulation between rooms in field. In this paper, the sound field analysis using the propagation mode expansion method proposed by the authors was applied to the test rooms where the dwelling rooms and the separating wall of an RC apartment building were reproduced. First, the proposed method was validated through the reconstruction of the expanded sound fields and the measurement of the incident directivity. Next, the comparison of the multiple geometries of the source room with two finishing conditions of the RC separating wall demonstrated the influence of the incident sound field on the sound insulation. The mechanism of the sound insulation between rooms was investigated using the proposed method. For the RC single wall, the sound field analysis showed an influence of mode matching between rooms on the low‐frequency transmitted sound. For the composite wall which was composed of the RC separating wall and the additional linings, it was demonstrated that the low‐frequency resonance transmission phenomenon is affected by the matching of the oblique incidence resonance frequency of the wall and the incident characteristics.

Acoustical intensity probe based on a polyvinylidene fluoride bimorph.

This study investigates complex acoustical intensity using a polyvinylidene fluoride (PVDF) bimorph. Analytical models of the open-circuit voltage outputs of an infinite-strip-shaped PVDF bimorph cantilever in an underwater sound field are developed. Results show that the sound pressure generates the sum of the outputs, while the particle velocity normal to the PVDF surface generates the difference. The sensitivities of the pressure- and velocity-generated voltage responses with respect to an incident plane sound field demonstrate uniform directivity in a low-frequency range, which is suitable for acoustical intensity determination. The higher velocity sensitivity confirms the advantage of using a PVDF bimorph as a velocity sensor, owing to its light weight and flexibility. An algorithm for determining the complex acoustical intensity normal to the surface is proposed by utilizing those voltage responses and the probe gain calibrated with a given angle of incident sound. This algorithm allows accurate determination of sound intensity of a plane wave field, where the reactive part of intensity is absent. However, a small error may exist when the reactive intensity is large and active intensity is small. This small discrepancy arises from the inherent variation in the phase directivity of the gains, which decrease with frequency.

Sensitivity of an infinite-strip-shaped polyvinylidene fluoride film in an underwater plane sound field

The sensitivity of an infinite-strip-shaped polyvinylidene fluoride (PVDF) film in an underwater plane sound field is analyzed in this paper. The high-frequency sensitivity is characterized by the resonances of the symmetrical in-plane stress modes with traction-free boundary conditions at the ends of the film and by the anti-resonances due to the superposition of the stress components. The low-frequency sensitivity exhibits a simple hydrostatic response. The directional property of the in-plane stress component, generated by the incident sound across the thickness, has two contributions. The first is from the aperture and dipolar functions of the incident sound pressure over the PVDF surfaces and at the two ends of the film. The second is from an amplitude modulation by an angle-dependent gain. The directional property of the in-plane stress component by the sound pressure at the two ends of the film is only controlled by the dipolar function, and that of the stress component in the thickness direction is only determined by the aperture function. The illustration of the frequency and directional features of the PVDF film may advance understanding of the mechanisms involved in generating the voltage output of PVDF by an incident sound field.

Cancelation of acoustic scattering from a smart hybrid ERF/PZT-based double‐wall composite spherical shell structure

A coupled multiphysics vibroacoustic model is developed for axisymmetric scattering of plane progressive time-harmonic sound waves from a fluid-filled and submerged smart hybrid double concentric composite spherical shell that incorporates noncollocated piezoelectric (PZT) and electrorheological fluid (ERF) actuator layers. The dynamic equations of motion for the PZT- and ERF-based sandwich shells are independently derived using Hamilton’s variational principle, Kirchhoff-Love thin shell formulation, Maxwell's electrodynamics equations, and Kelvin-Voigt viscoelastic constitutive model. The frequency-domain multi-input/multi-output (MIMO) sliding mode control (SMC) strategy is then applied to activate the sound scattering cancelation capability of the triply-coupled smart structure through collaborative active/semi-active operation of the built-in actuator elements. Extensive numerical simulations reveal the remarkable broadband attenuation of both types of wide and narrow resonance peaks arising in the calculated far-field backscattering spectra that are primarily linked to various types of interacting fluid- and shell-borne peripheral waves. Also, the near-field effectiveness and cloaking potentials of proposed smart hybrid design in realizing partial or complete acoustic invisibility with respect to the incident sound field are explored at selected frequencies. In particular, the superior 3 D acoustic cloaking capabilities of the PZT/ERF and ERF/ERF configurations are established noting the low actuation energy advantage of the ERF element.

Energy flow analysis of equivalent fluid models for porous media.

Based on a heat conduction analogy that enables the energy flow analysis (EFA) to describe the smoothed energy variations, the EFA has been applied to a variety of structures to predict vibrational responses at high-frequency regions. In this paper, energy equations in the form of heat conduction laws are derived to represent dilatational waves in rigid- and limp-frame porous media using equivalent fluid models. Homogeneous and inhomogeneous waves are considered. Within the EFA framework, the group velocity and loss factor included in the energy models for structures are replaced with the energy velocity and effective loss factor, respectively. The capabilities of the energy models are illustrated using configurations in which the porous layer backed by a rigid wall is in a normal and oblique incident sound field. The results of the numerical simulations confirm the feasibility of the energy model. From an energy perspective, the adequacy of using rigid- and limp-frame equivalent fluid models is also discussed. It is shown that the use of a rigid-frame equivalent model for predicting the energy distribution is more restrictive than predicting acoustic performance such as sound absorption.

Efficient prediction of construction equipment exterior and cabin interior noise over broad frequency range using novel SEA method

Statistical Energy Analysis (SEA) is commonly used for the prediction of interior cabin noise from construction equipment such as excavators, dump trucks, or graders. While traditional SEA method is computationally efficient and effective for the prediction of total radiated noise, it isn't suitable for prediction of sound diffraction around machinery and evaluation of spatial variations in sound field. As a result, prediction of cabin airborne interior noise transmission using SEA method typically requires experimental measurements in order to estimate incident sound field over the exterior boundary of the cab which makes it unsuitable for use in early stage design where test data isn't available. A novel SEA method that accounts for spatial gradients in the reverberant field has been developed and is introduced in this paper. It's usage for prediction of both exterior and cab interior noise over broad frequency range is demonstrated along with experimental validation for construction equipment under operating conditions.

Performance comparison of phased array transducers for inspection of dissimilar welds on nuclear reactor components

Trimetallic weld joint of steam generator is crucial and most critical component of Prototype Fast Breeder Reactor (PFBR). Ultrasonic examination of trimetallic welds are more challenging due to the coarse anisotropic grain structure of the weld metal and the mismatch of the acoustic impedance at the weld metal interface, which contributes to increase scattering, sound beam deviation and distortion, reflection and refraction of the incident sound field. This paper presents a comparative study on phased array ultrasonic examination of dissimilar weld joints (P91 – Alloy 800) using linear array (LA) and dual matrix array (DMA) transducer. Prior to the experimental investigations, wave propagation through the dissimilar weld joint and the defect sensitivity is understood by carrying out simulation based on ray-tracing and semi-analytical methods. Weld joint sample with artificial defect such as lack of side wall fusion was fabricated and studied using both transducer of Phased Array Ultrasonic Testing (PAUT).

A phenomenological extended-reaction boundary model for time-domain wave-based acoustic simulations under sparse reflection conditions using a wave splitting method

In environmental acoustics and in room acoustics, many surfaces exhibit extended-reaction (ER) behavior, i.e., their surface impedance varies with the angle of the incident sound wave. This paper presents a phenomenological method for modeling such angle dependent surface impedance properties in time-domain wave-based simulations. The proposed method has two attractive features: 1) it is general and can be used to model any type of surface as long as the angle dependent surface impedance is known, and 2) it adds little computational cost to the simulation. The method relies on the assumption of a low reflection density, rendering it suitable for modeling, e.g., outdoor sound propagation and early reflections in large rooms. A wave splitting technique is used to separate the reflected wave from the incident wave at the boundary for each time step of the simulation. Once separated, the angle of the incident sound field is determined and the surface impedance adjusted accordingly. The proposed method is validated analytically and experimentally for a single reflection case with different porous sound absorbers. A clear improvement in accuracy is observed, as compared to locally reacting boundary conditions.

Vibroacoustic response of stiffened thin plates to incident sound

A model was developed and applied for predicting the vibroacoustic response of stiffened plates excited by different incident sound fields. A relationship between the Joint Acceptance Function (JAF) and the modal radiation efficiency is derived. This allows for a direct calculation for both the vibration response and the sound transmission loss (TL) in the presence of a diffuse field whilst accounting for full vibroacoustic coupling. The response for different structural configurations, plane versus diffuse acoustic excitation and modelling approximations are highlighted, so presenting a systematic quantification of the factors influencing the vibroacoustic response. It is found that vibroacoustic coupling is enhanced by the presence of stiffeners, with the effects for different types of acoustic excitation and the contribution from the modal cross terms being most significant for the TL results. A coupled FE-BE numerical model of stiffened plate and other empirical formulae for unstiffened plate are subsequently used for the validation and verification of the proposed model. The results show that the computational cost of simulation can be significantly reduced with a small loss of accuracy when using the proposed model in comparison to the fully coupled FE-BE numerical model.

Method of Separation of Incidental Acoustic Field on Cylindrical Shell by Vector Processing

A theoretical and experimental study on the separation method of the incident sound field based on a small-scale vector sensor is proposed in this study, with the aim of resolving the problem of separation and acquisition of an incident sound field under the interference of near-field sound scattering from a cylindrical shell in water. The method of identifying and separating sound waves obtained under plane wave conditions is extended to complex sound-field conditions. Simulation and experimental results show that the vector separation method can greatly reduce the sound pressure amplitude and the phase deviation of the incident sound field, which is affected by near-field scattering from the cylindrical surface. The separation accuracy is related to the deviation angle and the distance from the target surface. The maximum deviation of the pressure amplitude is less than 1 dB, and the phase deviation is less than 3°. This method can effectively suppress the near-field scattering of the cylindrical shell and improve the separation accuracy of the incident sound field. The research results have reference value for a range of practical engineering applications.

3D focusing acoustic lens optimization method using multi-factor and multi-level orthogonal test designing theory

Focusing systems that consist of acoustic lenses exhibit higher controllability and focusing intensity when manipulating sound waves. Different parameters have different effects on the performance of the acoustic lens, so analyzing a variety of parameter combinations for the acoustic lens is difficult because of the complexity of the test. Therefore, we report an analytical method for studying the focusing performance of different factors on acoustic lenses at different levels, and the influence degree of each factor is investigated through range analysis. Results show that the factors that have the most significant influence on the focusing intensity, focal-area dimensions, and focal length are the incident sound field’s pressure P0, cell dimension c, and cell edges k, respectively. Moreover, the effects of other parameters, including the biased incident-wave angles, are obtained and analyzed through finite-element simulations. After analyzing and comprehensively comparing the influences of various parameters, the optimal parameter combination is obtained to achieve the best focusing performance of an acoustic lens. The experimental results show that the focusing intensity of the optimized acoustic lens is nearly 90% higher than the non-optimized one, which proves the effectiveness of the orthogonal test-optimization method in this paper.

Acoustical characteristics of segmented plates with contact interfaces

The possibility of shifting sound energy from lower to higher frequency bands is investigated. The system configuration considered is a segmented structure having non-linear stiffness characteristics. It is proposed here that such a frequency-shifting mechanism could complement metamaterial concepts for mass-efficient sound barriers. The acoustical behavior of the material system was studied through a representative two-dimensional model consisting of a segmented plate with a contact interface. Multiple harmonic peaks were observed in response to a purely single frequency excitation, and the strength of the response was found to depend on the degree of non-linearity introduced. The lower and closer an excitation frequency was to the characteristic resonance frequencies of the base system, the stronger was the predicted higher harmonic response. The broadband sound transmission loss of these systems has also been calculated and the low frequency sound transmission loss was found to increase as the level of the broadband incident sound field increased. The present findings support the feasibility of designing material systems that transfer energy from lower frequency bands, where a sound barrier is less efficient, to higher bands where energy is more readily dissipated.

An experimental analysis method for the incident sound field using propagation mode expansion: Study on the incident sound field in the insulation performance measurement for partition walls. part 1

In the measurement of the level difference between dwelling rooms, the low-frequency modal density of the source room is not high enough for the incident sound field to be assumed as a random incidence, and the sound field is expected to be strongly influenced by the standing waves of the room modes. In this paper, an experimental method is proposed to analyze the characteristics of the incident sound fields in dwelling rooms, where the propagation mode expansion is applied to the complex sound pressure observed on the two layers of planar microphone array. The proposed method is applied to numerical models to simulate the incident sound fields to obtain information on its accuracy and application limitations, and the applicability of the method to ordinary rooms is discussed. In addition, the effect of room absorption on incident characteristics is demonstrated using the proposed method.