Prediction Of Sound Field Research Articles

Geoacoustic inversion for bottom parameters in the deep-water area of the South China Sea

The properties of bottom acoustic parameters are greatly influenced on sound field prediction. Acoustic parameters in deep water are not well understood. We will discuss experience with geoacoustic inversion of transmission-loss (TL) data from 100 to 600 Hz from deep water regions. Bottom acoustic parameters are sensitive to the TL data in the shadow zone of deep water. Multiple steps TL inversion method is proposed to invertsound speed, density and attenuation in deep water. Both a uniform liquid half-space and a two-layer geoacoustic profile are assumed. Sound speeds of bottom are inverted by using TL data in the shadow zones at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea (SCS) in summer 2014. Meanwhile, bottom densities are estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the long range TL data by using the known sound speeds and densities as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. The inverted bottom parameters can be used to forecast the transmission loss in deep water area of SCS very well.

Localization of directional noise sources from high-performance military aircraft through subarray beamforming analysis

High-performance military aircraft noise is created by multiple sound generation mechanisms that need to be understood to guide noise reduction efforts and for adequate sound field predictions. Phased-array methods can be used to produce frequency-dependent equivalent acoustic source models. The Hybrid (beamforming) method [Padois et al., J. Sound Vib. 333 (2014)] is applied to an acoustical measurement along a 71-microphone ground-based array, spanning 32 m, placed in the vicinity of a high-performance military aircraft as the engine was operated at different powers. Application of the Hybrid method to the full-array creates an overall equivalent source model that is sufficient for predicting overall field radiation but fails to separate the different noise sources. Applying the Hybrid method to subarrays separates broadband shock-associated noise from the main radiation lobes of turbulent mixing noise. Results show that the subarray-based equivalent source distributions for the different types of noise originate from overlapping source locations. Further analysis of the subarray-based equivalent noise sources using coherence and directionality from the unwrapped phase of the cross-spectral source reconstructions identifies overlapping, frequency-dependent source regions with characteristics unique to broadband shock-associated noise and turbulent mixing noise. [Work supported by an Air Force Research Laboratory SBIR.]

Sound radiation from finite cylindrical shell excited by inner finite-size sources

The study of the characteristics of noise sources in cylindrical shells is the foundation of sound field prediction. Although noise sources are usually regarded as point sources to simplify the calculation model in noise source localization and waveguide sound propagation, the approximation is limited to far-field problems. For the near-field acoustics problems in engine room and ship cabin, the radiated noise possesses the spatial directivity because of the complex vibration distribution of the noise source surface. Moreover, the sound scattering on the surface of finite-size sources makes the noise source itself act not only as the energy input of sound field, but also as the scatterer to change the structure of sound field in the environment. These factors lead to large errors when the finite-size source is simplified into a point source. In order to explore the influence of finite-size source on the acoustic field inside and outside the underwater vehicle structure, the shell coupled equation is constructed by combining thin shell theory, equivalent source and Green function. The effects of source surface scattering and directivity on the acoustic field inside and outside the cylindrical shell are studied. The results show that the accuracy of finite-size source construction is related to the equivalent source location. It proves that equivalent source allocation should be arranged in the middle of the geometric center of sources and its structural surface. Sound scattering from the surface of the finite-size source will change the sound field inside the shell, and then the resonant peaks of the cavity are shifted to the high frequencies as the source volume increases, which causes a strong sound transmission phenomenon in some frequency bands. In addition, the directivity of the finite-size source has little effect on the intensity of the sound field inside and outside the shell, which is evident in changing the far-field directivity of the radiated sound field. The research results are valuable for noise prediction and noise control.

3D meshless FEM-BEM model for prediction of sound fields in cabins due to external sound disturbances

The Finite Element Method (FEM) and Boundary Element Method (BEM) are widely applied to predict the sound pressure level (SPL) in enclosed spaces for low frequency problems. However, a single method usually cannot fulfill the task for predicting the internal SPL in enclosures including objects in the interior due to external disturbances. Moreover, these methods have some disadvantages such as complex pre-processing, time-consuming and inevitable pollution effects. Based on these drawbacks, this paper attempts to combine the Meshless Method (MM), acoustical FEM and BEM into a hybrid method which can be applied to predict the SPL in an enclosed environment with external sound sources. Firstly, the hybrid theory for the acoustic problem and its implementation are illustrated. Next, numerical simulations and experiments are conducted to validate the peak value, SPL and computing efficiency using this method. Comparative results obtained from the proposed method, FEM and BEM using SYSNOISE are shown to be in agreement, and the proposed method is more efficient. Experimental results show that the average relative error of SPL in each location is less than 5.26 %. It is corroborated that the proposed method is applicable to the prediction of the internal SPL with the case of exterior sound sources existed.

Prediction of sound fields radiated by finite-size sources in room environments by using equivalent source models: three-dimensional simulation and validation

The use of equivalent source models (ESM) has recently been extended, with appropriate modifications, from its original application in acoustical holography to room acoustics simulations. This approach can serve as an alternative that is less computationally intense than the boundary element method and which is more flexible than traditional ray-tracing models in terms of dealing with non-compact sources and lower frequencies where the ray approximation is no longer accurate. Such room acoustics ESMs are based on the assumption that the room component of the total sound field can be represented by a number of equivalent sources of certain types, and that the total sound field can be predicted, with given free-space source information, by estimating the strengths of these equivalent sources based on a least-square match of the impedance boundary conditions on both the source and the room surfaces. However, to-date, the room acoustics ESMs have only been formulated and validated for two-dimensional cases. In the current work, the ESM formulation has been extended to three dimensions by replacing the two-dimensional equivalent sources with their three-dimensional counterparts, and its applications in a realistic three-dimensional room environment has been validated by successfully comparing the ESM and boundary element predictions.

Sound Field Prediction and Separation in a Moving Medium Using the Time-Domain Equivalent Source Method

Sound Field Prediction and Separation in a Moving Medium Using the Time-Domain Equivalent Source Method

Geoacoustic Inversion for Bottom Parameters in the Deep-Water Area of the South China Sea**Supported by the National Natural Science Foundation of China under Grant Nos 11434012, 41561144006, 11174312 and 11404366.

Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss (TL) data in the shadow zone of deep water. We propose a multiple-step TL inversion method to invert sound speed, density and attenuation in deep water. Based on a uniform liquid half-space bottom model, sound speed of the bottom is inverted by using the long range TL at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea (SCS) in summer 2014. Meanwhile, bottom density is estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the TL data in the shadow zones by using the known sound speed and density as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. The inverted bottom parameters can be used to forecast the transmission loss in the deep water area of SCS very well.

Comparison of holography, beamforming, and intensity-based inverse measurements on full-scale jet noise sources. Part II: Experimental results

Targeted reduction of high-amplitude jet noise is facilitated by accurate source imaging and sound field prediction models. To provide guidance for future jet noise reduction efforts, near-field acoustical holography, beamforming, and acoustic vector intensity-based inverse methods (AVIBIM) have been implemented in efforts to measure and predict full-scale high-performance aircraft noise sources. Patch-and-scan array measurements were taken in the near-field of a tethered tactical aircraft. From these data, equivalent source reconstructions are calculated using each method at multiple frequencies, and the source results are then propagated outward and compared with benchmark locations in the mid and far fields. The frequency ranges where each method’s reconstructions predict the measured sound pressure levels are identified and guidelines are provided for the design of measurements to optimize source reconstructions. These results are compared to those obtained in the numerical study (Part I) of this two-part investigation. [Work supported by USAFRL through ORISE.]

Shape Optimization of Acoustic Enclosures Based on a Wavelet–Galerkin Formulation

The problem of the shape optimization of acoustic enclosures is investigated in this paper. A general procedure, comprising a Wavelet–Garlerkin formulation and a so-called vertex-driven shape optimization is proposed to deal with the general problem of internal sound field prediction and the optimization of the boundary shape. It is shown that, owing to the compactly supported orthogonal property and the remarkable fitting ability, Daubechies Wavelet can be used as a global basis to approximate the unknown sound field on a relatively large interval globally instead of piecewise approximation like most of element based methods do. This feature avoids meshing the boundary of the enclosure, although vertex points are needed to define the boundary shape, whose positions keep updating during the shape optimization process. A rectangular enclosure is used as benchmark to assess and validate the proposed formulation, by investigating the influence of some key parameters involved in the formulation. It was shown that the sound pressure along the entire boundary of the rectangular enclosure can be accurately approximated without meshing. The same enclosure with an inner rigid acoustic screen is then used to reduce the sound pressure level within a chosen area through optimizing the shape of the screen, which shows the remarkable potentials of the proposed approach as a shape optimal tool for inner sound field problems.

Sound field prediction of ultrasonic lithotripsy in water with spheroidal beam equations**Project supported by the National Basic Research Program of China (Grant Nos. 2012CB921504 and 2011CB707902), the National Natural Science Foundation of China (Grant No. 11274166), the State Key Laboratory of Acoustics, Chinese Academy of Sciences (Grant No. SKLA201401), and the China Postdoctoral Science Foundation (Grant No. 2013M531313).

With converged shock wave, extracorporeal shock wave lithotripsy (ESWL) has become a preferable way to crush human calculi because of its advantages of efficiency and non-intrusion. Nonlinear spheroidal beam equations (SBE) are employed to illustrate the acoustic wave propagation for transducers with a wide aperture angle. To predict the acoustic field distribution precisely, boundary conditions are obtained for the SBE model of the monochromatic wave when the source is located on the focus of an ESWL transducer. Numerical results of the monochromatic wave propagation in water are analyzed and the influences of half-angle, fundamental frequency, and initial pressure are investigated. According to our results, with optimization of these factors, the pressure focal gain of ESWL can be enhanced and the effectiveness of treatment can be improved.

Propagation of wind turbine noise through the turbulent atmosphere

It is well known that turbulence can cause fluctuations in the resulting sound fields. In the issue of wind turbine noise, such effect is non-negligible since either the inflow turbulence from nearby turbine wakes or the atmospheric turbulence generated by rotating turbine blades can increase the sound output of individual turbines. In this study, a combined approach of the Finite Element Method (FEM) and Parabolic Equation (PE) method is employed to predict the sound levels from a wind turbine. In the prediction procedure, the near field acoustic data is obtained by means of a computational fluid dynamic program which serves as a good starting field of sound propagation. It is then possible to advance wind turbine noise in range by using the FEM/PE marching algorithm. By incorporating the simulated turbulence profiles near wind turbine, more accurate predictions of sound field in realistic atmospheric conditions are obtained.

Prediction of sound field from recoilless rifles in terms of source decomposition

In the present study, sound field from M40A1, a type of recoilless rifles, was measured using sound analyzers. Contrary to expectations based on the common characteristics of muzzle blast in previous studies, significantly different waveforms compared to those in other directions were observed in each acoustic pressure signal. In order to clarify how the surroundings affected the sound field, the temporal, spectral and directional characteristics were investigated using correlation functions and Fourier transforms. As a result, additional sources near the rear nozzle of the weapon were identified. Under the operating circumstances of recoilless weapons, blast and jet noise sources were found to be consistent with their acoustical characteristics. A hybrid model integrating the influences of blast and jet noise identified from source decomposition was derived according to the ISO standard 17201 and NASA empirical formulae. Since the predicted directivity pattern agrees moderately well with that measured in previous and present studies and the spectral distributions are in accordance with the measured ones, the hybrid model proposed in this study is considered to be appropriate for predicting the sound field from M40A1 recoilless rifle.

MODELING OF GREEN'S FUNCTION WITH BOTTOM REFLECTIVE PARAMETERS (P, Q) INSTEAD OF GA PARAMETERS

In shallow-water acoustic waveguide, the bottom of the sea (as the boundary condition that determines the Green's function of the sound propagation) plays an extremely important role in the sound field in water medium, especially at low frequency. Commonly the bottom is characterized by geoacoustic (GA) parameters and nearly all the existing numerical codes for sound field prediction use the GA parameters as the input of the bottom. However, the GA parameters are model based (needs a specified layered bottom model to mount the parameters in) which in practical applications may cause a series of problems such as model mismatching, extra uncertainty for field prediction and time consuming in MFP inverting. When the two bottom reflective parameters (P, Q) are used instead of GA parameters, which are model-free and directly determine the effect of the bottom on the sound field respectively in phase and strength, those problems will be solved naturally. In this paper, for range-independent waveguide using (P, Q) directly as input to construct the Green's function in terms of normal mode field is fully developed. For range-dependent waveguide, we propose an effective-equivalence based mapping between the bottom reflective parameters (P, Q) and the effective half-space (EHS) GA parameters to make (P, Q) as input parameters indirectly. Then all the existed numerical codes can still be used for the calculation of sound field without any modification except using P and Q as the input parameters of the bottom. The accuracy of this mapping is verified for both range-independent and range-dependent waveguide. At last the uncertainty analysis in GA space as well as in PQ space is briefly discussed to reveal the "extra uncertainty" due to the inter-coupling among the GA parameters.

Unstructured large eddy simulations of heated supersonic twin jets

Unstructured large eddy simulations are performed with the compressible flow solver ”Charles” developed at Cascade Technologies, for heated supersonic over-expanded jets issued from a twin nozzle. The study focuses on the modeling of the internal flow and its effects on the flow-field in the jet plume and ultimately on the radiated noise. In this work, near-wall adaptive mesh refinement, synthetic inflow turbulence and wall modeling are used inside the nozzle. In addition to the converging-diverging nozzle geometry, the computational domain includes the Y-duct, S-ducts, and angle adapter upstream of the nozzles, to realistically reproduce the elements of the experimental configuration where the internal flow conditions and wall modeling can be expected to affect the external flow-field and sound-field predictions. Comparisons between the numerical predictions and experimental PIV and far-field noise measurements from NASA Glenn Research Center will be presented and discussed.

Sound field alteration through cavity shape design using a Wavelet-Galerkin Method

The general problem of internal sound field prediction along with the manipulation of the cavity boundary shape to alter the sound pressure distribution is dealt with in this paper. Owing to the compactly supported orthogonal property of the wavelet and its extraordinary fitting capability, Daubechies wavelet scaling function is used as a global basis function to expand the unknown sound pressure under the general Galerkin framework. The proposed formulation is shown to offer high accuracy on the numerical calculation of the sound pressure field with the use of a remarkably small number of messing points. A genetic based shape optimization algorithm is proposed and demonstrated. As an example, an enclosure with an inner rigid acoustic screen is investigated. By optimizing the shape of the screen, the sound pressure level within a chosen area is successfully reduced. Results show the remarkable potentials of the proposed approach as a topology optimal tool for the general inner sound field problems.

Combined wave and ray based room acoustic simulations of audio systems in car passenger compartments, Part II: Comparison of simulations and measurements

The present series of papers summarizes the results of a three-year research project on the realistic simulation of sound fields in car passenger compartments using a combined Finite Element (FE) and Geometrical Acoustics (GA) approach. The simulations are conducted for the whole audible frequency range with the loudspeakers of the car audio system as the sound sources. The challenges faced during the project relate to fundamental questions regarding the realistic sound field simulation in small enclosures with strong modal and diffraction effects. While Part I of this series of papers focusses on the determination of the boundary and source conditions for the simulation model of the car compartment, the present paper, denoted here as Part II, presents extensive objective and subjective comparisons of the corresponding room acoustic measurement and simulation results.By applying the FE method to the low frequency part of the room transfer function (RTF) the study aims at the quantification of potential objective and subjective benefits with regard to the simulation quality in small rooms, when compared to a purely geometrical acoustics approach. The main challenges and limitations in the simulation domain are due to the very small volume, the difficult to determine source and boundary conditions and the considerable diffraction effects (especially at the seats) in the car passenger compartments. In order to keep the complexity of the FE simulations at a manageable level, all boundary conditions were described by acoustic surface impedances and no fluid-structural coupling was considered in the FE simulation model.While the results of the study reveal that an overall good agreement regarding the energy distribution in time and frequency domain is generally possible even in such complex enclosures, the results also clearly show the limitations of the impedance boundary approach in the FE domain as well as the strong sensitivity of the simulation results with regard to the uncertainty in the boundary and source conditions in both simulation domains. It can thus be concluded, that possible fields of application of the FE extension in room acoustic simulations lie in the prediction of the modally dominated low frequency part of the RTF of well defined rooms and in the prediction of sound fields that are strongly affected by near-field or diffraction effects as in the car passenger compartment. However, due to the considerable problems in the determination of realistic boundary conditions for the FE model, improved measurement techniques are urgently needed to further improve the overall simulation quality.

Comparison of sound field measurements and predictions in coupled volumes between numerical methods and scale model measurements

Prediction of sound fields in closed spaces can be achieved by various methods, either physical or numerical, based on different theoretical features. While the benefits and limitations of many methods have been examined for single volume spaces, there has been little effort in examining these effects for coupled volume situations. The present study presents a case study comparing theoretical, experimentally physical measurements on a scale model, and various numerical methods, namely boundary element method (BEM), finite-difference time-domain (FDTD) and ray-tracing through the commercial software CATT-Acoustic and ODEON. Although these numerical methods all use 3D numerical models of the architecture, each is different. Ray-tracing is more suitable to geometries with larger planes; BEM requires a more regular finer surface mesh; and FDTD requires a volumetric mesh of the propagation medium. A simple common geometry based on the scale model is used as a basis to compare these different approaches. Application to coupled spaces raises issues linked to later parts in the decay due to multi-slope decay rates, as well as diffraction phenomenon due to acoustic energy travelling between coupling surfaces from one volume to another. The ability of these numerical methods to adequately model these effects is the question under study.

Prediction of Exterior Sound Pressure Field of a Cylinder Using the Boundary Element Method

The prediction of sound pressure field in the acoustic domain caused by a diffracting object is of interest in many fields of acoustics. Applications in acoustics include detecting, locating and classifying objects that generate a scattered sound field excited by the incident sound waves. For example, relevant to these applications; the exterior sound pressure field in the acoustic domain due to incident plane waves on a cylinder can be determined analytically and numerically. For the numerical investigation the Finite Element Method (FEM) is widely used. However, for the exterior acoustic problem the FEM requires discretization everywhere in the entire domain, which results in increase of computational time to solve the problem. In order to overcome this problem, the Boundary Element Method (BEM) has been used in the current work to construct a suitable numerical model. The analytical results confirmed the accuracy of the numerical results obtained using the BEM.

Recent advances in sound propagation above a non-locally reacting ground

In the absence of atmospheric effects, the sound fields above a locally reacting ground can be accurately predicted by the Weyl-van der Pol formula. The solution is based on an asymptotic analysis to yield two terms: a direct and a ground reflected wave terms. The reflected wave term can be written as a product of a spherical wave reflection coefficient and the sound reflected from a rigid ground. In the contrary, it is more challenging to derive a similar formula for the sound fields above non-locally reacting grounds. In the past, an approximation in the same form as the Weyl-van der Pol formula has been used which becomes inadequate for layered grounds. In this presentation, a brief review of the asymptotic analysis will be discussed. An overview of the analytical and numerical approaches will be presented for obtaining accurate prediction of sound fields above the non-locally reacting ground. It will be further demonstrated that the reflection coefficient can be split exactly into two terms - a plane wave reflection coefficient and a ground wave term involving the boundary loss factor. The correlation between the numerical distance and the location of the surface wave pole will be examined.

Implementation and evaluation of a diffusion equation model based on finite difference schemes for sound field prediction in rooms

In this paper, the use of finite difference schemes for the acoustic diffusion equation model is introduced. Their features and limitations are analysed to select the adequate scheme based on both the stability conditions and the error order. The air absorption effects on the implementation are also discussed in terms of stability. To investigate the validity of the implementation, a set of simulations was conducted in a cubic room with four different absorption distributions. This evaluation was done by increasing either the spatial or the temporal resolutions of the studied scheme. The predicted values are compared with the statistical theory and geometrical models. The simulations suggested an empirical criterion for predicting the spatial and temporal resolutions that maximise the performance of the finite difference scheme.