Prediction Of Acoustic Field Research Articles

Series expansion for the sound field of rotating sources

A method is derived for the fast, exact prediction of acoustic fields around rotating sources by using a series expansion which generalizes a previously published method for a circular piston. The technique gives exact predictions for the field outside the sphere containing the rotor in a computational time two orders of magnitude less than that required for direct numerical evaluation of the acoustic integrals. Its use is demonstrated by application to two sample problems characteristic of real aircraft propellers.

Approximating acoustic field uncertainty in underwater sound channels

Precise acoustic field prediction relies on accurate knowledge of many environmental parameters. Uncertainty in any of the inputs to a propagation routine leads to uncertainty in the predicted field. The most accurate but time-consuming method of assessing the resulting field uncertainty is Monte Carlo simulation, which requires calculating the field with many possible combinations of inputs. This presentation covers a method to approximately determine the uncertainty in the predicted field, using only one additional field calculation for each uncertain parameter, when the input uncertainties are relatively small. The method relies on identifying how variations in uncertain input parameters can be mapped into spatial displacements in the original field calculation. Simultaneous uncertainty in multiple input parameters is handled using a net effective spatial displacement. In addition to expected values and standard deviations, the method allows for the generation of complete probability density functions for the uncertain predicted field. Calculated field amplitude results for both ideal and more realistic range-independent sound channels having small depth and sound speed uncertainty are shown and compared to Monte Carlo simulations at several source-receiver ranges. [Work sponsored by the Office of Naval Research, Code 321OA.]

Refraction of Sound in a Horizontally Inhomogeneous, Time-Dependent Ocean

Measurements of the three-dimensional (3-D) structure of a sound-speed field in the ocean with the spatial and temporal resolution required for prediction of acoustic fields are extremely demanding in terms of experimental assets, and they are rarely available in practice. In this study, a simple analytic technique is developed within the ray approximation to quantify the uncertainty in acoustic travel time and propagation direction that results from an incomplete knowledge or purely statistical characterization of sound-speed variability in the horizontal plane. Variation of frequency of an acoustic wave emitted by a narrowband source due to a temporal variation of environmental parameters is considered for deterministic and random media. In a random medium with locally statistically homogeneous, time-dependent 3-D fluctuations of the sound speed, calculation of the signal frequency and bearing angle variances as well as the travel-time bias due to horizontal refraction is approximately reduced to integration of respective statistical parameters of the environmental fluctuations along a ray in a background, range-dependent, deterministic medium. The technique is applied to acoustic transmissions in a coastal ocean, where tidally generated nonlinear internal waves are the prevailing source of sound-speed fluctuations, and in a deep ocean, where the fluctuations are primarily due to spatially diffuse internal waves with the Garrett-Munk spectrum. The significance of 3-D and four-dimensional (4-D) acoustic effects in deep and shallow water is discussed

Investigation on holographic algorithm and experiment of combined wave superposition approach

The general nerar-field acoustic holography (NAH) cannot reconstruct the surface acoustic information of each coherent acoustic source in coherent acoustic field,which has more than one acoustic sources with the same frequency, and they cannot realize holographic reconstruction and prediction of the independent acoustic field generated by one coherent acoustic source too.So the holographic reconstruction and prediction of coherent acoustic field has become the foremost problem to be resolved in the application of NAH. On the basis of the proposed combined wave superposition approach, an experiment is carried out to realize the holographic reconstruction and prediction of a coherent acoustic filed generated by two sound boxes.By this experiment,the feasibility and accuracy of combined wave superposition approach are demonstrated,the shortcoming of general wave superposition approach in holographic construction and prediction of coherent acoustic field are also demonstrated.The Tikhonov regularization method is proposed to control the ill-posedness of inverse acoustic problem, and the principle of filter coefficient selection is also studied.The experiment indicates that Tikhonov regularization method with appropriate filter coefficient can improve the precision of the holographic reconstruction and prediction.

A probability density function method for acoustic field uncertainty analysis

Acoustic field predictions, whether analytical or computational, rely on knowledge of the environmental, boundary, and initial conditions. When knowledge of these conditions is uncertain, acoustic field predictions will also be uncertain, even if the techniques for field prediction are perfect. Quantifying acoustic field uncertainty is important for applications that require accurate field amplitude and phase predictions, like matched-field techniques for sonar, nondestructive evaluation, bio-medical ultrasound, and atmospheric remote sensing. Drawing on prior turbulence research, this paper describes how an evolution equation for the probability density function (PDF) of the predicted acoustic field can be derived and used to quantify predicted-acoustic-field uncertainties arising from uncertain environmental, boundary, or initial conditions. Example calculations are presented in one and two spatial dimensions for the one-point PDF for the real and imaginary parts of a harmonic field, and show that predicted field uncertainty increases with increasing range and frequency. In particular, at 500Hz in an ideal 100m deep underwater sound channel with a 1m root-mean-square depth uncertainty, the PDF results presented here indicate that at a range of 5km, all phases and a 10dB range of amplitudes will have non-negligible probability. Evolution equations for the two-point PDF are also derived.

Comparison study on spherical wave superposition method and spherical wave source boundary point method for realizing nearfield acoustic holography

In the light of the concept of spherical wave source, the theoretical model of nearfield acoustic holography (NAH) based on the spherical wave superposition method (SWSM), including reconstruction of expansion coefficients, prediction of acoustic field, error sensitivity analysis, regularization method and a searching method with dual measurement surfaces for determining the optimal number of expansion terms, is established. Subsequently, the spherical wave source boundary point method (SWSBPM) and its application in the NAH are introduced briefly. Considering the similarity of the SWSM and the SWSBPM for realizing the NAH, they are compared. The similarities and differences of the two methods are illuminated by a rigorous mathematical justification and two experiments on a single source and two coherent sources in the semi-free acoustic field. And, the superiority of the NAH based on the SWSBPM is demonstrated.

Theoretical and experimental investigation of holographic reconstruction and prediction of multi-source hybrid acoustic field

Study on a multi-source hybrid acoustic field is very important for nearfield acoustic holography (NAH) to be applied in the practical engineering field. In this paper, two methods based on the distributed source boundary point method (DSBPM) for holographic reconstruction and prediction of multi-source hybrid acoustic field, namely combination method with single surface measurement and combination method with multi-surface measurement, are proposed. Then, an experiment on two coherent speakers is carried out. By comparison of the reconstructed and predicted, results and the actual measured results, the validity of the two methods and the superiority of the combination method with multi-surface measurement are proved. Considering the high sensitivity of the reconstructed results to measurement errors the Tikhonov regularization method is proposed to stabilize the reconstruction procedure and restrict the influence of errors. And then, the optimal reconstructed results can be obtained and the reliability of holographic reconstruction and prediction is assured.

An iso-deviant strategy for efficient ambient noise predictions with EAGLE

Transmission loss (TL) computations in littoral areas require a dense spatial and azimuthal grid to achieve acceptable accuracy and detail, which is very slow. This problem of accuracy versus speed led to a new concept, OGRES (Objective Grid/Radials using Environmentally sensitive Selection), which produces sparse, irregular acoustic grids, with controlled accuracy. Recent work to further increase accuracy and efficiency with better metrics and interpolation led to EAGLE (Efficient Acoustic Gridder for Littoral Environments). On each iteration, EAGLE produces an acoustic field with approximately constant spatial uncertainty (hence, iso-deviance), yielding TL predictions with ever-increasing resolution and accuracy. This work adapts EAGLE to ambient noise computations. The Dynamic Ambient Noise Model (DANM) allows accurate, detailed estimation of the mean and variance of ambient noise in both space and time, but its TL computations are too slow for many applications. In the present work, a series of EAGLE acoustic field predictions was used by DANM (and compared to the dense full-grid solution) to determine the relationship between transmission loss uncertainty and noise-field uncertainty for a complex littoral area. An example is presented where approximately an order of magnitude efficiency improvement (over regular grids) is demonstrated. [Work sponsored by ONR under the LADC project.]

Reconstruction and prediction of multi-source acoustic field with the distributed source boundary point method based nearfield acoustic holography

In a multi-source acoustic field, the actual measured pressure is a scalar sum of pressures from all the sources. The pressure belonging to every source cannot be separated out with the existing techniques. Consequently, routine formulas cannot be used to reconstruct the acoustic source and predict the acoustic field directly. In this paper, a novel theoretical model of reconstruction and prediction of multi-source acoustic field in the distributed source boundary point method (DSBPM) based nearfield acoustic holography (NAH) is established. Three different methods, namely combination method with single surface measurement, combination method with multi-surface measurement and elimination method with multi-surface measurement, are proposed to realize the holographic reconstruction of sources. With these methods, the problem of reconstruction and prediction of acoustic field existing multiple coherent sources synchronously is solved effectively. Using the particular solutions constructed by the DSBPM to establish the vibro-acoustic transfer matrix, the calculation time, calculation precision and calculation stability are improved. These methods are valuable in localizing acoustic source and predicting acoustic field in engineering field.

Prediction of acoustic fields radiated into a damped cavity by an N-port source through ducts

The use of two parameters—source impedance and source strength—to model a fluid machine radiating fluid-borne sound via ducts has led to excellent predictions when the source, a ventilator, propagates in one or two plane-wave ducts. Can such previously published methods successfully be applied to the case of a multi-port source radiating via ducts into a damped cavity? The case under study here is a car ventilation/heating unit and the aim was to predict the pressure spectrum inside the passenger compartment caused by the noise propagated through the ventilation ducts. The progressive validation procedure used indicated how and why as the system increases in complexity, predictive accuracy diminishes. The final results are nevertheless convincing and the hypotheses, which can be further refined to reflect the reality better and provide higher quality results, are clearly defined.

An advanced time approach for acoustic analogy predictions

This paper deals with a new interpretation of the retarded time approach that is widely used in the prediction of acoustic fields from moving sources. A hierarchical inversion between the emission time and the reception time leads to advanced time approach. This consists in projecting the current status of a source in the observer time domain where the received signal is progressively built. The practical relevance of this methodology lies on two statements: no retarded time equations must be solved; an aerodynamic noise prediction can be processed parallelly to the aerodynamic computation. Theoretically, the advanced time approach differs from the retarded time approach only in one aspect. A signal emitted at a given instant by a point source, moving at subsonic as well as supersonic velocity, is received only one time by an observer moving at subsonic velocity. Consequently, only one value of the advanced time corresponds to a value of the emission time. The advanced time approach is herein applied to a retarded time solution of the Ffowcs Williams and Hawkings equation proposed by Farassat. The noise radiated by elementary acoustic sources in complex motion is then computed and checked against analytical solutions.

A numerical prediction of acoustic fields generated by wind turbines

A numerical prediction of acoustic fields generated by wind turbines

Modal adiabaticity in shallow water. Part I: Volume interaction

To assess the modal adiabaticity of sound propagation in the ocean is crucial both for acoustic field prediction (forward problem) and tomographic retrieving (inverse problem). Most of the criteria in the literature is too restrictive (conservative). A new criterion of the modal adiabaticity for volume interaction is proposed in this paper. The adiabaticity of two strong volume interaction cases in shallow water has been estimated based on the new criterion: (1) the Polar front case, and (2) the solitary wave case. The results assessed by the criterion have been verified numerically by processing the modal decomposition of the PE field. It is found that the adiabatic modes can survive in a narrow belt along the ‘‘cutoff’’ line in the ‘‘m−f’’ (mode number−frequency) plan for these two strong volume interaction cases. [Work supported by ONR and NOAA.]

Estimation of Geoacoustic Parameters from Narrowband Data Using a Search-Optimization Technique

Geoacoustic parameters were estimated for vertical array data from the matched-field inversion benchmark data sets. Separate inversions were performed for narrowband data at 25 Hz, 50 Hz and 75 Hz, using a matching function consisting of the incoherent sum of the Bartlett outputs for the five vertical arrays at ranges of 1, 2, 3, 4 and 5 km. Parameter estimation was performed using a parabolic equation sound propagation algorithm to generate the replica fields, and a search-optimization technique to obtain estimates of the optimized parameter values. This technique involved an initial search stage in which the parameter space was sampled, and a second optimization stage in which each of a specified number of the best matches found in the search stage was used as the starting point for optimization. This approach provided multiple independent estimates of the geoacoustic parameters, and allowed assessment of the non-uniqueness of the problem and the sensitivity of the matching function to the individual parameters. A method was developed to combine the results for several frequencies to estimate the parameters. It used a weighted average with weights computed on the basis of the relative sensitivities at those frequencies; these sensitivities were estimated by the root-mean-square (RMS) gradient observed during the optimizations. Strong interdependencies among the parameters were found in the analysis, particularly between the sediment thickness and the sound speed at the bottom of the sediment. For the single-frequency matching function used here, it was observed that the inversion problems were ill-posed in that sets of parameter values from a wide region of the parameter space gave essentially perfect matches. The consistency of the parameter estimates was greatly improved by including a regularization term in the matching function. Regularized search-optimization provided an efficient method for estimating an effective geoacoustic model for acoustic field prediction.

Acoustic cavitation field prediction at low and high frequency ultrasounds

Sonochemistry, or chemistry under ultrasound, has increased in interest in the past few years. Electricité de France is involved in scaling up this new technology to industrial plants. The propagation of a power ultrasound wave (from 20 to 800 kHz) through a liquid initiates a little-known phenomenon called acoustic cavitation. Inceptions and germs grow into bubbles which collapse, possibly giving rise to extreme conditions of temperature and pressure (assessed to be up to 10 000 K and 500 atm). These conditions initiate and greatly enhance chemical reactions. Thus, it is important to know where bubbles are and how intense cavitation is, depending on geometric and acoustic factors. Then, mathematical modelling may help to predict acoustic cavitation fields. Two computations, the first one based on linear acoustic equations and the second one based on fluid dynamics equations, are presented. Calculated pressure fields are obtained in the case of a cylindrical sonoreactor at 28 and 500 kHz. The comparison with experimental observations and measurements shows good agreement with both theory and calculations.

A new non-linear solution procedure for one, two and three dimensional sound propagation

A new numerical procedure is described for acoustic field predictions under fully non-linear conditions capable of capturing strong shocks. The analysis employs a particularly novel implementation of new variables which highlight the acoustic components.

Acoustic sensing of the upper crust

Acoustic interaction with the ocean bottom is dominated by the volcanic upper crust in virtually the entire Pacific and large portions of the Atlantic Ocean. Previously reported measurements by numerous investigators revealed very low sound speeds (∼2500 m/s) in recently formed crust, compared to ultrasonic measurements in dredged hand held samples (∼6000 m/s between 0–60 m.y.), hypothetically due to high large scale crack density. Recent measurements of the coherent component of the reflection coefficient in 40–60 m.y. old, nearly sediment-free (<40 m) crust, provide accurate measures of critical angles, and by inference, compressional and shear speeds. In the absence of fracture zones, the estimated compressional sound speeds in the older crust are generally only slightly greater than in young crust. Shear speeds at some sites in this age range exceed Vw, the speed of sound in water, whereas in young crust Vs < Vus. As a result, at sufficiently low frequencies and small grazing angles, where roughness effects are minimal, reflectivity is low (near zero when Vs ≲ Vw) in young crust and may be near unity in 40–60 m.y. old crust. Implications of these results for acoustic field prediction and future research in acoustic sensing of the crust will be discussed. [Work supported by ONR.]

Prediction of acoustic field perturbed by a cylindrical tank using finite element method approach.

The finite element method is applied to acoustic field around a cylindrical tank in order to analyze the diffraction effects. The method used in this study is the so-called hybridtype infinite element method. The cylindrical tank situated in a finite domain perturbs the acoustic field radiated from a point source. The finite domain is divided into many finite elements, which are combined with the infinite elements on the inter-element boundary using an arbitrary interpolation function. The numerical result is compared with an experimental result which was obtained for acoustic field around a water tank of a thermal power plant. From these studies, it can be seen that the hybrid technique using the combined finite and infinite elements is useful for the analysis and prediction of noise around some obstacle like a water tank.

An indirect boundary integral equation method for architectural acoustics

A numerical method and the resulting computer program for prediction of interior acoustic fields are described. The method is based upon an indirect boundary integral equation formulation of the Helmholtz equation using hybrid layer potentials. The computer program incorporates general boundary conditions directly, and utilizes piecewise constant variable approximations. It is capable of predicting the steady‐state response of enclosures with finite impedance boundaries, vibrating boundaries, and multiple interior sources. Numerical results are presented for the acoustic field inside a rectangular prism under various boundary conditions and with various excitations. Apparent limitations of the current program are discussed, and various potential applications in architectural acoustics and noise control are suggested. [Work performed at the Institutt for Teleteknikk, Norges Tekniske Hogskole, Trondheim, Norway and supported by NTNF.]