Combining acoustic imaging techniques to localize and identify sources

Standard acoustic imaging techniques, such as beamforming or near-field acoustical holography, are now widely used in engineering contexts. However, large arrays of microphones are sometimes required to have a good resolution. Besides new challenges arise, particularly in the field of non-stationary sources, which need to be identified and solved. Cyclostationary sound sources, a specific kind of non-stationary signals, are characterized by statistical properties evolving periodically in time. In practice the first-order statistical properties contain some periodic components while the second orders may be random with a periodic flow of energy. The present work tackles the acoustic imaging of cyclostationary sources with a scanning microphone, ie. without any array. Cylindrical surfaces, adapted to standard rotating machines, are considered. The difficulty of reconstructing non-stationary acoustic sources from discrete measurements is then alleviated suing their cyclostationary properties. A cyclic sound field is hence extracted that can show how sources are evolving in space and time. Finally, a Bayesian formulation, gathering both physical and probabilistic information on the inverse problem, is used to back-propagate the sound over the radiating surface.

In recent years some progresses have been made in acoustic source localization (ASL) in highly anisotropic plates when the plate material properties are not known. Some simplifying assumptions in many of the earlier source localization techniques that the wave propagates along a straight line from the source to the receiving sensor is not correct for many anisotropic plates. Wave energy in anisotropic solids generally propagate along curved lines and forms non-circular wave fronts. As a result, for highly anisotropic solids the acoustic source localization techniques that assume straight-line propagation of waves from the source to the receiver are bound to give erroneous results. In this paper earlier techniques as well as new developments for acoustic source localization in homogeneous isotropic and anisotropic plates are discussed. The newest source localization techniques in anisotropic plates analyze the non-circular shapes of the wave front. Following this analysis, the acoustic source has been successfully localized in some anisotropic plates without knowing the plate properties. Limitations of the current techniques that still exist and require further investigation are discussed.

The acoustic holography (AH) method with single measuring plane has been well known as the conventional method and can be implemented by far field measurement with simple instruments. However, the noise source resolution of the AH is not sufficient. In order to improve the resolution in the noise source identification, several kinds of the acoustic holography methods have been so far proposed. For example, the near field acoustic holography (NAH) can provide high and accurate resolution of the holography by the nearfield measurement. However, the nearfield measurement within one wave length is sometimes impossible in the actual circumstances. The Acoustic Double Holography (A D H) proposed in this paper is a simplified approach with higher resolution of the noise source locations than that of the conventional AH methods. The ADH method basically uses dual measuring planes and does not require nearfield measurement. The sound pressure data detected on the rear plane are transformed into the virtual pressure data on the front plane taking into account of the distance between the plane and the object. Comparing the virtual pressure data with the actual data measured on the front plane, resolution on holography can be improved significantly. Computer simulation and an experiment with two loud speakers were executed in order to confirm the fundamental feature of the proposed method. Several advantages on the method with respect to resolution over the conventional AH method were discussed. Furthermore, the ADH measurement was carried out on running engine under the full load operation. Through these results, the highly noise radiating areas on the engine surface were detected and reduced with noise shielding material. The overall engine noise level was reduced by 1.5dBA as the first stage in this noise control work.

Source localization problem in near-field acoustical signals on solids surfaces is one of the computationally challenging tasks. This paper concentrates on a solution to this problem. The modeling of the presence of noises while measuring the acoustic pressure field produced due to the sound sources in the vicinity is done using acoustical holography method. The pressure field produced is modeled mathematically using Rayleigh-Sommerfield Algorithm and Helmholtz integral. Near-field acoustic holography (NAH) is performed using the conventional method termed as Fourier Transform based Acoustical Holography. Mathematically modeled pressure field is super positioned and NAH is used to synthesize the original pressure field. In this paper, we have done a simulation model for localizing acoustical sources on the solid surfaces. A pipelining method is described for super-positioning the truncated pressure field to match the actual measured pressure field. Windowing technique is adopted to address spectral leakage effects and the abnormality is caused to the proximity of the boundaries. The results reveal that the proposed methodology can apply to localize sources on diverse kind of surfaces.

In this paper, the basic principle of planar near field acoustic holography is expatiated and the reconstruction formula of sound source localization is given. We take theoretical results of sound pressure radiating from a point source to the holographic plane as the experimental data, and then substitute it in the reconstruction formula to reconstruct the sound source information. By changing holographic plane size, spacing and holographic measurement distance, we compared the sound source reconstruction results with the known sound source to see how these measured conditions parameters affect the reconstruction accuracy. Test results show that precise result can be got when the sound bandwidth exceed 5Khz, the hologram plane size is 6 times greater than the sound wavelength, the spacing of measurement point on holographic plane is less than 1/3 of the wavelength and the holographic measurement distance is less than 1/34 of the wavelength. We applied the planar near field acoustical holography in plant disease stress monitoring, and choose the signal with frequency of 2KHz to carry out the reconstruction experiment. The holographic plane was 0.6m side length square, measuring point spacing was 0.06m and the distance between hologram plane and reconstruction plane was 0.015m. Three sound sources were finally identified. By modifying the parameter setting and comparative analysis, we seek the best selection for simulation and use it as a reference in the ultrasonic signal tests of the diseases stressed plant. Having collected acoustic emission signals, we identified the sound source by near field acoustic holography and located plant diseases stress. Consequently, we can conduct a new crop disease forecast and precision spraying of pesticides.

Source localization of EEG signals identifies the loci of neural activity that generates the EEG voltage or power distribution on the scalp. This chapter provides a practical guide to source localization and its usage in EEG analysis in the frequency domain. Techniques proposed to overcome the inverse problem of source localization have progressed from equivalent dipole modeling to distributed source modeling, such as sLORETA and eLORETA. This chapter discusses the rationale behind these techniques, as well as the advantages and disadvantages of using different techniques for source localization. Recent advances in high-resolution structural MRI make it possible to accurately delineate and model the tissues inside the head. This chapter discusses the importance of using realistic head models constructed with anatomical MRIs for EEG source localization. Finally, it reviews the applications of source localization as a neuroimaging tool in EEG frequency analysis.

Small, finite measurement apertures are a major impediment for accurate and fast application of near‐field acoustic holography (NAH). In theory, FFT‐based NAH methods are very powerful and acquire extremely fast inverse calculations of the sound field. However, in practice the usually small amount of sensors in a measurement array limit the possibilities of classical implementations of NAH, especially near the edges of the array or measurement grid. An alternative method to, for example, spatial windowing, patch NAH, SONAH, etc. is the recently introduced border‐padding technique. This method makes it possible to apply FFT‐based NAH with a large increase of accuracy compared to the original methods, specifically at the measurement grid edges, while maintaining the high speed of the inverse process PNAH is known for. This work focuses on the application of border‐padding and the practical implementation in a newly developed PNAH system. It illustrates a large variety of extended possibilities for PNAH that border‐padding enables. Apart from the earlier introduced standard border‐padding, experimental results are shown for a highly improved border‐padding method. Measurements and calculations on industrial products illustrate the wide applicability of the presented method.

Nearfield acoustic holography (NAH) is an indirect technique for identifying noise sources and visualizing acoustic field. Recently, several different methods, such as the spatial Fourier transform method, the boundary element method (BEM) and the Helmholtz equation-least squares (HELS) method, have been used to realize the NAH successfully. In the paper, a novel numerical method, the distributed source boundary point method (DSBPM), is proposed to realize the NAH. In the method, the transfer matrices from the reconstructed surface to the hologram surface are constructed indirectly by a set of particular solution sources located inside the vibrating structure, and their inverses are carried out by singular value decomposition (SVD) technique. Additionally, considering the high sensitivity of the reconstructed solution to measurement errors, the Tikhonov regularization method is implemented to stabilize the reconstruction procedure and the regularization parameter is determined by L-curve criterion. Compared with the BEM-based NAH, the variable interpolation, the numerical quadrature, and the treatments of singular integral and nonuniqueness of solution are all avoided in the proposed method. Two numerical examples and an experiment are investigated to validate the feasibility and correctness of the proposed method.

<div class="htmlview paragraph">This paper describes a study to demonstrate the feasibility of developing an acoustic encapsulation to reduce airborne noise from a commercial diesel engine. First, the various sources of noise from the engine were identified using Nearfield Acoustical Holography (NAH). Detailed NAH measurements were conducted on the four sides of the engine in an engine test cell. The main sources of noise from the engine were ranked and identified within the frequency ranges of interest. Experimental modal analysis was conducted on the oil pan and front cover plate of the engine to reveal correlations of structural vibration results with the data from the NAH.</div> <div class="htmlview paragraph">The second phase of the study involved the design and fabrication of the acoustical encapsulation (noise covers) for the engine in a test cell to satisfy the requirements of space, cost and performance constraints. The acoustical materials for the enclosure were selected to meet the frequency and temperature ranges of interest. The “noise covers” were designed to reduce the airborne sound power level from the engine by 2-5 dB in the frequency range of 500-2000 Hz. The effectiveness of the treatments was done by measuring the sound pressure and sound power of the engine with and without the encapsulation in accordance with ISO 6798. The results of the study showed that the overall sound power level from the engine was reduced by 3-5 dB depending on the type of materials used. Recommendations are made for the commercial production and implementation of the acoustic treatments on a practical vehicle.</div>

Acoustic holography is a technique to visualize the sound field propagating from a sound source. It is used to analyse and locate sources of noise, and even allows to predict the propagation of sound in the environment. However, this technique involves simultaneous measurements of up to 256 microphones, arranged in an array. We propose an alternative approach with 4 or less microphones which are re-positioned using an articulated robot arm. Provided the measurement conditions are stable, this alternative approach with consecutive measurements allows to make use of acoustic holography to investigate various sources of noise. The case of fan noise is demonstrated where the focus is put on practical aspects conducting near-field acoustic holography experiments using a robot arm, as well as post-processing measurement data. As a result, 2 complimenting techniques, static and dynamic near-field acoustic holography, are introduced and their merits are discussed.

The source localization problem concerns the detection and localization of an emitter whose transmission is observed by geographically separated receivers. The passive multistatic radar (PMR) problem concerns the detection and localization of a target that scatters an illumination signal to geographically separated receivers. By modeling the scattering target as an emitter, the techniques of source localization can be applied to the PMR problem. Indeed, this approach has recently been introduced in the literature. However, the exact relationship between the two problems has not been made explicit. In this work, we derive a centralized generalized likelihood ratio test (GLRT) detector that performs the processing characteristic of both source localization and PMR. This detector is used to assess the relative detection benefit provided by source localization in PMR. We show that source localization techniques are of limited utility in PMR due to the SNR regimes of the target-scattered and direct-path signals typical of the PMR signal environment.

Since its conception in 1980, nearfield acoustic holography (NAH) has become an accepted analysis tool with a number of commercial packages now available. One limitation of NAH is that the acoustic field is reconstructed through a spatial sampling. Therefore, this technique becomes less efficient at higher frequencies where microphone array spacing must be reduced. Analytical models have indicated that an NAH method relying on energy-based measurements will provide the same reconstruction resolution of current NAH methods with significantly fewer measurement locations. This would lead to a considerable savings in data acquisition time for scanning array systems and reduce the inefficiency at high frequencies. The energy-based reconstruction method will be introduced. Experimental results will be presented for a planar test case and the resulting reconstruction accuracy will be compared to the analytical prediction model.

In the conformal near-field acoustic holography (NAH) using the boundary element method (BEM), the transfer matrix relating the vibro-acoustic properties of source and field depends solely on the geometrical condition of the problem. This kind of NAH is known to be very powerful in dealing with the sources having irregular shaped boundaries. When the vibro-acoustic source field is reconstructed by using this conformal NAH, one tends to position the sensors as close as possible to the source surface in order to get rich information on the nonpropagating wave components. The conventional acoustic BEM based on the Kirchhoff-Helmholtz integral equation has the singularity problem in the close near field of the source surface. This problem stems from the singular kernel of the Green function of the boundary integral equation (BIE) and the singularity can influence the reconstruction accuracy greatly. In this paper, the nonsingular BIE is introduced to the NAH calculation and the holographic BIE is reformulated. The effectiveness of nonsingular BEM has been investigated for the reduction of reconstruction error. Through interior and exterior examples, it is shown that the resolution of predicted field pressure could be improved in the close near field by employing the nonsingular BIE. Because the BEM-based NAH inevitably requires the field pressure measured in the close proximity to the source surface, the present approach is recommended for improving the resolution of the reconstructed source field.

Near‐field acoustical holography (NAH) techniques can be optimized if the method capitalizes on the geometry of the noise source under investigation. Helmholtz‐equation least squares method (HELS) uses the solutions of Helmholtz equation in spherical coordinates as basis functions for the pressure field. HELS is an efficient NAH technique if the source and the measurement surfaces are spherical in nature. For nonspherical cases, such as radiation from a plate or bar, it takes a large number of functions to represent the field. In these cases, there is also a question about where to place the origin of the wave functions. In search of a HELS‐type method that could be applied to nonspherical sources, a study into the features of conical coordinates has been conducted. Because the Helmholtz equation is separable in conical coordinates, the solutions can be used, in a manner similar to HELS, as basis functions to represent the pressure field. For conical coordinates, the basis functions are spherical Hankel functions and Lame functions. This HELS‐type formulation in conical coordinates could be a natural choice for NAH on conical sources and may be appropriate for NAH on jet noise. [Work supported by Blue Ridge Research and Consulting and Air Force Research Laboratory.]

Near-field Acoustical Holography is a tool that is conventionally used to visualize sound fields through an inverse process in a three-dimensional space so that either sound field projections or sound source localization can be performed. The visualization is conducted by using sound pressure measurements taken in the near-field region close to the surface of the unknown sound source. Traditional Fourier-based Near-field Acoustical Holography requires a large number of measurement inputs to avoid spatial truncation effects. However, the use of a large number of measurements is usually not feasible since having a large number of microphones is costly, and usually the array is limited in size by the physical environment, thus limiting the practicality of this method. In the present work, because of the desire to reduce the number of microphones required to conduct acoustical holography, a method known as Statistically Optimized Near-field Acoustical Holography initially proposed by Steiner and Hald was analyzed. The main difference between the present work and the concept mentioned by Steiner and Hald is the cylindrical coordinate system employed here for the purpose of experimenting on a bladeless fan, which resembles a cylindrical structure and which could be assumed to be a cylindrical source. The algorithm was first verified via simulations and measurements, and was then applied to experimental data obtained via pressure measurements made with a cylindrical microphone array. Finally, suggestions for noise control strategies for the bladeless fan are described, based on the measurement results.