Near-field Acoustic Holography Research Articles

Application of Bayesian Regularization Criterion in near Field Acoustic Holography Based on Equivalent Source Method

In order to choose an optimal regularization parameter for near field acoustic holography (NAH) based on equivalent source method (ESM), Bayesian regularization criterion method was introduced to solve the inverse problem. Comparison simulations were conducted with generalized cross validation method (GCV), L-curve method and Hald regularization formula method at various sound source frequencies, hologram distances, and Signal-to-Noise Ratios (SNR). The results show that GCV, L-curve, and Hald regularization formula method cannot achieve the optimal regularization parameter at large hologram distances, high noise levels, high frequencies and input SNR errors. However, Bayesian regularization criterion method is not affected by these restrictions and has excellent reconstruction performance in wide frequency band. Further experiment validates the correctness and effectiveness of the application of Bayesian regularization criterion method in ESM-based NAH, thus providing a new guidance for the solving of inverse problem in ESM-based NAH.

Near-field electromagnetic holography for high-resolution analysis of network interactions in neuronal tissue

BackgroundBrain function is dependent upon the concerted, dynamical interactions between a great many neurons distributed over many cortical subregions. Current methods of quantifying such interactions are limited by consideration only of single direct or indirect measures of a subsample of all neuronal population activity. New methodHere we present a new derivation of the electromagnetic analogy to near-field acoustic holography allowing high-resolution, vectored estimates of interactions between sources of electromagnetic activity that significantly improves this situation. In vitro voltage potential recordings were used to estimate pseudo-electromagnetic energy flow vector fields, current and energy source densities and energy dissipation in reconstruction planes at depth into the neural tissue parallel to the recording plane of the microelectrode array. ResultsThe properties of the reconstructed near-field estimate allowed both the utilization of super-resolution techniques to increase the imaging resolution beyond that of the microelectrode array, and facilitated a novel approach to estimating causal relationships between activity in neocortical subregions. Comparison with existing methodsThe holographic nature of the reconstruction method allowed significantly better estimation of the fine spatiotemporal detail of neuronal population activity, compared with interpolation alone, beyond the spatial resolution of the electrode arrays used. Pseudo-energy flow vector mapping was possible with high temporal precision, allowing a near-realtime estimate of causal interaction dynamics. ConclusionsBasic near-field electromagnetic holography provides a powerful means to increase spatial resolution from electrode array data with careful choice of spatial filters and distance to reconstruction plane. More detailed approaches may provide the ability to volumetrically reconstruct activity patterns on neuronal tissue, but the ability to extract vectored data with the method presented already permits the study of dynamic causal interactions without bias from any prior assumptions on anatomical connectivity.

Sound source identification and sound radiation modeling in a moving medium using the time-domain equivalent source method.

Planar near-field acoustic holography has been successfully extended to reconstruct the sound field in a moving medium, however, the reconstructed field still contains the convection effect that might lead to the wrong identification of sound sources. In order to accurately identify sound sources in a moving medium, a time-domain equivalent source method is developed. In the method, the real source is replaced by a series of time-domain equivalent sources whose strengths are solved iteratively by utilizing the measured pressure and the known convective time-domain Green's function, and time averaging is used to reduce the instability in the iterative solving process. Since these solved equivalent source strengths are independent of the convection effect, they can be used not only to identify sound sources but also to model sound radiations in both moving and static media. Numerical simulations are performed to investigate the influence of noise on the solved equivalent source strengths and the effect of time averaging on reducing the instability, and to demonstrate the advantages of the proposed method on the source identification and sound radiation modeling.

Detecting building leakages using nearfield acoustic holography technique: A numerical simulation

A crack on a building wall and sound generated inside the building using an artificial sound source were simulated using Matlab. Various noise types affecting the pure sound signal, such as background noise in the room, were simulated. The forward problem of sound propagation from the crack surface to a virtual microphone array was simulated using the free-space green’s function. Nearfield acoustic holography (NAH) was applied to perform the inverse problem to calculate the sound pressure at the crack surface and determine the crack location on the building wall. Effects of various errors and noise on the sound pressure reconstruction using NAH were studied. The reconstructed sound pressure levels on the wall surface were compared with the originally simulated sound pressure data at the crack surface. Various crack sizes and shapes were simulated to determine the correlation between the size of the crack and the reconstructed pressure field.

Characterization of the smart-phone radiated noise using nearfield acoustic holography

Smart phones have dramatically evolved on the functional and aesthetic fronts in recent years. However, as smart phones get slimmer, problems arise where the noise radiated from the smart phone, albeit small in SPL, interferes with the conversation and degrades the call quality. The noise radiation is primarily due to the vibration of the internal electronics components and its transmission through the circuit board and the casing. In this paper the nearfield acoustic holography (NAH) is used to characterize the 3-D sound field of the smart-phone radiated noise, which is tailored specifically to deal with the small size and the low SPL of the sound radiator. Quantities such as acoustic pressure, particle velocity, and intensity at the smart phone surface are obtained and utilized to identify parts that are responsible for noise generation. In this respect, the NAH proves to be an effective tool for the characterization and control of the smart phone acoustics.

Inverse Patch Transfer Functions Method as a Tool for Source Field Identification

Many methods to detect, quantify, or reconstruct acoustic sources exist in the literature and are widely used in industry (near-field acoustic holography, inverse boundary element method, etc.). However, the source identification in a reverberant or nonanechoic environment on an irregularly shaped structure is still an open issue. In this context, the inverse patch transfer functions (iPTF) method first introduced by Aucejo et al. (2010, “Identification of Source Velocities on 3D Structures in Non-Anechoic Environments: Theoretical Background and Experimental Validation of the Inverse Patch Transfer Functions Method,” J. Sound Vib., 329(18), pp. 3691–3708) can be a suitable method. Indeed, the iPTF method has been developed to identify source velocity on complex geometries and in a nonanechoic environment. However, to obtain good results, the application of the method must follow rigorous criteria that were not fully investigated yet. In addition, as it was first defined, the iPTF method only provides source velocity while wall pressure or intensity should also give useful information to engineers. In the present article, a procedure to identify wall pressure and intensity of the source without any additional measurement is proposed. This procedure only needs simple numerical postprocessing. Using this new intensity identification, the influence of background noise, evanescent waves, and mesh discretization are illustrated on numerical examples. Finally, an experiment on a vibrating plate is shown to illustrate the iPTF procedure.

Reconstructing excitation forces acting on a baffled plate using near-field acoustical holography

This paper presents numerical simulations of using nearfield acoustical holography technology to reconstruct the excitation forces acting on the surface of a baffled plate. The reason for choosing a baffled plate is that analytic solutions to the vibration responses and acoustic radiation are readily available. To acquire a better understanding of the interrelationship between excitation forces and vibro-acoustic responses of the plate, the structural intensities and power flows due to individual excitation forces and their interactions are calculated. The relative contributions of the excitations toward overall structural vibration responses of the plate are analyzed by taking the normal modes transform into the wavenumber domain. Results show that the wavenumber contents of vibration responses of a plate due to a line, point, and distributed forces have very distinct characteristics. Therefore, by analyzing the vibration responses in the wavenumber domain, it is possible to reveal the impacts of individual excitation forces on the resultant structural vibration of a plate.

Analyzing panel acoustic contributions toward the sound field inside the passenger compartment of a full-size automobile

The Helmholtz equation least squares (HELS)-based nearfield acoustical holography (NAH) is utilized to analyze panel acoustic contributions toward the acoustic field inside the interior region of an automobile. Specifically, the acoustic power flows from individual panels are reconstructed, and relative contributions to sound pressure level and spectrum at any point of interest are calculated. Results demonstrate that by correlating the acoustic power flows from individual panels to the field acoustic pressure, one can correctly locate the panel allowing the most acoustic energy transmission into the vehicle interior. The panel on which the surface acoustic pressure amplitude is the highest should not be used as indicative of the panel responsible for the sound field in the vehicle passenger compartment. Another significant advantage of this HELS-based NAH is that measurements of the input data only need to be taken once by using a conformal array of microphones in the near field, and ranking of panel acoustic contributions to any field point can be readily performed. The transfer functions between individual panels of any vibrating structure to the acoustic pressure anywhere in space are calculated not measured, thus significantly reducing the time and effort involved in panel acoustic contributions analyses.

Building leakage detection and quantification using statistically optimized nearfield acoustic holography technique

This paper presents a building infiltration detection and quantification technique using statistically optimized nearfield acoustic holography (SONAH) technique. A model building with known cracks on its wall was investigated in this study. The model building housed a synthetic acoustic source. A hologram measurement was performed outside the model building and the sound pressure levels on the walls were reconstructed. The correlation between the reconstructed pressure levels and the area of the crack was obtained. It was found that the acoustic technique can successfully detect and quantify the leakages from the building model. Two different frequencies of reconstructions are compared and it was found that the lower frequency reconstructions were more accurate. Effects of various regularization methods, phase matching, and quantization error are discussed. Various methods to suppress the wrap-around error in the reconstructions will be addressed.

Equivalent sources method for supersonic intensity of arbitrarily shaped geometries

Supersonic acoustic intensity is utilized to locate radiating regions on a complex vibrating structure. The supersonic intensity is obtained by a special process that removes the evanescent waves from the near-field acoustical holography measurement. The filtering process is well understood for separable geometries, but unfortunately, there are few results for arbitrarily shaped objects. This work proposes a methodology based on a stable invertible representation of the radiated power operator. The power operator is approximated numerically by the equivalent source formulation and the appropriate complete spectral basis is employed to form the stable invertible operator. The operator is formed with the most efficient radiation modes and these modes are utilized to obtain the supersonic solution for the near-field holographic problem. This concept is tested using numerically generated data in a spherical geometry and the results are validated with the spherical harmonic, supersonic filter. Finally, a vibrating ship-hull structure provides a physical example for application and validation of the proposed methodology in a more complex geometry.

Identification of active sources inside cavities using the equivalent source method-based free-field recovery technique

In previous studies, an equivalent source method (ESM)-based technique for recovering the free sound field in a noisy environment has been successfully applied to exterior problems. In order to evaluate its performance when applied to a more general noisy environment, that technique is used to identify active sources inside cavities where the sound field is composed of the field radiated by active sources and that reflected by walls. A patch approach with two semi-closed surfaces covering the target active sources is presented to perform the measurements, and the field that would be radiated by these target active sources into free space is extracted from the mixed field by using the proposed technique, which will be further used as the input of nearfield acoustic holography for source identification. Simulation and experimental results validate the effectiveness of the proposed technique for source identification in cavities, and show the feasibility of performing the measurements with a double layer planar array.

Multisource statistically optimized near-field acoustical holography.

This paper presents a reduced-order approach to near-field acoustical holography (NAH) that allows the user to account for sound fields generated by multiple spatially separated sources. In this method, an equivalent wave model (EWM) of a given field is formulated to include combinations of planar, cylindrical, spherical, or other elementary wave functions in contrast to an EWM restricted to a single separable coordinate system. This can alleviate the need for higher-order functions, reduce the number of measurements, and decrease error. The statistically optimized near-field acoustical holography (SONAH) algorithm is utilized to perform the NAH projection after the formulation of the multisource EWM. The combined process is called multisource statistically optimized near-field acoustical holography (M-SONAH). This method is used to reconstruct simulated sound fields generated by combinations of a vibrating piston in a sphere and linear arrays of monopole sources. It is shown that M-SONAH can reconstruct near-field pressures in multisource environments with lower errors and fewer measurements than a strictly plane or cylindrical-wave formulation using the same simulated measurement.

改良近距離音響ホログラフィ法の開発（機構部品を用いた実証実験）

改良近距離音響ホログラフィ法の開発（機構部品を用いた実証実験）

Vibration and noise source identification methods for a diesel engine

In this study, we used a four-cylinder inline diesel engine as an example to explain how to use test methods and multi-body dynamic techniques to identify the exact source of noise and vibration. A 1 meter noise test was conducted to indicate that the engine front end is the main source of noise. The near-field sound intensity method and the near-field acoustic holography technique were used to identify the chain drive system as the source of noise and vibration. A 3D dynamic analysis model of the timing chain drive system was established using RecurDyn software. Results of the analysis show that the hydraulic tensioner is incompatible with the timing drive system of the bush roller chain. Hence, the timing chain system causes a loud noise and strong vibration. Some strategies on how to optimize the timing chain drive system are presented at the end of this paper.

수중반사파 환경에서의 근접음장 홀로그래피 적용에 대한 오차 고찰

수중반사파 환경에서의 근접음장 홀로그래피 적용에 대한 오차 고찰

The Simulation and Accuracy Analysis of Sound Filed Reconstruction Based on Fourier Transform-Source Simulation Technique

Acoustic transform algorithm plays an important role in sound pressure reconstruction. According to a new sound pressure reconstruction of near-field acoustic holography based on Fourier transform-source simulation technique, this article through a simulation experiment with the models of Simply Supported Plate Sound Source, verify the feasibility of the method. Furthermore, in order to verify the effectiveness of this method, this paper analyzes the impact factors that affect the accuracy of the Reconstruction. And it does simulation analysis mainly from the parameters of the Virtual Source Plane, the Distance of the Holographic Measurement and the parameters of the Holographic plane. Therefore this study provided referencing basis for later study on complex field of sound pressure reconstruction based on this technology.

Scaling of plane-wave functions in statistically optimized near-field acoustic holography.

Statistically Optimized Near-field Acoustic Holography (SONAH) is a Patch Holography method, meaning that it can be applied in cases where the measurement area covers only part of the source surface. The method performs projections directly in the spatial domain, avoiding the use of spatial discrete Fourier transforms and the associated errors. First, an inverse problem is solved using regularization. For each calculation point a multiplication must then be performed with two transfer vectors--one to get the sound pressure and the other to get the particle velocity. Considering SONAH based on sound pressure measurements, existing derivations consider only pressure reconstruction when setting up the inverse problem, so the evanescent wave amplification associated with the calculation of particle velocity is not taken into account in the regularized solution of the inverse problem. The present paper introduces a scaling of the applied plane wave functions that takes the amplification into account, and it is shown that the previously published virtual source-plane retraction has almost the same effect. The effectiveness of the different solutions is verified through a set of simulated measurements.

A multisource-type representation statistically optimized near-field acoustical holography method

A reduced-order approach to near-field acoustical holography (NAH) that accounts for sound fields generated by multiple spatially separated sources of different types is presented. In this method, an equivalent wave model (EWM) of a given field is formulated based on rudimentary knowledge of source types and locations. The statistically optimized near-field acoustical holography (SONAH) algorithm is utilized to perform the NAH projection after the formulation of the multisource EWM. The combined process is called multisource-type representation SONAH (MSTR SONAH). This method is used to reconstruct simulated sound fields generated by combinations of multiple source types. It is shown that MSTR SONAH can successfully reconstruct the near field pressures in multisource environments where other NAH methods result in large errors. The MSTR SONAH technique can be extended to general sound fields where the shapes and locations of sources and scattering bodies are known.

Transient nearfield acoustical holography

Transient Nearfield Acoustical Holography Sean F. Wu Department of Mechanical Engineering, Wayne State University, Detroit, MI, 48202 This paper presents the general formulations for reconstructing the transient acoustic field generated by an arbitrary object with a uniformly distributed surface velocity in free space. These formulations are derived from the Kirchhoff-Helmholtz integral theory that correlates the transient acoustic pressure at any field point to those on the source surface. For a class of acoustic radiation problems involving an arbitrarily oscillating object with a uniformly distributed surface velocity, for example, a loudspeaker membrane, the normal surface velocity is frequency dependent but is spatially invariant. Accordingly, the surface acoustic pressure is expressible as the product of the surface velocity and the quantity that can be solved explicitly by using the Kirchhoff-Helmholtz integral equation. This surface acoustic pressure can be correlated to the field acoustic pressure using the Kirchhoff-Helmholtz integral formulation. Consequently, it is possible to use nearfield acoustic holography to reconstruct acoustic quantities in entire three-dimensional space based on a single set of acoustic pressure measurements taken in the near field of the target object. Examples of applying these formulations to reconstructing the transient acoustic pressure fields produced by various arbitrary objects are demonstrated.

Acoustic building infiltration measurement system

Systems and methods of detecting and identifying a leak from a container or building. Acoustic pressure and velocity are measured. Acoustic properties are acquired from the measured values. The acoustic properties are converted to infiltration/leakage information. Nearfield Acoustic Holography (NAH) may be one method to detect the leakages from a container by locating the noise sources.