Patch Near-field Acoustical Holography Research Articles

Data extrapolation and sound field reconstruction based on the boundary element method

Near-field acoustic holography (NAH) based on the boundary element method (BEM) is a powerful tool for noise source identification which is the premise of noise control. For large-scale structures, the measurement aperture should be large enough to ensure that the reconstruction results are accurate. However, the layout of field points often cannot meet the conditions necessary for reliable measurements. The patch NAH methods proposed in previous literature do not explore the influence of the initial and extrapolated aperture on the reconstruction of the vibration velocity. In this paper, the field distribution modes are redefined by the BEM and applied to extrapolation technology. First, the sound pressure of the field points on an expanded hologram is obtained by a data extrapolation method. The expanded data can then be used to reconstruct the vibration velocity of the structure and the sound pressure in the sound field via BEM-based NAH. A modified Tikhonov regularization is utilized in the inverse process. Different sound sources with simple or complex radiated sound fields are used for simulations to explore the influence of the initial and extrapolated aperture on the reconstruction. The application scope of the proposed extrapolation technology is identified and the conclusions are verified by experiments.

Identification of panel acoustic contribution based on patch nearfield acoustic holography

To identify panel acoustic contribution to the interior sound field, a new method using a modified sound field separation technique based on patch nearfield acoustic holography is presented. An improved sound field separation technique with single semi-closed surface covering the panel of interest is proposed to simplify the measurement process. The outward and inward pressures relative to the hologram surface can be separated directly by establishing the relationship between the measured acoustic pressures and calculated values based on the equivalent source method. The acoustic quantities on the vibrating panel surface can be reconstructed from the free radiated field by getting rid of the inward and the scattered field on the surface. The acoustic power on the panel is obtained as a criterion to evaluate the relative contributions to the interior sound field. A numerical simulation of a scaled car cabin model is investigated and the results demonstrate that the proposed method is an available approach for identifying panel acoustic contribution on the vibrating surface of interest to the interior sound field inside a cavity.

An improvement of the generalized discrete Fourier series based patch near-field acoustical holography

Patch near-field acoustic holography (NAH) is an effective tool for reconstructing sound field with a small microphone array. The generalized discrete Fourier series based near-field acoustic holography (GDFS-NAH) is a natural patch NAH method. Although Fourier-based, the spectral leakage is greatly reduced in GDFS-NAH by using generalized discrete Fourier series rather than discrete Fourier transform, as more wavenumber components are used in the reconstruction. However, more wavenumber components also lead to the instability in a noisy case. In this study, the GDFS-NAH is improved by remodeling the reconstruction processing and introducing the sparse regularization. Different from the original GDFS-NAH, in which the pressure on the measurement plane is decomposed into discrete Fourier series, the reconstruction model is reformulated by decomposing the particle velocity on the reconstruction plane, which will avoid the inversion in the original model. Meanwhile, the problem is solved by using sparse regularization, which will suppress the influence of high wave number components and promote the stability. Numerical results show that the proposed method can effectively improve the reconstruction accuracy within 100–2000 Hz, and is capable of interpolating the sound filed simultaneously. The advantages of the proposed method are also verified by experiments.

Research on patch near-field acoustic holography based on two-level iteration

In order to solve the “window effect” and “aperture repetition effect” caused by limited measurement aperture, a patch near-field acoustic holography method based on two-level iteration is proposed. After analyzing the principle and process of patch near-field acoustical holography based on orthogonal spherical wave, a patch near-field acoustical holography method based on two-level iteration is proposed. The validity of the method is verified by simulation and experiment. Compared with the reconstruction results of patch near-field acoustical holography based on orthogonal spherical wave, it shows the superiority of patch near-field acoustical holography based on two-level iteration.

Data extension near-field acoustic holography based on improved regularization method for resolution enhancement

In recent years, patch near-field acoustic holography (NAH) has gradually emerged for the reconstruction of sound field radiated from large-scale sound source under the condition of small holographic aperture in engineering practice. However, this technique has not solved the problem that when the projection of large-scale sound source is located on the edge of the expanded hologram, high-precision reconstruction results are unavailable. In addition, the reconstruction of sound field is a typical inverse problem, so the existence of measurement errors makes the solution of the inverse problem be ill-posed and the accuracy of sound field reconstruction is greatly reduced. To overcome these difficulties, a new technique combining an improved method of orthogonal spherical wave superposition, data extension and modified regularization method is proposed to enhance the spatial resolution of reconstructed images. First, the sound pressure is reconstructed on an extended hologram using the improved method orthogonal spherical wave superposition and data extension. At the same time, the method modified Tikhonov regularization (MTR) is introduced to solve the edge error and other ill-posed reconstruction problems. Through iteration the accuracy of the regenerated data is improved. Finally, the extended data are used to reconstruct the sound field. The results of numerical simulation and experiments show that this technique can effectively improve the spatial resolution of the reconstructed image and the reconstruction is more robust.

Hybrid patch near-field acoustic holography based on Kalman filter

The wave superposition method (WSM) is a classical method for reconstructing the sound field of an arbitrary sound source, but its main drawback is the uncertainty of equivalent source configuration. To overcome this problem, a hybrid patch near-field acoustic holography based on Kalman filter (KF-HPNAH) is proposed. The algorithm consists of three steps: (a) the calculation of the three-dimensional (3D) virtual motion trajectory of sound source using the projected coordinates of the sound source on multiple hologram planes; (b) the application of KF to the trajectory, thus improving the accuracy of 3D localization of the sound source; (c) the formulation of the equivalent source configuration strategy of the WSM based on the 3D coordinates. The KF-HPNAH can effectively reduce the dimension of a WSM matrix and improve the accuracy of sound field reconstruction. Numerical simulations and experiments are carried out to verify the effectiveness of the proposed method. The results show that the KF-HPNAH solves the uncertainty problem of equivalent source configuration of WSM successfully. Compared to the existing methods, KF-HPNAH has the best reconstruction accuracy. The proposed method can thus be used to localize the sound source and reconstruct the sound field radiated from an arbitrary sound source at frequencies between 200 Hz and 900 Hz.

Measurement extension limits of patch nearfield acoustic holography

Patch nearfield acoustic holography is widely used when the measurement aperture is smaller than the area of the sound source. This paper discusses the factors affecting the limit of aperture extension. One step patch nearfield acoustic holography based on equivalent sources model was considered in this study. Available literature provide constant estimations on the ratio of measurement area to the area of the source. However, the present study shows that this extension limit is a variable depending on a few parameters. Wave number, hologram distance and choice of regularization technique were found to affect the extended area of reconstruction significantly. By choosing the right combination of these parameters, accurate reconstructions were achieved even when the measurement area was only ten percent of the source surface in size. This paper provides a systematic comparison of reconstruction errors for cases with various parameter combinations. A model relating the parameters and the extension limit is under development.Patch nearfield acoustic holography is widely used when the measurement aperture is smaller than the area of the sound source. This paper discusses the factors affecting the limit of aperture extension. One step patch nearfield acoustic holography based on equivalent sources model was considered in this study. Available literature provide constant estimations on the ratio of measurement area to the area of the source. However, the present study shows that this extension limit is a variable depending on a few parameters. Wave number, hologram distance and choice of regularization technique were found to affect the extended area of reconstruction significantly. By choosing the right combination of these parameters, accurate reconstructions were achieved even when the measurement area was only ten percent of the source surface in size. This paper provides a systematic comparison of reconstruction errors for cases with various parameter combinations. A model relating the parameters and the extension limit is ...

A patch near-field acoustical holography procedure based on a generalized discrete Fourier series

Planar near-field acoustical holography (NAH) can be used to reconstruct a three-dimensional sound field from sound pressure data measured by a planar microphone array. The conventional planar NAH makes use of the discrete Fourier transform (DFT) to process the measured data, yielding a low computational cost. However, if the measurement aperture does not fully cover the sound source extension, the spatial windowing will lead to severe reconstruction errors. Many patch NAH methods have been proposed to allow measurement apertures smaller than the source size, such as the statistically optimized NAH (SONAH), which is not based on the DFT. These methods have proven to outperform the conventional NAH for small measurement apertures, but with an increased computation time and more complex implementation. This paper introduces an alternative patch procedure for planar NAH that replaces the DFT with a so-called “generalized discrete Fourier series” (GDFS). Unlike the DFT, the periods of the two-dimensional GDFS and the number of Fourier coefficients are made larger than the measurement aperture and the number of microphones, respectively. Then, the Fourier coefficients are evaluated in the least-norm sense. This reduces the spectral leakage due to the spatial windowing, improving the NAH results. As a numerical example, a simply supported plate driven by a point force is considered, and patches of the plate normal velocity are estimated from simulations of the radiated sound pressure on a small microphone array. It is shown that the GDFS-based method might lead to reconstructed velocity fields as accurate as SONAH, or even more accurate. However, unlike SONAH, the proposed method presents a low computational cost and a straightforward implementation. Therefore, it is a worthy alternative to the currently available patch procedures for planar NAH.

Fast wideband acoustical holography.

Patch near-field acoustical holography methods like statistically optimized near-field acoustical holography and equivalent source method are limited to relatively low frequencies, where the average array-element spacing is less than half of the acoustic wavelength, while beamforming provides useful resolution only at medium-to-high frequencies. With adequate array design, both methods can be used with the same array. But for holography to provide good low-frequency resolution, a small measurement distance is needed, whereas beamforming requires a larger distance to limit sidelobe issues. The wideband holography method of the present paper was developed to overcome that practical conflict. Only a single measurement is needed at a relatively short distance and a single result is obtained covering the full frequency range. The method uses the principles of compressed sensing: A sparse sound field representation is assumed with a chosen set of basis functions, a measurement is taken with an irregular array, and the inverse problem is solved with a method that enforces sparsity in the coefficient vector. Instead of using regularization based on the 1-norm of the coefficient vector, an iterative solution procedure is used that promotes sparsity. The iterative method is shown to provide very similar results in most cases and to be computationally much more efficient.

Patch nearfield acoustic holography combined with sound field separation technique applied to a non-free field

The conventional nearfield acoustic holography (NAH) is usually based on the assumption of free-field conditions, and it also requires that the measurement aperture should be larger than the actual source. This paper is to focus on the problem that neither of the above-mentioned requirements can be met, and to examine the feasibility of reconstructing the sound field radiated by partial source, based on double-layer pressure measurements made in a non-free field by using patch NAH combined with sound field separation technique. And also, the sensitivity of the reconstructed result to the measurement error is analyzed in detail. Two experiments involving two speakers in an exterior space and one speaker inside a car cabin are presented. The experimental results demonstrate that the patch NAH based on single-layer pressure measurement cannot obtain a satisfied result due to the influences of disturbing sources and reflections, while the patch NAH based on double-layer pressure measurements can successfully remove these influences and reconstruct the patch sound field effectively.

Patch nearfield acoustic holography in a moving medium

To realize the accurate reconstruction of sound field in a moving medium under the condition of limited holographic aperture, a patch nearfield acoustic holography (NAH) in a moving medium is proposed. The proposed method not only reduces the influence caused by the limited aperture effects through sound field extrapolation, but also perfectly suits for sound field reconstruction in a moving medium by improving the shape of the modified Tikhonov regularization filter and the noise estimation method in accordance with flow effects. In the method, two cases that the flow direction is parallel to and perpendicular to the hologram surface are considered. Especially in the perpendicular case, the expression of the wavenumber component in the z direction is improved to make the proposed method suitable for the moving medium at a high Mach number. Simulations are investigated to examine the performance of the proposed method and show its advantages by comparing with NAH in a moving medium and the conventional patch NAH. It is found that, the proposed method is effective and robust at different flow velocities of the medium and different frequencies of the sound source.

Development of Near-Field Acoustical Holograph and its Review

Near-field acoustic holography (NAH) is a powerful technique for identifying noise sources and visualizing acoustic field.The theory and algorithm of NAH techniques are introduced , and it is proved by experiment and simulation. The researches on near field acoustical holography (NAH) are reviewed，including the sound field separation technique and Patch NAH technique arisen in recent years.The difficulties in NAH and research on current situations are discussed , Finally,some engineering applications are introduced by detailed examples.

Patch nearfield acoustic holography based visualization of spatial distribution of sound quality objective parameters

Psychoacoustic research reveal that sound pressure level reduction in cabin does not mean improvement of human’s subjective auditory perception. This paper presents Patch Nearfield Acoustics Holography-Sound Quality (Patch NAH-SQ) combined analysis method with the advantages of NAH and SQ. This methodology breaks the limit of measurement aperture of conventional NAH and can visualize of spatial distribution of sound pressure and sound quality objective parameters of entire sound field by using a small number of microphones. From the 3D images of spatial distribution of sound pressure and sound quality objective parameters, the sound sources which most contribute to human’s auditory perception can be located. In this paper, the impact of the choice of parameters such as overlap ratio of Patch holographic surface, SNR, stand-off distance and frequency on reconstruction error of sound pressure and sound quality objective parameters is examined. Finally, the experiment of Patch NAH-SQ methodology proposed is validated in anechoic camber, the results are presented and analyzed.

Uncertainty of array-based measurement of radiated and absorbed sound intensity

Patch near-field acoustic holography (NAH) coupled with an array of sound intensity probes allows separating the sound field incident on a surface from the one radiated by the surface itself. Although the measurement principle has been successfully used to separate the noise source contribution from disturbing sources and/or noise reflections, the method accuracy has not been investigated in the literature. We describe the results of experiments meant to evaluate the uncertainty in the identification of noise radiated by vibrating panels with different absorption characteristics in presence of an incident acoustic radiation using the statistically optimized near-field acoustic holography. Measurement errors were evaluated through tests performed in controlled acoustic conditions. Results evidenced that the measurement uncertainty depends on the accuracy of the microphone array positioning and on the incident sound field. These conclusions were in agreement with the results obtained by simulations in the phase of instrument optimization.

Underwater patch near-field acoustical holography based on particle velocity and vector hydrophone array

One-step patch near-field acoustical holography (PNAH) is a powerful tool for identifying noise sources from the partially known sound pressure field. The acoustical property to be reconstructed on the surface of interest is related to the partially measured pressure on the hologram surface in terms of sampling and bandlimiting matrices, which cost more in computation. A one-step procedure based on measuring of the normal component of the particle velocity is described, including the mathematical formulation. The numerical simulation shows that one-step PNAH based on particle velocity can obtain more accurately reconstructed results and it is also less sensitive to noise than the method based on pressure. These findings are confirmed by an underwater near-field acoustical holography experiment conducted with a vector hydrophone array. The experimental results have illustrated the high performance of one-step PNAH based on particle velocity in the reconstruction of sound field and the advantages of a vector hydrophone array in an underwater near-field measurement.

Aperture extension for near-field acoustical holography applied to jet noise

Near-field acoustical holography (NAH) techniques are used to investigate noise source characteristics of high-power jets on military aircraft. Sound field reconstruction of large sources, measured with an aperture of limited size, may generally be performed with the use of patch NAH methods. Patch methods, such as statistically optimized near-field acoustical holography (SONAH), help to mitigate the effects of a truncated measurement aperture by avoiding the use of the spatial discrete Fourier transform operation. However, the lack of information outside the measurement aperture may lead to other errors, particularly when large propagation distances are required. Some missing data must be recovered to propagate beyond the immediate measurement region. Numerical aperture extension methods, in conjunction with SONAH, are employed to characterize high-power jet noise sound fields. These methods include complex interpolation and extension, analytic continuation, and in-plane holographic projection. [Work supported by Air Force SBIR.]

Research on Patch Near-Field Acoustic Holography

Support vector regression is used to establish a kind of patch near-field acoustic holography. The regression functions are constructed by treating the measured data on the patch holography as training samples, and then the data outside the measurement aperture are extrapolated. The experimental results show that the extrapolation of the sound pressure outside the smaller initial hologram aperture may be realized easily and effectively, and the reconstruction accuracy is satisfactory.

Hologram pressure field weighted norm extrapolation method

Hologram pressure extrapolation is a key process of Patch nearfield acoustic holography. In this paper, a pressure extrapolation method based on the weighted norm extrapolation is proposed, in which the auto-power spectrum of hologram pressure estimated by the measured data is used as a weighting function to build the weighted norm, and the hologram pressure extrapolation is realized by minimizing the weighted norm. Because both the spectrum shape and the bandwidth information about hologram pressure are utilized during the extrapolation, a more accurate extrapolated result can be achieved. The validity of the proposed method is proved by a numerical simulation and an experiment study. The simulation results show that the proposed method has obviously advantages in accurcy and computational efficiency compared with the classic wave number domain extrapolation method. Furthermore, the proposed method is used to extrapolate a pressure field generated by a camped steel plate in experiment and a satisfactory result is obtained.

Patch Near-field Acoustic Holography by Extrapolation Using Orthogonal Spherical Wave Source

In order to reduce the reconstructed errors, the near-field acoustic holography requires the measurement aperture larger than the sound source area. This limitation causes heavy measurement work and makes the NAH difficult to implement on large-scale structure. To solve this problem Patch near-field acoustic holography based on hologram pressure extrapolation is developed. A new patch near-field acoustic holography (PNAH) based on exploration using orthogonal spherical wave source is proposed. According to the solution of Helmholtz equation in spherical coordinates, the sound field can be approximately expressed as a linear sum of a series of orthogonal spherical wave function, so that the sound field generated by vibrator is approximated by superposing fields generated by a series of orthogonal spherical wave sources of different orders and the pressure exploration is realized by radiating process of these spherical wave sources. After near-field exploration the fast Fourier transform (FFT) method is implemented for reconstructing the normal velocity or the field on and near the radiator. The results of numerical simulations show that the method can work well in the small measurement aperture, reduce the requirement of measurement aperture area, and is convenient for application.

Patch nearfield acoustic holography based on the distributed source boundary point method

Patch nearfield acoustic holography based on the distributed source boundary point method (DSBPM) is proposed to reduce an error of the reconstructed result obtained from the pressure measured on a small aperture. The extrapolated data on a larger holographic surface is obtained through the measured data on the small holographic surface using the DSBPM, then the extrapolated data is used to reconstruct sound source. The simulation results and the experimental results show that the method can provide a good reconstructed result under the condition of the small holographic aperture.