Near-field Acoustic Holography Method Research Articles

Measurement of structural intensity using patch near-field acoustical holography

Measurement of power flow in a structure, called the structural intensity (SI), is essential for vibration control and noise reduction. The near-field acoustical holography (NAH)-based measurement method is suitable to analyze the interrelationship between SI and acoustic intensity (AI) because NAH-based methods provide measurements of SI and AI simultaneously. Use of NAH requires the measurement of a pressure field over a complete surface located exterior to the structure. Therefore, if the measurement aperture is smaller than the structure, reconstructed results from the pressure on the finite aperture are seriously contaminated. This finite aperture effect prevents implementation of this NAH-based method on an actual large-scale structure such as a ship. Patch NAH and regularization method for SI measurement are applied to overcome this difficulty. This method enables implementation of the NAH-based SI measurement method in a large-scale structure. The effectiveness of this method is demonstrated by experiment.

Laser Doppler vibrometry and near-field acoustic holography: Different approaches for surface velocity distribution measurements

Nowadays there are several direct or indirect measurement methods for the determination of the surface velocity in vibrating structures, but two of them seem to be the most promising and interesting, in particular for vibro-acoustic problems: laser Doppler vibrometry (LDV) and near-field acoustic holography (NAH). While LDV is a direct laser-based vibration measurement technique, NAH allows the determination of the particle surface velocity starting from simultaneous microphone measurements performed on a plane array positioned near the vibrating object, although with some limitations. In this work the two structural and acoustic techniques are compared on a simple laboratory case, specifically a plate, in order to carefully and quantitatively assess the measurement uncertainty in the indirect NAH method used to estimate the vibration velocity. Advantages and disadvantages of the two methods are discussed briefly. This study was conducted within the European Growth Project “ACES” (Optimal Acoustic Equivalent Source Descriptors for Automotive Noise Problems) GRD1-1999-11202.

Experimental Results of Converted NAH Method

Among sound source localization methods, the acoustic intensity (AI) method is widely used in industry. On the other hand, nearfield acoustic holography (NAH) method, which is also a localization method, has not spread to industry use because of its difficulty in measuring high frequency measurement. For high frequencies, the NAH method requires a very large number of measurement points, which results in a prolonged measurement time and very large size. I have proposed a theoretically converted NAH method(1),(2), which uses the extra spatial-domain spectrums in the backpropagating computation of NAH method. This method can measure the sound field using a reasonable number of measurement points even for high frequencies. As a result, the workload of measurement is reduced to the same level as that using acoustic intensity method. In this paper, the theory of the proposed method is explained, and some experimental results are presented to verify this method.

706 改良近距離音響ホログラフィ法による平面音波の計測結果

I have proposed a converted Nearfield Acoustic Holography (NAH) method which lightens the workload of measurement at high frequency. In former researches, the theoretical background and the effectiveness of proposing method are explained, and are verified in numerical simulations and experimentations. In former research, the numerical simulations and experimentations are applied to the point sound source(s). However the NAH method is originally based on the plane wave theory. In this research, the experimentations are performed to the plane wave sound field created by vibrated plate. As a result, it is verified that the proposing method can reconstruct almost similar images compared with original NAH method, with lesser workload.

Implementation of a nearfield acoustic holography system based on pressure and velocity measurements

Analytical models have indicated that a pressure and velocity-based nearfield acoustic holography (NAH) method 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 of NAH for high spatial frequencies. The pressure and velocity-based NAH method will be introduced. Experimental results for a simply supported plate and cylinder will be presented. These results will be compared to results obtained using conventional NAH methods.

334 改良近距離音響ホログラフィ法の実験結果について

As a localization method, the Acoustic Intensity method is widely used. On the other hand, the Nearfield Acoustic Holography method is rarely used in actual industry though this method has high performance. There are some reasons about this. I am proposing converted Nearfield Acoustic Holography method. The purpose of this method is to lighten the workload of measurement. The workload of measurement is one of main reason that the Nearfield Acoustic Holography method is not used widely. In this paper, the experimentations are performed. And as a result of it, it is verified that the Nearfield Acoustic Holography method can be measured by same workload with the Acoustic Intensity method.

Impedance measurements around grazing incidence for nonlocally reacting thin porous layers.

For locally reacting materials with a constant surface impedance, a classical method based on the work of Chien and Soroka [J. Sound Vib. 43, 9-20 (1975)] for measuring this impedance in situ around grazing incidence is currently used. A generalization of this work to include thin nonlocally reacting materials with a surface impedance noticeably dependent on the angle of incidence is performed. It is shown that the model by Chien and Soroka can be used, though the constant surface impedance must be replaced by the impedance at grazing incidence for the evaluation of the numerical distance. Measurements performed on a thin porous layer using this method are compared with measurements performed using the near-field acoustical holography method [M. Tamura, J. Acoust. Soc. Am. 88, 2259-2264 (1990)]. Other measurements performed on a fibrous layer are in good agreement with the predicted values of the impedance at grazing incidence.

1222 音響ホログラフィ法を用いた空力騒音計測に関する研究第 2 報 : 離散周波数騒音への応用

1222 音響ホログラフィ法を用いた空力騒音計測に関する研究第 2 報 : 離散周波数騒音への応用

K-1245 音響ホログラフィ法を用いた空力騒音計測に関する研究 : 計測システムの構築と精度検討(J06-6 流体関連の騒音と振動(6))(J06 流体関連の騒音と振動)

K-1245 音響ホログラフィ法を用いた空力騒音計測に関する研究 : 計測システムの構築と精度検討(J06-6 流体関連の騒音と振動(6))(J06 流体関連の騒音と振動)

F-1034 十字アレーを用いた近距離音響ホログラフィ法の開発(G10-5 音響・騒音制御)(G10 機械力学・計測制御部門一般講演)

F-1034 十字アレーを用いた近距離音響ホログラフィ法の開発(G10-5 音響・騒音制御)(G10 機械力学・計測制御部門一般講演)

2416 近距離場音響ホログラフィーにおけるWaveletの応用(OS14-2/音・振動と設計(2))(OS14/音・振動と設計)

An important issue in noise suppression of structures, machinery such as automobiles and aircrafts is found by the precise information on source locations and noise radiation characteristics and to work out the countermeasure plans effectively. In the point of view of noise reductions, noise source identification has to be the basic issue. Experiment data of Nearfield Acoustical Holography (NAH) include all kinds of noise. Therefore it is impossible to get precise result without removing noise. In this paper, the basic theory of discrete wavelet analysis and techniques to identify the multiple noise sources are investigated. And the validity of the system of NAH method are made sure by comparison of Hanning Window method with Fast Wavelet Transform (FWT).

9732847 Study on analysis method of vehicle exterior noise - development and application of integrated near-field acoustical holography method Takuo Nishi, Hiroshi Takata, Weikang Jiang (Isuzu Motors Limited)

9732847 Study on analysis method of vehicle exterior noise - development and application of integrated near-field acoustical holography method Takuo Nishi, Hiroshi Takata, Weikang Jiang (Isuzu Motors Limited)

Reduction of requirements and restrictions for near-field acoustic holography in cavities

Recently, Williams and Houston developed a near-field acoustic holography (NAH) method for generic cylindrical cavities [Proc. Internoise 96, 2553–2558]. Previously, cavity NAH methods required restrictions or gross assumptions, which limited their use to specific cases. The Williams–Houston method, on the other hand, is valid inside any practical cylindrical boundary. The method, though, requires prior measurement of the near field of all boundaries, whether they contain sources or not, even when boundaries are locally reacting with a known acoustic impedance. Also, because of singularities in the Green’s function used, reconstructions could not be performed at some frequencies. This work is based on the Williams–Houston method, and presents several significant improvements and contributions. Most importantly, scattering off locally reacting cavity boundaries is accounted for in the Green’s function, reducing the required number measurements and processing, for a given resolution. As well, Green’s functions are solved for with consideration given to the uniqueness and existence of the acoustic field to allow reconstructions to be performed over a continuous range of frequencies. A set of cavity NAH Green’s functions are developed in Cartesian coordinates, extending the method to parallel walled rectangular (box) cavities. Reconstruction results comparing the methods will be presented.

A Study of Subsidiary Source Image Reduction in Acoustic Double Holography Method.

A sound source identification technique based on the acoustic double holography method was proposed in the previous paper. This method can achieve high resolution in sound source identification compared with the acoustic holography method. Furthermore, this method is more convenient than the nearfield acoustic holography method for measurement and calculation. However, this method still has the problem that comparatively high-level subsidiary source images appeared in the reconstructed image patterns which might have caused misjudgment. In this paper, a specific zone filtering technique applied to reconstructive calculation for reducing the subsidiary source image is proposed. By this technique, the level of subsidiary source image can be reduced without decrease in resolution of a real noise source image. Finally, in order to examine the effectiveness of the filtering technique, computer simulation and experimentation are carried out.

Near‐field acoustic holography for enclosures with unknown Green’s functions

A method for predicting the acoustic field in acoustically complex cavities is developed based upon current near‐field acoustic holography (NAH) techniques. Current NAH methods make use of analytical Green’s functions to generate predictions of global acoustic fields. For this reason, NAH is easily applied to generate predictions in free fields, and in cavities with simple geometries and uniform boundary impedances. For acoustically complex cavities, however, the absence of a known analytical Green’s function, renders current NAH methods either not solvable, or in cases where approximate Green’s functions are used, not accurate. The method requires no prior knowledge of the acoustically complex cavity’s Green’s function. As well, no approximations about the boundary conditions are required. Instead, the method takes advantage of the discrete nature of NAH predictions, in conjunction with an initial measurement of a cavities acoustic pressure field, to generate a cavity specific discrete Green’s function. Computational and experimental results will be presented along with a detailed explanation of the method. [Work supported by NASA Langley Research Center.]

Application of diffraction tomography to acoustic intensity measurement

Diffraction tomography is applied to measure acoustic intensity in three‐dimensional space. With Rytov's approximation, the homogeneous Helmholtz equation is obtained. The backpropagation algorithm is used to reconstruct the scattered field. Taking the special case of incident plane waves, the image kernel is expressed in convolution form, and reconstruction becomes a deconvolution process. An experimental setup is proposed. The object is a vibrating plate driven at its center. The acoustic pressure is measured from various angles. The pressure measurement acts as a diffraction tomogram. The particle velocity is reconstructed. The structural intensity (SI) is then computed using the finite difference method in terms of particle velocity. A cross‐section image of the SI is obtained, which is an advantage over the nearfield acoustic holography method of J. D. Maynard, E.G. Williams, and Y. Lee [J. Acoust. Soc. Am. 78, 1395–1413 (1985)], which can only give the acoustic intensity as contours and three‐dimensional plots. From the cross‐section image, one will be able to clarify the outstanding theoretical problems of active intensity and reactive intensity.