• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.05a
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• pp.292-292
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• 2004

There are many difficulties to get the scattered field generated by obstacle which has arbitrary shape or irregular surface impedance by using analytic solution or numerical methods. In this study, we propose experimental method of acoustic scattering holography that can predict the far-field scattered field based on nearfield measurements. First of all, we express scattered field using K-H integral equation and compare the differences of which green's function we use. Also we consider analytic solution of scattered field by infinite cylinder to analysis for the errors due to apply cylinderical holography. So the errors which caused by holography due to frequency (ka) and microphone spacing are also analyzed by numerical simulation.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.19 no.8
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• pp.816-827
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• 2009

This paper deals with a bearing faults localization technique based on holographic approach by visualizing sound radiated from the faults. The main idea stems from the phenomenon that bearing faults in a moving vehicle generate impulsive sound. To visualize fault signal from the moving vehicle, we can use the moving frame acoustic holography [Kwon, H.-S. and Kim, Y.-H., 1998, "Moving Frame Technique for Planar Acoustic Holography," J. Acoust. Soc. Am. Vol. 103, No. 4, pp. 1734${\sim}$1741]. However, it is not easy to localize faults only by applying the method. This is because the microphone array measures noise(for example, noise from other parts of the vehicle and the wind noise) as well as the fault signal while the vehicle passes by the array. To reduce the effect of noise, we propose two ideas which utilize the characteristics of fault signal. The first one is to average holograms for several frequencies to reduce the random noise. The second one is to apply the partial field decomposition algorithm [Nam, K.-U., Kim, Y.-H., 2004, "A Partial Field Decomposition Algorithm and Its Examples for Near-field Acoustic Holography," J. of Acoust. Soc. Am. Vol. 116, No. 1, pp. 172${\sim}$185] to the moving source, which can separate the fault signal and noise. Basic theory of those methods is introduced and how they can be applied to localize bearing faults is demonstrated. Experimental results via a miniature vehicle showed how well the proposed method finds out the location of source in practice.

• Journal of KSNVE
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• v.7 no.6
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• pp.919-929
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• 1997

Induction motors are used in many areas to transform electrical energy to mechanical energy. In the design of an induction motor, not only energy efficiency but also noise becomes an important factor. To effectively address the noise problem, it will be convenient if one can see where and how noise is generated and propagated. In this study sound radiation by an induction motor is visualized using cylindrical acoustic holography. To minimize the bias error by window effect Minimum Error Window(MEW) is used. Its performance is verified by numerical simulations. Based on these theoretical understanding, sound pressure measurement with an induction motor are performed. Not only sound radiation are visualized but sound pressure level and sound power level are also estimated. Results show that the main source is located at nearly bottom part of the motor and the total sound pressure level is 49dB, which satisfies the guideline value suggested by the KS C 4202.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.11a
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• pp.33-36
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• 2007

Acoustic holography exhibits the spatial distribution of sound pressure in time or frequency domain. The obtained picture often contains far more than what we need in practice. For example, when we need to know only the locations and overall propagation pattern of sound sources, a method to show only what we need has to be introduced. One way of obtaining the necessary information is to use envelope in space. The spatial envelope is a spatially slowly-varying amplitude of acoustic waves which contains the information of sources' location. A spatial modulation method has been theoretically developed to get a spatial envelope. By applying the spatial envelope, not only the necessary information is obtained but also computation time is reduced during the process of holography. The spatial envelope is verified as an effective visualization scheme in time domain by being applied to complicated sound fields.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.19 no.10
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• pp.987-995
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• 2009

As an experimental technique to analyze the far-field characteristics of underwater cylindrical array sensors, cylindrical acoustic holography is studied. Inside an laboratory water tank, far-field directivity patterns as well as near-field source images are reconstructed from the measured hologram by hydrophone array. Approximate equation for far-field directivity estimation is derived based on stationary phase method. The simulation and experiment show well usefulness of the proposed method in application of underwater array sensors.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2009.04a
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• pp.111-116
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• 2009

As an experimental technique to analyze the far-field characteristics of underwater cylindrical array sensors, cylindrical acoustic holography is studied. Inside an laboratory water tank, far-field directivity patterns as well as near-field source images are reconstructed from the measured hologram by hydrophone array. Approximate equation for far-field directivity estimation is derived based on stationary phase method. The simulation and experiment show well usefulness of the proposed method in application of underwater array sensors.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.2
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• pp.251-259
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• 1994

Nearfield acoustic holography is known as a powerful tool to study sound radiation from a structure. In this work, the so called backward propagation of sound pressure field is studied to obtain the structure velocity distribution. The results, which were obtained using FFT algorithms, are presented for a finite plate excited at the frequencies above and below coincidence. These results illustrate the effect of stand-off distance and noise. An optimum cutoff frequency in wavenumber domain was suggested to reduce the effects of evanescent wave in the backward propagation. The experimental results were also included for a plate to demonstrate the effectiveness of the suggested cutoff frequency. The optimum cutoff frequency to exclude the unwanted noise in the process of reconstruction of the velocity field gives the good results in both simulations and experiments.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.05a
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• pp.1020-1025
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• 2002

This paper exploits how vortex noise is generated around a cylinder. This is done by utilizing an acoustic holography. In this experiment. compressed air was Infected to tile cylinder through a hose. Therefore, major noise sources were at a hose end as well as around the cylinder： vortex shedding noise. The holography results show a resultant noise picture there it is not easy to clearly sort out the shedding noise and what is generated at the end of the tube. We attempted to separate those noise by the method we developed : contribution analysis scheme. The method, in fact, was found to be efficient and practical to separate the noise field into independent noise sources. The highlights of the results are. we believe, that lift and drag noise picture are now available. This procedure does not limit its application. therefore we may use this to visualize any noise field that we want to understand.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.10a
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• pp.643-648
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• 2011

This paper deals with the ESM(equivalent source method) with the continuation of acoustic near-field for NAH(near-field acoustic holography) to overcome the finite measurement aperture effect and reconstruct the normal velocity on an arbitrarily shaped structure surface. The continuation method is an extension of the measured sound field into a region outside and is based on the Green's function relating acoustic quantities on the two conformal surfaces. This algorithm is not limited to planar surfaces and can be applied to arbitrarily shaped surfaces. The ESM is an alternative approach of BEM-based NAH for the reconstruction on a general structure. In ESM the acoustic field is represented by a set of point sources located over a surface that is close to the structure surface. The simulation results of this study shows that the reconstruction error of particle velocity on the source surface is 11% and 16% for planar and cylindrical sources separately.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.17 no.8
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• pp.693-700
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• 2007

We can predict spatial acoustic pressure distribution on the plane of interest by using acoustic holography. However, the information embedded in the distribution plot is usually much more than what we need: for example, source locations and their overall propagation pattern. One possible candidate to solve the problem is complex envelope analysis. Complex envelope analysis extracts slowly-varying envelope signal from a band signal. We have attempted to extend this method to space domain so that we can have spatial information that we need. We have to modulate two dimensional data for obtaining spatial envelope. Although spatial modulation basically follows the same concept that is used in time domain, the algorithm for the spatial modulation turns out to be different from temporal modulation. We briefly describe temporal complex envelope analysis and extend it to spatial envelope of 2-D acoustic field by introducing geometric transformation. In the end, the results of applying the spatial envelope to the holography are envisaged and verified.