• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.680-690
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• 2020

In ocean environment, the sound speed gradient of seawater has an important influence on far field sound propagation. The FEM/BEM is used to decouple the vibroacoustic radiation of the spherical shell, and the Green function of the virtual source chain is adopted for decoupling. For far field radiated Sound Pressure Level (SPL), the Beam Displacement Ray normal Mode (BDRM) is employed. The vibration and near-/far-field radiated SPL of spherical shell is analyzed in shallow sea uniform layer, negative/positive gradient, negative thermocline environment, and deep-sea sound channel. Results show that the vibroacoustic radiation of spherical shell acted at 300Hz can be analogous to dipole. When the radiated field of the spherical shell is dominated by large-grazing-angle waves, it can be analogous to vertically distributed dipole, and the far field radiated SPL is lower; while similar to horizontally distributed dipole if dominated by small-grazing-angle waves, and the far field SPL is high.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.23 no.2
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• pp.167-175
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• 2013

The wave propagation characteristics of an acoustic metamaterial composed of periodically repeated one-dimensional Helmholtz resonator array was investigated considering the effects of dimensional changes of the resonator geometry on the transmission coefficient and band gap. The effective impedance and transmission coefficient of the acoustic metamaterials are obtained based on the acoustic transmission line method. The designed acoustic metamaterials exhibit band gaps and negative bulk modulus that are non-existent properties in the nature. The band gap of the acoustic metamaterial is strongly dependent on the geometry parameters of Helmholtz resonators and lattice spacing. Also, a new type of metamaterial that is periodically constructed with two different resonators was designed to open the local resonance band gap without change of Bragg scattering.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.1
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• pp.53-60
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• 2010

This paper dealt with the acoustic detection of partial discharge (PD) in insulation oil for insulation diagnostics of oil immersed transformers. Electrode systems such as needle to plane, plane to plane, and floating were fabricated to simulate some defects in transformers. A wide band acoustic emission(AE) sensor with the frequency ranges of 100 kHz~1 MHz and a narrow band AE sensor with the resonant frequency of 140 kHz were used in the experiment. Also, a decoupler and an amplifier were designed to detect and amplify the acoustic signal only. The decoupler separates acoustic signal from DC source without any distortion, and the amplifier has the gain of 40 dB in frequency ranges of 11 kHz~4 MHz. In the experiment, frequency components and propagation characteristics of acoustic signal were analyzed, and an algorithm of positioning of PD occurrence by the time difference of arrival was proposed. From the results, the frequency components of the acoustic signal exist from 50 kHz to 200 kHz and the positioning error of PD calculated by three AE sensors was within 1%.

• Journal of the Institute of Convergence Signal Processing
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• v.10 no.4
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• pp.220-224
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• 2009

The authors have investigated the acoustic field analysis using the Constrained Interpolation Profile(CIP) Method recently proposed by Yabe. The present study has examined the calculation accuracy of the three-dimensional (3-D) acoustic field analysis using the type-M CIP method. In this paper we show phase error of type-M CIP method and the dependence on the wave-propagation direction in the type-M CIP acoustic field analysis, and then demonstrate that it is effective for acoustic field analysis, compared with the FDTD and the exact solution. We show the dependency on the propagation angle in the CIP acoustic field analysis.

• Computers and Concrete
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• v.18 no.2
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• pp.267-278
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• 2016

In order to observe the internal damage of concrete in real time, we introduced acoustic emission nondestructive detecting technology into a series of fracture tests; the test results revealed the whole process that concrete undergoes when it sustains damage that leads to failure, according to the change rules of the acoustic emission parameters. The results showed that both the initiation and unstable loads can be accurately determined using the abrupt change of the acoustic emission rate curves and the turning point of the acoustic emission parameters' accumulative curves. The whole process, from damage to failure, includes five phases, beginning with damage, such as cracking, a stable crack growth process, a critical unstable stage, and unstable propagation. The brittle fracture characteristics of concrete change when steel bars are joined, because the steel bars and the concrete structure bond, which causes an increase in the acoustic emission signals within the fracture process of the reinforced concrete. The unstable propagation stage is also extended. Our research results provide a valid methodology and technical explanations, which can help researchers to monitor the cracking process of concrete structures, in real time, during actual projects.

• Journal of the Korea Institute of Military Science and Technology
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• v.10 no.4
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• pp.52-61
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• 2007

Temporal and spatial variations of sea water largely affect on the pattern of underwater sound propagation. Acoustic environmental changes and their effects on underwater sound propagation in the northern part of the East Sea, which have been poorly studied mainly due to lack of observations, are investigated by analyzing the hydrographic data acquired since 1993. Severe changes in acoustic environments are associated with various physical processes such as deep convection, thermal fronts, and eddies in the northern part of the East Sea. Spatio-temporal variations of sound speed field and the layer of the maximum sound speed are categorized into six typical cases. Using a sound source of 5 kHz, acoustic transmission losses are calculated range-independently for the six typical cases. Significant differences among the patterns of transmission loss in the six cases suggest that a different tactics are required when we operate in the northern part of the East Sea.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1998.04a
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• pp.554-560
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• 1998

Acoustic performance of the absorptive mufflers used in air-conditioning and ventilating ducts, gas turbines, industrial fans influenced by the design parameters such as the length of mufflers, lining thickness, properties of the acoustic lining, and so forth. So we predicted the acoustic performance according to the design parameters. For the analysis, we assumed the radial propagation constant is locally reacted to the acoustic lining, and analyzed the acoustic performance based on the plane wave theory by using four-pole parameters.

• Journal of Advanced Marine Engineering and Technology
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• v.27 no.5
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• pp.590-599
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• 2003

In this study. object transport method based on ultrasonic flexural vibration is presented. Ultrasonic vibration generates ultrasonic traveling waves on the surface of elastic medium. Objects are transported through the interaction with traveling waves propagating in medium. Two types of transport methods are studied： frictional drive and acoustic levitation. With frictional drive, objects are transported in contact with the beam in the opposite direction of wave propagation whereas with acoustic levitation, objects are acoustically levitated above the beam surface and transported in the wave propagation direction. Transport characteristics are experimentally investigated using objects of different shapes and sizes. The transition from acoustic levitation mode to frictional drive mode is also examined. and it is found to occur when the ratio of mass to area of an object exceeds the threshold ratio of mass to area. It is envisaged that this feasibility study will serve as a stepping-stone for ultrasonic vibration to become an effective industrial material handling device in the future.

• Proceedings of the IEEK Conference
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• summer
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• pp.110-113
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• 2004

Underwater acoustic(UWA) communication is one of the most difficult field because of several factors such as multipath propagation, high temporal and spatial variability of channel conditions. Therefore, it is important to model and analyze the characteristics of underwater acoustic channel such as multipath propagation, transmission loss, reverberation, and ambient noise. In this paper, UWA communication channel is modeled with a ray tracing method and applied to image transmission. Quadrature phase shift keying(QPSK) and multichannel decision feedback equalizer(DFE) are utilized as phase-coherent modulation method and equalization technique, respectively. The objective is to improve the performance of vertical sensor array than that of single sensor in the viewpoint of bit error rate(BER), constellation output, and received image quality.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.73-78
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• 2000

On this study, the fatigue crack propagation of super duplex stainless steel is investigated in conditions of various volume fraction of austenite phase by changing heat treatment temperature. And we analysed acoustic emission signals during the fatigue test by time-frequency analysis methods. As the temperature of heat treatment increased, volume fraction of austenite decreased and coarse grain was obtained. The specimen heat treated at $1200^{\circ}C$ had longer fatigue life and slower rate of crack growth. As a result of time-frequency analyze of acoustic emission signals during fatigue test, main frequency was $200{\sim}300kHz$ having no correlation with heat treatment and crack length, and 500kHz was obtained by dimple and separate of inclusion