• The Journal of the Acoustical Society of Korea
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• v.41 no.6
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• pp.601-609
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• 2022

In August 2007, coastal upwelling occurred off the east coast of Korea, and vertical water temperature and salinity data were obtained from a real-time surface ocean buoy. Based on the time series observation data, a vertical sound velocity structure was calculated before, during, and after the occurrence of the coastal upwelling, and how the coastal upwelling affects the sound propagation and detection environment through acoustic modeling considering the horizontal scale and actual seabed topography. As a result of comparing and analyzing the low-frequency (500 Hz) sound transmission loss and the target detection range by depth using the parabolic equation model, it was analyzed that if coastal upwelling occurs, a detection gain of up to about 10 dB can be expected. In addition, through this study, it was confirmed that the characteristics of sound propagation can be greatly changed even in a short period of about 2 to 3 days before and after coastal upwelling.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.2
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• pp.78-86
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• 1990

The vertical attenuation of structureborne noise in a ship structure is studied by means of the wave guide theory. When modeling a ship structure as an acoustic wave guide system the cross mode is derived from the assumption about the boundary of the system, i.e. the constraint due to transverse frames. In this paper, the constraint is relaxed so that the displacement at the boundaries could take place. The numerical result shows better agreement with the measured one than that of the previous assumption of fixed boundary condition in the low frequency region. The effect of local changes of mass and damping factors on the attenuation losses are also investigated numerically.

• The Journal of the Acoustical Society of Korea
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• v.39 no.4
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• pp.270-278
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• 2020

In this paper, sound transmission of Micro-Perforated Plates (MPPs) installed in an impedance tube with a circular cross-section is described using an analytic method. Vibration of the plates is expressed in terms of an infinite series of modal functions, where modal function in the radial direction is given by the Bessel function. Under the plane wave assumption, a low frequency approximation is derived, and a formula for the sound transmission coefficient of multi-layered MPPs is presented using the transfer matrix method. The Sound Transmission Losses (STLs) of single and double MPPs are computed using the proposed method and compared with those done by the Finite Element Method (FEM), which shows an excellent agreement. As the perforation increases, the STL is degraded, since the STL becomes dominated by the perforation ratio rather than by vibration of the plate. The STL shows dips at natural frequencies as well as at the mass-spring-mass resonance frequency. The proposed model for the STL prediction in this study can be applied to an arbitrary number of MPPs, where each MPP may or may not have a perforation.

• Journal of the Korea Institute of Military Science and Technology
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• v.18 no.2
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• pp.201-211
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• 2015

Acoustic propagation in shallow water with changing environments is a major concern of navy. Temporal and spatial variability of acoustic propagation in the northern East China Sea (ECS) is studied, using the 11 years hydrographic data and the Bellhop acoustic model. Acoustic propagation in the northern ECS is highly variable due to extensive interaction of various ocean currents and boundaries. Seasonal variations of transmission loss (TL) with various source depths are highly affected by sharp gradient of sound speed and bottoms interaction. Especially, various bottom sediment types lead to severely degrading a waterborne propagation with bottom loss. In particular, the highly increased TL near the ocean front depends on the source position, and the direction of sound propagation.

• The Journal of the Acoustical Society of Korea
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• v.14 no.1
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• pp.108-114
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• 1995

Cylindrical chamber mufflers with an extended inlet or outlet are extensively used in many application fields to reduce the propagated noise in ducts. The basic attenuation effectivencess in the low frequency region can be explained by the reactive wave action inside the expansion chamber associated with the geometric configurations of the inlet and outlet locations, and the area expansion of the jacket. In this study, an acoustic analysis is carried out for a concentric extended pipe inserted into a simple expansion chamber. An algebraic equation is derived by using the eigenfuction expansion and orthogonality principle in which the acoustic pressures and particle velocities defined on each subdivided surface are expressed by the separable coordinates. By using the proposed analytical method, transmission losses are predicted for several configurations of the concentric extended systems and they agree very well with experimental results.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.10
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• pp.1648-1655
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• 1997

The intake noise, a major source of vehicle noises, has rapidly become a noticeable, and has been studied to reduce the level. Traditionally, the intake system has been developed through a experiment, namely, the trial and error process. This approach requires very high cost and long time consuming to develop the system. Recently, FEM and BEM are becoming useful in analysis of the intake system, and te results of analysis are very valid. But because these techniques also require high cost and long analysis time, these are generally not practical tool at the early stage of the development of an intake system. In this study, the software was developed to predict and analyze the acoustic characteristics of an intake system. It was based on the Transfer Matrix Method and operated to analyze a simplified intake system in a personal computer. It can be used early in the design stage of development of the intake system. This study presented an improvement to reduce the level of an intake noise. It was to select the optimum position of a resonator and verified by NIT/SYSNOISE, FE analysis commercial software, and testing a prototype.

• Transactions of the Korean Society of Automotive Engineers
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• v.14 no.5
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• pp.93-99
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• 2006

Flexible couplers are widely used for exhaust transmitted vibration reduction in vehicles. This paper describes an investigation into the acoustical characteristics of exhaust flexible coupler by the simulation and testing. Computational acoustic simulation is carrying out to investigate resonance frequency and transmission loss of decoupler using the boundary element method and transfer matrix approach. To confirm the acoustical simulation results of exhaust decoupler, we compare with measured experimental results by the test of transmission loss measurement system. In the comparison with simulation results and tests results, there is correctly fit the resonance frequency and the trend of transmission loss. Also, we show that the acoustical structure of decoupler is analogous to the expended tube or side branch resonator. The characteristics of exhaust decoupler have a marked increase in the acoustic attenuation at the specified frequency bend. Therefore the decoupler is applied to develop the exhaust system not only for the vibration isolator but also for the noise attenuator.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.327-330
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• 2004

파워흐름해석법은 진동에너지 흐름 형태에 따른 에너지지배방정식을 이용하는 방법으로서, 진동에너지의 공간적인 분포와 전달 경로 등을 제시할 수 있는 방법이다. 이러한 파워흐름해석법은 고주파수 대역의 진동을 해석하기 위하여 효과적으로 적용될 수 있다. 본 연구에 서는 평판 구조물 진동에 대한 파워흐름해석법의 예측 결과를 실험 결과와 비교하여 분석하였다. 분석할 진동 특성들은 손실계수, 주파수 응답함수 등을 포함한다. 그리고 이러한 분석 결과로부터 파워흐름해석법이 고주파수 진동에 효과적으로 적용될 수 있음을 보였다.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.11a
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• pp.870-873
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• 2003

This research is about the sound attenuation in the duct with lining sound absorbing material in it. Many previous researches assumed the property of lining material as locally-reacting. As the thickness of lining material thickens or the upper limit of the interested frequency range goes higher, there is a growing tendency for the experiment results to deviate from the theoretical results based on the locally reacting assumption. In this paper, the acoustic performance of the two-dimensional dissipative silencer with the propagation of sound in the absorbent was derived theoretically and calculated. The effect of increase of sound absorbing material is also considered. These results are compared from the previous results with using the locally-reacting property of sound absorbing material.

• Proceedings of the Acoustical Society of Korea Conference
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• spring
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• pp.451-454
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• 2002

HYREV (Hanyang univ. Reverberation model)은 천해에서 적용 가능한 고주파 단상태 잔향음 모델이다. 천해에서 발생하는 잔향음은 외해에 비해 경계면 산란 영향이 크기 때문에 경계면 산란 영향을 정확히 포함한 잔향음 모델 개발이 필요하다. 본 모델에서는 고유음선(eigenray) 계산을 통하여 음원과 산란체까지의 도달시간과 전달손실을 계산 하였으며 경계면 산란 예측은 복합 거칠기(composite roughness) 모델을 이용하였다. 모델의 검증을 위하여 GSM(generic sonar model) 잔향음 모의 신호 및 실측 잔향음 신호와 비교하였으며 비교 결과 GSM 보다 HYREV 모델이 천해 잔향음 예측에 적합함을 확인할 수 있었다.