• The Journal of the Acoustical Society of Korea
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• v.38 no.6
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• pp.661-669
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• 2019

High frequency active sonar is appropriate for detecting small targets such as a diver in coast environment. In case of using high frequency active sonar in shallow coastal environment, a false alarm rate is high due to clutters caused by marine biological noise, ship noise, wake, etc. In this paper, we propose an algorithm for target detection which is robust against clutter in active sonar system in shallow coastal environment. The proposed algorithm increases the rate of reduction clutter using calculation of statistical characteristics of signal and a clustering method. The algorithm is evaluated and analysed with sea trial data, as a result, that shows the rate of reducing rate of clutter of 96 % and over.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.10
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• pp.401-409
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• 2018

This paper presents means of reducing noise in NSD using path control methods for Light Tactical Vehicles (LTV). NSD is applied to the rear axle of LTVs for enhancing mobility. NSD can improve mobility of vehicles with a high locking ratio but causes noise under certain conditions due to its mechanical structure. This noise results from contact between gears due to the differential role of NSD. The noise affects users, so users have continually requested noise reductions. Though the noise doesn't affect product performance and durability, and satisfies the National Defense's noise condition standards, users request for noise reduction is the focus of this research. Eliminating the source of sound for LTVs is realistically limited, so this research applies a path control method to reduce noise by controlling the path which transmits the noise. This study improves the structural delivery system and examines methods of reducing noise in LTV systems.

• The Journal of the Acoustical Society of Korea
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• v.30 no.1
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• pp.9-16
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• 2011

When a hydrofoil ship plies at high speed, there exist possibilities of collision with ocean mammals dwelling near the surface. An active sonar located within the immersed hydrofoil structure that provides the lift for the vessel, can be used for early warning of their presence. The proper functioning of the active sonar system depends on its ability to reject noise and pick up the target signal. In this article, we measured the noise on a hydrofoil of an operating ship with two flush-mounted hydrophones. The measurements were conducted for the purpose of (1) identifying the effect of operating state of machinery likes engine, cooler and generator (2) observing the change of noise depending on the measuring position (3) observing the change of noise with increasing ship speed. To verify our experiment, experiments were performed three times and the measured results are compared with other investigations and they show similarity to each other. The results are analyzed with frequency domain in order to apply to operating active sonar detecting system and focus on high frequency band within sonar's operating frequency region. Through these experiments and analysis, it is expected that we can identify the generated noise around hydrofoil where active sonar is installed and these results lead us to design active sonar that could distinguish target signal from noise more effectively.

• Journal of Korean Society of Transportation
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• v.19 no.4
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• pp.19-33
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• 2001

In the developed countries, traffic noise is one of most serious problems faced by people's lives. So the importance of the traffic noise is quite well recognized by the infrastructure planners as well as the people. The traffic noise is valued in monetary terms in some countries and it is reflected in estimating the net present value or benefit/cost ratio. On the contrary, the effects of traffic noise are not reflected in the assessment of infrastructure in most cases in Korea. However, as the income level has been increasing, more people have been becoming to put more importance on their living conditions. The purpose of this paper is to estimate the value of traffic noise in the Seoul metropolitan area. The housing price were surveyed to use the quasi-hedonic price technique. By this way, two housing prices at the same floor level in different 128 complexes in the Seoul metropolitan area were surveyed. the actual traffic noise level was also measured. The differences of housing prices and noise levels were analyzed using the various types of regression models. The value is quite different by size of house. The value of large house is higher than that of small house. Since the income level of people in large house is higher than that in small house. it might be said that value of traffic noise for high income people is higher than that for low income people. Moreover, the increase of 1dB(A) noise affects the house price by about 0.3% in Seoul metropolitan area.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.18 no.1
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• pp.20-25
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• 2008

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.

• Journal of KSNVE
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• v.4 no.3
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• pp.273-277
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• 1994

1990년 7월 이후 #2 베어링에서 발생한 오일 휩 현상을 감지하고서도 그 주원인을 일반적으로 흔히 발생하는 사항에서만 해결방안을 찾으려 노력한 결과 3년 반의 시간이 경과되었고, 뜻하지 않게도 저널 직경의 불균일에 의한 오일 휩 현상임 을 뒤늦게 알게 되었다. 철저한 점검의 필요성을 강조하지 않을 수 없다. 증기터빈 의 Overhaul 작업완료 후 Oil Flusning 절차가 있는데 이 Oil Flusning 방법에 문제 가 있다. 보통은 작업완료후 로터를 정지한 상태에서 베어링 앞에 Oil Strainer(Fine Mesh)를 설치하고 오일 펌프를 기동하여 Flushing을 하는데 이때 Strainer를 통과한 작은 이물질이 베어링내에 축적되었다가 터빈 기동시 이 이물질과 로터가 접촉되어 저널이 손상되게 된다. 인천화력3호기 #2 베어링의 오일 휩을 일으키게 한 저널의 편마모 현상도 이와같은 경우임에 틀림없다. 왜냐하면 '90, O/H시 #2 베어링 저널의 직경은 원통형이었기 때문이다. 따라서 Oil Flusning은 터빈 구조상 불가능한 경우 도 있겠지만 베어링 by-pass Oil Flushing 시스템으로 개조 운영되어야 저널 및 베어링 마멸현상을 제거할 수 있고 이로인한 오일 휩 현상도 예방할 수 있을 것으로 판단된다.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.25 no.1
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• pp.40-47
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• 2015

Dynamic response of any small structure is always affected by the mass of the attached accelerometer. This paper predicts the natural frequencies and frequency response functions by removing the mass loading effect from the accelerometer. This mass loading is studied on a simple cantilever beams by varying the location of accelerometer. By using sensitivity analysis with iteration method, accelerometer mass and location are obtained. The predicted natural frequencies of the small cantilever beam without the accelerometer's mass show good agreement with the structural re-analysis.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.11a
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• pp.722-726
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• 2007

In this paper we deal with a design of the integral sliding mode controller to suppress the disturbance force acting on the suspension system of the magnetically levitated train system. One of the important factors that cause the disturbance force acting on the suspension system comes from the low propulsion speed of linear induction motor. In this paper integral sliding mode controller is employed to reject the disturbance force produced by the propulsion system of the linear induction motor. In order to show the effectiveness of the designed controller a dynamic simulation is utilized and the sliding mode controller without integral compensator is compared with the proposed integral sliding mode controller to suppress the disturbance force.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.11a
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• pp.159-162
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• 2007

뉴테이션(Nutation, 장동)은 자이로스코프와 같은 큰 축 대칭 물체의 회전축에 대해 작은 불규칙적인(어원적으로 "nodding") 운동이다. 위성체가 회전상태에 돌입하게 될 때 위성체가 순수한 회전 운동이 아닌 뉴테이션 운동을 일으킬 경우, 이를 제거하기 위하여 뉴테이션댐퍼(NutationDamper)를 사용한다. 유도탄에 들어가는 부품 중에서 자이로조립체 내에 들어 있는 뉴테이션댐퍼의 개발을 하고자 본 연구를 수행 하게 되었으며, 자이로조립체 내에서 뉴테이션댐퍼의 역할은 자이로조립체의 원주상에 환형 튜브 형상으로 장착되어 자이로조립체가 세차운동(Precession)을 할 때 부가적으로 수반되는 바람직하지 못한 떨림(Wobble motion)인 뉴테이션을 감쇠시키는 기능을 수행한다.

• Journal of KSNVE
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• v.11 no.1
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• pp.49-56
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• 2001

In this paper, two plant models, of which one is newly developed and the other one is the conventional one, of the focus servo system of DVD drive are presented and a two-degree-of freedom controller consisted of Inverse dynamics feedforward and LQG/LTR feedback controller is designed. The newly developed plant model is used to design the feedforward controller and the conventional model is used for the design of feedback controller. The output of newly developed model is the displacement of objective lens and the output of conventional model is the focus error of the DVD focus servo system. The displacement of the objective lens is estimated by the dynamics model of the DVD focus servo system. The disturbance rejection performance of the two-degree-of freedom controller is compared with that of an LQG/LTR one.