• 소음진동
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• 제9권6호
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• pp.1123-1130
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• 1999

A system which is composed of a membrane and an air cavity is studied. To analyze the low frequency characteristics of a single membrane-cavity system, a plane wave model is derived. The relations among system variables, such as tension, density and stiffness, are investigated. Absorption coefficient has a maximum value at a peak frequency. In addition, a membrane-cavity system absorbs the low frequency noise with a band around peak frequency. This band is primarily determined by damping effect of the system. Furthermore, a multiple membrane-cavity system is investigated by using the transfer matrix method. To show the practical applicability of the proposed model, extensive experiments were conducted. Results show that a multiple membrane-cavity system can have broader noise reduction in the low frequency range than single.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2009년도 추계학술대회 논문집
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• pp.167-168
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• 2009

The noise sources in ship and offshore structure have an influence on adjacent receiving area through a partition between noise sources and receiving area. The partition in ship is usually made of stiffened plate. Sound transmission loss (STL) of the partition at high frequency could be improved by additional installation of insulation or wall panel. At low frequency, however, it is very difficult and needs an increase of plate thickness which causes a considerable weight increase of ship. In this paper, we have investigated the effect of the bulkhead boundary condition. From measurement result, we found that the bulkhead boundary condition can affect a lot in STL, especially at low frequency range. Finally, we get the 5dB increase in STL through the modification of boundary condition.

• 한국군사과학기술학회지
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• 제17권5호
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• pp.629-636
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• 2014

In the proposed paper, we designed low phase noise frequency synthesizer for Ku-band. The proposed up-mixing frequency synthesizer consists of narrow local oscillation part and variable frequency oscillation part. To improve the phase noise of frequency synthesizer, we analyze how the configuration of frequency synthesizer affect the phase noise. The implemented frequency synthesizer reduce the phase noise. The phase noise is -95.18dBc/Hz at 7kHz frequency offset in 16GHz and -94.27dBc/Hz at 7kHz frequency offset in 16.125GHz.

• 한국소음진동공학회논문집
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• 제15권8호
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• pp.924-930
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• 2005

Sound absorptive materials have good performance in high frequency range, not at low frequencies. Therefore it has been great challenge to develop a sound absorbing structure that is good at low frequency. We propose to use a Helmholtz resonator array panel for this purpose. A Helmholtz resonator is one of noise control elements widely used in many practical applications. The resonator is a simple structure composed of a rigid-walled cavity with a neck, but it has very high performance at resonance frequency. This paper discusses the sound absorption of Helmholtz resonator array panels at normal and random incidence. First, various experimental results are introduced and studied. Secondly, we theoretically predict the absorptive characteristics of the resonator away panel. The theoretical approach is based on the Fourier analysis for a periodic absorber. We believe that this method can be used to design a panel for low frequency noise control.

• 전기학회논문지
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• 제66권6호
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• pp.898-904
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• 2017

DFT(Discrete Fourier Transform) is one of the most widely used method to estimate the phasor of a relaying signal. The harmonics are eliminated by the DFT. However, high frequency components, except for harmonics, are not removed and cause an error in DFT-based phasor estimation process. This paper suggests high frequency noise reduction method by using a newly designed low-pass filter to estimate a signal phasor. When selecting a stop-band cut-off frequency of the low-pass filter, high frequency components generated by faults are considered. To reduce the phasor estimation delay caused by a low-pass filter, this paper proposes a low-pass filter whose settling time is reduced. An adverse effect of high frequency noise on DFT-based phasor estimation is reduced. To evaluate the performance of the proposed method, signals which are collected under a fault condition at a 345[kV] transmission system modeled by EMTP-RV are used.

• 한국음향학회지
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• 제32권5호
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• pp.369-376
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• 2013

Lowson의 음향상사식을 이용하여 시간영역에서 풍력터빈의 저주파수 소음을 예측 하였고, 관련 소음원들의 기여도를 분석하였다. 소음원으로서 날개-깃 상 평균 압력 분포를 구하기 위하여 XFOIL를 이용하였다. 이 때, 소음 예측 시 입력 값 인 유한 요소 상의 힘을 계산하기 위해 날개-깃을 여러 개의 요소로 분할하였다. 소음원을 힘 섭동항, 가속도항, 속도항으로 분리하여 주파수 기여도를 분석하였다. 끝으로, 예측 스펙트럼을 운용 중 인 풍력터빈에 대하여 측정한 저주파수 소음과 비교하였고, 그 결과 풍속 증가에 따라 힘 섭동 성분이 저주파수에서 크게 기여하는 것을 확인하였다.

• Tribology and Lubricants
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• 제19권6호
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• pp.342-348
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• 2003

The purpose of the paper is to investigate the noise characteristics of cylindrical roller bearings. For the sake of simplicity, it is assumed that the cylindrical roller bearing is infinitely long, and there is no outside force acting on the bearing. The effects of radial clearance of the bearing, viscosity of the lubricant and number of the roller on the noise of the bearing are also examined. Results show that the fundamental frequency of the bearing noise corresponds to the multiplication of number of the roller and whirling frequency of the roller center or the retainer. The acoustical frequency spectra of the roller bearing are pure tone spectra, containing the fundamental frequency of the bearing and its super­harmonics. The low viscosity of the lubricant, high radial clearance of the bearing, and low number of the roller decrease the bearing noise. The results and discussions of the present paper could aid in the low­noise design of the cylindrical roller bearing.

• 한국위성정보통신학회논문지
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• 제6권1호
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• pp.45-49
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• 2011

본 논문에서 위성통신 단말에 적용할 PLL 주파수 합성기의 잡음 특성을 분석하였고, 주파수 간격(step size)을 작게하면서 저 위상잡음 특성을 갖도록 설계 및 구현하였다. 구현된 주파수 합성기 모듈은 950~1450 MHz 대역에서 -2dBm 이상의 출력을 보였으며, 위상잡음은 10kHz offset에서 -101dBc/Hz로 측정되었다. 주파수 합성기의 위상잡음 특성을 분석한 결과 루프대역 내에서의 위상잡음은 분주기 값을 작게 하기위한 LO의 특성을 따라 감을 알 수 있었다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 춘계학술대회논문집
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• pp.1693-1698
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• 2000

In operating test of medium speed diesel engine, the large noise over 110dBA would be occurred, and silencer should be needed to prevent the transmission of noise through exhaust duct. A near neighborhood of medium speed engine test shop, outbreak of low frequency noise was reported. From the result of noise measurement, it was found that the coupling of engine noise and air column between workshops was main cause of annoying low frequency noise. From this study, 3 ways of reformation methods were proposed; insertion of plenum chamber, placement of baffles, and alteration of direction of exhaust. As a result of these modification, low frequency noise was cancelled out.

• 한국소음진동공학회논문집
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• 제17권8호
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• pp.736-740
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• 2007

A laboratory study on low frequency noise assessment has been carried out to evaluate the relevance of the weighting curve. Especially, the A-weighting curve which is used in most noise assessments has been evaluated using the acceptability limit in this study. The acceptability limit is one of the indicators in which the subjective responses were well-reflected. For the measurement of the acceptability limit, pure tone stimuli were used in the frequency range between 20 and 200 Hz. The measurement was proceeded in the anechoic chamber to minimize the background noise level. A total of 29 test subjects, who were aged between 19 to 33 years, participated in this study. They had been exposed to various stimuli for about 1 hour by supra-aural earphone. The measurement consisted of two listening sessions: hearing threshold and the acceptability limit session. The results showed that the tendency of the acceptability limit curve was approximately equal to C-weighting curve which had been found to be superior to A-weighting curve in assessment of low frequencies.