• 한국소음진동공학회:학술대회논문집
• /
• 한국소음진동공학회 2002년도 추계학술대회논문집
• /
• pp.863-868
• /
• 2002

Operational Modal Analysis also known as Ambient Modal Analysis has an increasing interest in mechanical engineering. Especially on big structures where the excitation and not less important the determination of the forces is most often a problem. In a structure like a wind turbine wing where the modes occur both close in frequency and hi-directional the ambient excitation has big advantages. In this paper modal parameters are identified from the wing by operational modal analysis. For the parameter identification both parametric and non-parametric techniques are used. Advantages and disadvantages are discussed and results from the different techniques are compared

• Journal of Mechanical Science and Technology
• /
• 제15권8호
• /
• pp.1174-1180
• /
• 2001

Various experimental data, including mixing areas, cross correlation factors, surface flow patterns on nozzle walls, and far field noise spectra, was used to draw a noise control mechanism in a supersonic jet. In the underexpanded case, mixing of the jet air with ambient air was significantly enhanced as presented before, and mixing noise was also dramatically reduced. Screech tones, in the overexpanded case, were effectively suppressed by trailing edge modifications, although mixing enhancement was not noticeable. From mixing and noise performance of nozzles with modified trailing edges, enhancing mixing through streamwise vortices seems an effective way to reduce mixing noise in the underexpanded flow regime. However, screech tones in the overespanded flow regime is well controlled or suppressed by making shock cells and/or spanwise large scale structures irregular and/or less organized by a proper selection of trailing edges. The noise field in the overexpanded flow regime was greatly affected by the symmetricity of the nozzle exit geometry. In the underexpanded flow regime, the effects of the symmetricity of the nozzle exit on mixing were negligible.

• 한국소음진동공학회:학술대회논문집
• /
• 한국소음진동공학회 2006년도 춘계학술대회논문집
• /
• pp.946-950
• /
• 2006

This paper proposed the ways for reduction of the prominent peak of the tone noise in air conditioning units. In order to find out the sources of the tone noise, the resonant frequencies and modes of the fans were investigated. Also, the effects of the ambient temperatures and material properties of the fans on the tone noise were studied. From the experiments, it was shown that the tone noise was greatly influenced by the torsional resonance of the fan and motor system and commutation frequency. In other words, the torsional resonance of the fan and motor system has not to be close to the commutation frequency (torque ripple frequency) which creates a rate of change in the angular acceleration in order to reduce the tone noise in air conditioning units.

• Journal of electromagnetic engineering and science
• /
• 제4권2호
• /
• pp.79-86
• /
• 2004

In this paper, the uncertainty in the calibration of coaxial thermal noise sources using a noise figure measuring (NFM) equipment is evaluated. Contributions to the uncertainty such as the calibration uncertainty of the standard noise source, mismatch, measurement of adapter efficiency, ambient temperature variation, and repeatability are evaluated in the frequency range of 10 MHz to 18 ㎓. Results show that the expanded uncertainty(k=2) is 0.23 ㏈ for the noise sources of 5 ㏈ and 15 ㏈ ENR, and 0.27 ㏈ for those of 21 ㏈.

• 수산해양기술연구
• /
• 제23권4호
• /
• pp.163-168
• /
• 1987

1984, 85, 87년에 걸쳐 7~8월에 부산402호(380GT, 850PS)를 이용하여 한국동해안 주요항구 입구와 외해에서 해중소음과 선박기관소음의 해중음압 분포와 그 음의 스펙트럼 특성을 해황별로 환경소음과 선박기관진동소음으로서 비교.검토한 결과를 요약하면 다음과 같다. 1. 풍속 0~2m/sec일 때 깊이 5m층에서의 환경소음의 음압준위는 부산 동항에서는 108dB, 부산 남항에서는 106dB, 구룡포항에서는 101dB로서 항내에 소음원이 많을 때는 높고, 그렇지 않을 때는 낮았다. 한반도와 울릉도 중간의 외해에서는 다같이 100dB로서 항구 부근보다 낮았다. 또 외해에 있어서 깊이에 따른 음압의 분포는 깊이 20m, 70m 층에서는 5m층에서 보다 1~2dB 낮고, 100m층 이하에서는 100dB로 거의 일정했다. 2. 풍속 10~15m/sec일 때, 외해에서의 환경소음의 음압준위는 5m층에서 108dB이고 그 후 점차 낮아져 70m 층에서 100dB이며, 그 이하에서는 100dB로 모두 같았다. 선박기관진동소음의 음압준위는 깊이 5, 50, 100, 150, 200m층에서 각각 146, 125, 112, 110, 104dB로서 깊어짐에 따라 점차 낮아졌다. 3. 풍속 10~15m/sec일 때 외해의 깊이 5m층에서 10Hz, 100Hz, 1KHz, 10KHz의 주파수대에 있어서의 스펙트럼 레벨은 해중소음이 각각 86, 75, 61, 32dB, 선박진동소음이 각각 105, 95, 86, 55dB로서 후자가 전자보다 20dB 정도 높았다. 200m층에 있어서는 전자가 80, 68, 47, 26dB, 후자가 82, 70, 59, 31dB로서 후자가 전자보다 4dB 정도 높았다.

• Structural Engineering and Mechanics
• /
• 제17권3_4호
• /
• pp.429-444
• /
• 2004

The ambient vibration measurement is a kind of output data-only dynamic testing where the traffics and winds are used as agents responsible for natural or environmental excitation. Therefore an experimental modal analysis procedure for ambient vibration testing will need to base itself on output-only data. The modal analysis involving output-only measurements presents a challenge that requires the use of special modal identification technique, which can deal with very small magnitude of ambient vibration contaminated by noise. Two complementary modal analysis methods are implemented. They are rather simple peak picking (PP) method in frequency domain and more advanced stochastic subspace identification (SSI) method in time domain. This paper presents the application of ambient vibration testing and experimental modal analysis on large civil engineering structures. A 15 storey reinforced concrete shear core building and a concrete filled steel tubular arch bridge have been chosen as two case studies. The results have shown that both techniques can identify the frequencies effectively. The stochastic subspace identification technique can detect frequencies that may possibly be missed by the peak picking method and gives a more reasonable mode shapes in most cases.

• Transactions on Control, Automation and Systems Engineering
• /
• 제3권2호
• /
• pp.133-138
• /
• 2001

This paper describes a system that can used to recognize an unknown material regardless of the change of ambient tem-perature using temperature response curve fitting and fuzzy neural network(FNN). There are some problems to realize the recogni-tion system using temperature response. It requires too many memories to store the vast temperature response data and it has to be filtered to remove noise which occurs in experiment. And the temperature response is influenced by the change of ambient tempera-ture. So, this paper proposes a practical method using curve fitting the remove above problems of memories and nose. And FNN is propose to overcome the problem caused by the change of ambient temperature. Using the FNN which is learned by temperature responses on fixed ambient temperature and known thermal conductivity, the thermal conductivity of the material can be inferred on various ambient temperature. So the material can be recognized by the thermal conductivity.

• 수산해양기술연구
• /
• 제26권2호
• /
• pp.180-183
• /
• 1990

한국 남해안에서 높게 보고되고 있는 수중소음준위에 대하여 그 소음원을 규명하고자 부산근해의 두 해역에서 파고 1~1.5m의 비교적 조용한 해상 상태일 때 시간별, 깊이별로 10KHz까지 수중소음의 변화를 관측했다. 전체적인 소음준위는 매우 높게 나타났으며, 선박통행이 비교적 뜸했던 A 정점에서는 이들의 편차가 1~2dB 이내로 작은 값을 보였는데 비해 선박 통행이 빈번했던 B 정점에서는 전 주파수대에 걸쳐 그 편차가 각각 4dB와 3dB까지 크게 나타났다. 이것은 그동안 500Hz 이상의 주파수대에서 주 소음원으로 여겨졌던 수면파 외에도 이 해역에서는 선박 소음이 주요 소음원임을 의미하고 있다.

• 한국음향학회지
• /
• 제23권2호
• /
• pp.134-145
• /
• 2004

본 연구는 수중과 수상의 송수신자간의 음성 통화를 위한 수중 무선 전화기 설계 제작과 그 성능에 관한 연구이다. 배경잡음, 다중경로 및 잠수자의 흡배기음과 같은 수중음향통신채널의 변수들과 수중 무선 전화기의 통화성능과의 관계를 해석하여 단측파대 방식으로 시스템을 제작하였다. 반송 주파수는 배경 잡음에 대한 S/N비를 높이기 위해 약 33 kHz로 하였으며 송신 음압 준위는 131 dB re 1 μPa이다. 해양에서의 성능평가 결과 거리 300 m 이하에서 문장 이해도는 80% 이상이었다.

• Wind and Structures
• /
• 제16권2호
• /
• pp.157-178
• /
• 2013

Output-only modal parameter identification is based on the assumption that external forces on a linear structure are white noise. However, harmonic excitations are also often present in real structural vibrations. In particular, it has been realized that the use of forced acceleration responses without knowledge of external forces can pose a problem in the modal parameter identification, because an external force is imparted to its impulse acceleration response function. This paper provides a three-stage identification procedure as a solution to the problem of harmonic and white noise excitations in the acceleration responses of a linear dynamic system. This procedure combines the uses of the mode indicator function, the complex mode indication function, the enhanced frequency response function, an iterative rational fraction polynomial method and mode shape inspection for the correlation-related functions of the force-embedded acceleration responses. The procedure is verified via numerical simulation of a five-floor shear building and a two-dimensional frame and also applied to ambient vibration data of a large-span roof structure. Results show that the modal parameters of these dynamic systems can be satisfactorily identified under the requirement of wide separation between vibration modes and harmonic excitations.