• The Journal of the Acoustical Society of Korea
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• v.29 no.7
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• pp.411-421
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• 2010

A time-reversal mirror (TRM) is useful in diverse areas, such as reverberation ing, target echo enhancement and underwater communication. In underwater communication, the bit error rate has been improved significantly due to the increased signal-to-noise ratio by spatio-temporal focusing. This paper deals with two issues. First, the optimal number of array elements for a given environment was investigated based on the exploitation of spatial diversity. Second, an algorithm was developed to determine the optimal location of the given number of array elements. The formulation is based on a genetic algorithm maximizing the contrast between the foci and area of interest as an objective function. In addition, the developed algorithm was applied to the matched field processing with ocean experimental data for verification. The sea-going data and simulation showed almost 3 dB improvement in the output power at the foci when the array elements were optimally distributed.

• The Journal of the Acoustical Society of Korea
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• v.20 no.8
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• pp.44-57
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• 2001

This paper analyses the performance of FFTSA (Fast Fourier Transform Synthetic Aperture) in the effects of temporal coherence and oscillatory towed course, which is one of the techniques for passive synthetic aperture SONAR process using linearly distributed towed array. Also this paper proposes the FFTSA technique using towed array having conformal shape to alleviate the performance degradation for estimating the incident angle under inconsistent under water environments. And this paper analyses the performance of the proposed FFTSA technique making use of conformal structure throughout exhaustive computer simulations.

• The Journal of the Acoustical Society of Korea
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• v.17 no.2
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• pp.32-42
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• 1998

본 논문은 비균일 분포 수동형 견인 배열 센서를 사용하여 입사각 추정은 물론, 도 플러 주파수를 동시에 추정해내기 위한 기법을 제안한다. 균일 선형 센서 배열을 사용하는 전통적인 수동형 견인 센서 배열 처리기법은 센서간의 등 간격 구조 및 단순한 표본 과정에 기인하여 활용 분야의 한계성을 가지며, 주된 응용 분야가 방사된 음향신호의 입사각을 추 정하는데만 국한되어 사용될 수 있다는 문제점을 내포하고 있다. 이러한 사실 때문에, 수중 천해 환경 하에서 견인함의 등속 이동에 의해 발생하는 도플러 주파수들에 대한 정보는 목 표물에 대한 식별이나 음향신호 전달 경로의 모델링에 중요한 매개변수로 작용하며, 일반적 인 수동형 견인 센서 배열의 사용에 의한 처리 방식으로는 이러한 매개변수의 추출이 불가 능하게 된다. 본 논문은 이러한 제한성을 극복하고 방사 신호들의 개별적인 입사각 및 도플 러 주파수의 동시 추정을 가능케하도록 비균일 분포의 선형 센서 배열과 Sample Skipping 기법(Sample Skipping Technique;SST)을 적용한 새로운 형태의 수동형 견인 배열 센서 처 리 기법을 제안한다.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1997.04a
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• pp.524-532
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• 1997

수중음향표적 특히 선박방사소음을 탐지하거나 식별하는 군사적 목적의 수동소나는 수중청음기 배열로 구성되며 각 배열센서에 수신된 신호에 배열 신호처리기술을 적용하여 선박의 거리, 방위 탐지는 물론 선박의 음향적 특징을 식별하는 고도의 음향장치이다. 그러나 이러한 장치운용자의 선박탐지, 식별이나 새로운 수동소나 개발, 나아가 스텔스 능력의 선박 설계를 위해서는 선박방사소음의 측정, 분석 및 예측에 관한 이해가 선행되어야 할 것이다. 본 연구는 대표적인 선박방사소음 측정시스템의 소개, 방사소음발생기구, 측정자료의 분석 및 예측에 관한 기초기술을 연구 분석한 내용이다.

• Proceedings of the Acoustical Society of Korea Conference
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• spring
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• pp.163-166
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• 2004

표면 반사파가 존재하는 무향수조 내에서 근접 음향 홀로그래피의 적용 가능성을 살펴보기 위하여 여러 변수의 변화에 따른 음장 예측 수치해석을 수행하였다. 수직 배열 센서 및 수평 배열 센서를 이용하는 경우에 대해 해석하였으며 수직 배열센서를 이용하여 한 면에서 측정하는 경우 표면 반사파에 의한 영향은 무시할 만큼 작은 것을 확인하였다. 수평 배열센서를 이용하는 경우에는 표면 반사파의 영향이 상대적으로 크게 나타나 반사파를 고려하기 위해 두개의 측정면에서의 음압값이 필요하며 직접파와 반사파를 모두 고려하는 경우 음압 예측치의 오차가 적게 나타남을 알 수 있었다.

• The Journal of the Acoustical Society of Korea
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• v.37 no.1
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• pp.53-59
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• 2018

In this study, a new design method is developed to optimize the structure of an underwater sparse array sensor. The purpose of this research is to design the structure of a sparse array that has the performance equivalent to a fully sampled array. The directional factor of a sparse planar array is derived as a function of the structural parameters of the array. With the derived equation, the structure of the sparse array sensor is designed to have the performance equivalent to that of the fully array sensor through structural optimization of the number and location of transmitting and receiving elements in the array. The designed sparse array sensor shows beam patterns very close to those of the fully array sensor in terms of PSLL (Peak Side Lobe Level) and MLBW (Main Lobe Beam Width), which confirms the effectiveness of the present optimal design method. Further, the validity of the analytic beam patterns is verified by comparing them with those from the FEA (Finite Element Analysis) of the optimized sparse array structure.

• 대한공업교육학회지
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• v.33 no.2
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• pp.273-286
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• 2008

Underwater acoustic sensor array can detect acoustic signal in underwater and the sensor array can be mounted in each left, right or front side of the UUV(Unmanned Underwater Vehicle). The sensor array could be conformal array and effected turbulent boundary layer flow noise. Therefore, in this paper numerical simulations were performed to know the how the outer flow noise affect the hydrophone which embedded in the elastomer. Corcos wall pressure model was used as turbulent boundary layer flow noise and this model was applied to the frequency density function. Characteristics of transfer function according the kx wave number were simulated and design parameters were thickness of elastomer, density, and modulus of elasticity. Based on the simulation results when increasing the thickness of elastomer noise reduction was increased. This results can be applied to the design of conformal array of UUV.

• Journal of Sensor Science and Technology
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• v.8 no.2
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• pp.102-107
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• 1999

In this paper, to develop the fiber optic hydrophone for the use of low frequency applications, two channels TDM(Time Division Multiplexing) fiber-optic hydrophone array was fabricated and their acoustic charateristics were investigated in the acoustic water tank. A fiber length of the order of 150m is wounded at the hollow cylinder type aluminum mandrel and the fundamental natural frequency of the mandrel maintained above 10kHz. An unbalanced interferometer (discrete Mach-Zehnder type) was used. Sound detection performance is tested in the underwater tank with 3kHz continuous sound source. Finally, it is shown that two channels TDM fiber-optic hydrophone array can detect 3kHz sound stably. This results can also applicable for the development of multi-channel fiber optic hydrophone array.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.10 no.12
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• pp.2151-2156
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• 2006

Beamforming for underwater acoustic communication (UAC) is affected by the broadband feature of UAC signal, which has relatively low currier frequency as compared to the signal bandwidth. The narrow-band assumption does not hold good in UAC. In this paper, we discuss a broadband FIR beamformer for UAC using the baseband equivalent way signal model. We consider the broadband FIR beamformer for QPSK UAC with carrier frequency 25kHz and symbol rate 5kHz. Array geometry is a uniform linear way with 8 omni-directional elements and sensor spacing is the half of the carrier wavelength. The simulation results show that the broadband n beamformer achieves nearly optimum signal to interference and noise ratio (SINR) and outperforms the conventional narrowband beamformer by SINR 0.5dB when two-tap FIR filter is employed at each sensor and the inter-tap delay is a quarter of the symbol interval. The broadband FIR beamformer performance is more degraded as the FIR filter length is increased above a certain value. If the inter-tap delay is not greater than half of the symbol period, SINR performance does not depend on the inter-tap delay. More training period is required when the inter-tap delay is same as the symbol period.

• The Journal of the Acoustical Society of Korea
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• v.24 no.8
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• pp.454-461
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• 2005

Time spread due to the multipath in underwater acoustic channel, induces ISI (Inter-Symbol Interference) which degrades the performance of the underwater acoustic communication system. The passive phase conjugation (PPC) which improves the signal to multipath interference ratio (SMR) and therefore reduces the frequency selectivity. is a diversity communication technique giving a less ISI under multipath fading channel. Its frequency selectivity depends on the number of receiver array sensors and time varying source to receiver range. In this study, frequency selectivity of the PPC and its effects on bit error of underwater acoustic communication is analyzed by numerical simulation.