• The Journal of the Acoustical Society of Korea
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• v.32 no.1
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• pp.1-13
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• 2013

Salinity does not generally affect sound speed because it shows very small variations in the ocean. However, low salinity water appears in the Western Sea of Jeju Island every summer so that sound speed and sound propagation can change near sea surface. We calculated Sound Speed Profile (SSP) using vertical profiles of temperature and salinity, which were averaged over years of normal salinity and low salinity (<28 psu) from 30 years (1980~2009) at 3 sites of Korea Oceanographic Data Center (KODC). As a result, sound speed variation by low salinity alone was -5.36 m/s at sea surface and -1.35 m/s at 10m depth for low salinity environments. Gradient of SSP was positive down to 5 m depth due to decrease of sound speed near surface, leading formation of haline channel. Simulation of acoustic propagation using a ray model (Bellhop) confirmed the haline channel. Haline channel has formed 4 times while hydrostatic channel controlled by only pressure has formed 9 times for 30 years. The haline channel showed larger critical angles of rays than hydrostatic channel. Haline channel was also formed at some sites among 20 measurement sites in low salinity water mass which appeared on August $1^{st}$ 2010.

• The Journal of the Acoustical Society of Korea
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• v.34 no.1
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• pp.1-11
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• 2015

Salinity affects sound speed in the low salinity environment, in the seas where freshwater from large rivers and flows into the marginal sea area near the Yangtze River and the Niger River. In this paper, SSC (Surface Sound Channel) formed by low salinity water was investigated in the East China Sea and the Gulf of Guinea of rainy season. The data from KODC (Korea Oceanographic Data Center) in the East China Sea and from ARGO (Array for Real-time Geostrophic Oceanography) in the Gulf of Guinea of the tropical area were used for analysis. SSC haline channel was formed 14 times among 32 SSC occurrences when the 90 data from 9 points were analyzed during a decade (2000 ~ 2009) in the East China Sea. In the Gulf of Guinea, haline channel was formed 18 times among 20 SSC occurrences during 3 years (2006 ~ 2009). When the sound speed gradient was analyzed from temperature-salinity gradient diagram, the gradients of both salinity and temperature affect SSC formation in the East China Sea. In contrast, the salinity gradient mostly affects SSC formation due to the least change of temperature in the well-developed mixed layer in the Gulf of Guinea. Their acoustic characteristics show that channel depth is 6.5 m, critical angle is $1.5^{\circ}$ and difference of transmission loss between surface and thermocline is 11.5 dB in the East China Sea, while channel depth is 18 ~ 24 m, critical angle is $4.0{\sim}5.4^{\circ}$ and difference of transmission loss is 21.5 ~ 27.9 dB in the Gulf of Guinea. These results are expected to be used as a basic understanding of the acoustic transmission changes due to low salinity water at the estuaries and the ocean with heavy precipitation.

• Information and Communications Magazine
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• v.33 no.8
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• pp.63-70
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• 2016

본고에서는 최근 활발하게 연구되고 있는 수중 음향통신을 위한 물리계층 기술에 대하여 알아 본다. 수중음향통신은 지상의 전파를 이용한 무선 통신 기술과 달리 음파를 이용한다. 음파는 수중에서 약1500m/s로 매우 저속이고 시간에 따른 다중 경로와 해수면과 해저면에서의 반사가 발생한다. 또한 수온, 염분, 수압, 해류와 해저지형 등에 의해 신호의 왜곡 및 손실이 일어나기 때문에 수중음향통신은 지상에서 전파를 이용한 통신에 비하여 매우 어려운 일이다. 본고에서는 이러한 수중음향채널의 특성을 살펴보고 링크버짓 계산을 한다. 그리고 수중음향통신을 위한 물리계층 변조기법을 살펴본다. 특히 OFDM 변조기법에 대하여 자세히 설명하고 실해역 측정을 통한 수중채널 특성을 기반으로 채널을 모델링하고 OFDM 변조기법을 위한 파라미터 선정 및 성능비교를 하였다.

• The Journal of the Acoustical Society of Korea
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• v.8 no.4
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• pp.13-22
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• 1989

Underwater channels can be regarded as time-varying systems in view of the acoustical characteristics due to the fact that the characteristics of the channel are affected by the environmental and geometrical conditions. Especially in shallow water case, the surface and bottom conduct as a waveguide so echo effect due tu the multipath reflections are severe. Therefore in shallow water communications, it is very important to equalize the transmitted signals distorted by the underwater channels with time-varying multipath fading. In this paper an equalizer system which employs the frequency domain adaptive filter to equalize the channels using inverse modeling technique is introduced.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2013.10a
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• pp.402-404
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• 2013

The wireless communication channel different speed by depth of water or salt and it is influenced by multi-path according underwater. In the paper, MIMO(Multi-input-Multi-Output) system used turbo Equalizer combining Equalizer with Turbo codes for data rates by multi-path channel. we proposed and simulated that the Decision-Directed method used for phase offset. The simulation of proposed method show that the bit-error rate performance can be severely affected by phase errors.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.1
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• pp.16-24
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• 2011

The sound speed of seawater can be calculated by the empirical formula as a function of temperature, salinity and pressure. It is little affected by salinity because the average salinity is 34 psu and varies within a few psu seasonally and spatially in the ocean. Recently, low-salinity water of 24 psu flows into the western sea area of Jeju Island due to the flood of the Yangtze River in China during summer, affecting sound speed profile. In this paper, it was analyzed how environmental changes affected to the underwater communication - the sound speed of low-salinity water was calculated, and the communication channel was estimated by the simulated acoustic rays while the transmitting and receiving depth and the range were varied with and without the low-salinity layer. And The BER (Bit error rate) was calculated by BPSK(Binary phase shift key) modulation and the effects of the low-salinity water on the BER was investigated. The sound speed profile was changed to have positive slope by the low-salinity layer at the sub-surface up to 20 m of depth, forming acoustic wave propagation channel at the sub-surface resulting in the decrease of most of the BER Consequently, this paper suggests that it is important to consider changes of the ocean environment for correctly analyzing the underwater communication and the detection capability.

• The Journal of the Acoustical Society of Korea
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• v.33 no.5
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• pp.273-281
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• 2014

This paper analyzes the effect of a thermocline on the long-range acoustic signal propagation using the experimental data acquired in the shallow water near Jeju island. Temperature and salinity measurement data in Korea Oceanographic Data Center (KODC) show that the seasonal thermocline exists near Jeju island, and, under the thermocline, the bottom loss property strongly affects the long-range propagation of acoustic signal along the down-ward refractive paths. We estimate the bottom loss under the thermocline using experiment data obtained near Jeju island in May, 2013. The result shows that the estimated bottom losses are below 3 dB and the higher level signal is received at the deeper receiver depths. This shows that the acoustic trapping under the thermocline can be a viable long-range signal transmission channel in the shallow water with a thermocline.

• Composites Research
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• v.31 no.4
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• pp.145-150
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• 2018

Various structural health monitoring (SHM) systems have been suggested for aerospace industry in order to increase its life-cycle and economic efficiency. In the case of aircraft structure madden with metal, a major concern was hot spots, such as notches, bolts holes, and where corrosion or stress concentration occurs due to moisture or salinity. However, with the increasing use of composites in the aerospace industry, further advanced SHM systems have been being required to be applied to composite structures, which have much complex damage mechanism. In this paper, a method of acoustic emission localization for composite structures using Q-switched laser and multiple Amplifier-integrated PZTs have been proposed. The presented technique aims at localization of the AE with an error in distance of less than 10 mm. Acoustic emission simulation and the localization attempt were conducted in the composite structure to validate the suggested method. Localization results, which are coordinates of detected regions, grid plots and color intensity map have been presented together to show reliability of the method.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.22 no.2
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• pp.311-318
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• 2021

Underwater, signals are transmitted by sound waves. Sound waves are transmitted through a multipath, either directly or through reflection, due to the variety of underwater environmental characteristics. In such diverse and complex underwater environments, tests must be conducted to determine the extent of the hazard from the survivability and pitfalls of submarines by measuring the underwater radiated noise. Usually, the sound source level measurement of underwater radiated noise should be made within the closest point (CPA: Closest Point of Approach) ± a few meters between the measurement sensor and the submarine. In this study, FDOA and TDOA methods were proposed to estimate the underwater source range. A simulation based on the underwater channel model confirmed the performance of the proposed method.