• Korean Journal of Crystallography
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• v.16 no.2
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• pp.141-148
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• 2005

It is well known that the general metals have a lot of grain boundaries. The grain boundaries play a negative role to increase the resistivity and to decrease the conductivity. The small resistivity and the large conductivity have been a goal of the material scientists, and no signal noise, perfect signal transfer, and the realization of the real sound are the dream of electronic engineers and audio manias. Generally, oxygen free copper (OFC) and Ohno continuous casting (OCC) copper cables have been used for the purpose of the precise signal transfer and low noise. However they still include a lot of grain boundaries. In our study, we have grown the single crystal by the Czochralski method and succeeded to produce single crystal wires from the crystal in the dimension of $0.5{\times}0.5{\times}2500mm$. The produced wire still possesses very good single crystal properties. We observed the structure of the wire, and measured the resistance and impedance. Glow Discharge Spectrometer (GDS) was used for analyzing the compositions of copper single crystals and commercial copper. Current-Voltage curve, resistance, total harmonic distortion and speaker frequency response were measured for comparing electrical and acoustic properties of two samples.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.1
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• pp.10-16
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• 2013

This paper discussed a procedure for noise certification of Aircraft and predicting the full scale over-flight noise of propeller from acoustic wind tunnel measurement of small scale propeller. Noise Certification Procedures is established from International Civil Aviation Organization(ICAO). The data manipulations are then discussed in extrapolation to simulation flight distance and flight simulation. One of the most important point of flight simulation is adjustments for differences between wind tunnel test conditions and flight test conditions. To simulated the noise level estimation procedure for noise data post-process, simulate procedures from data of the wind tunnel noise measurement and the flight noise measurement by using a 7kg degree UAV. This study confirmed an effectively noise estimation procedures by wind tunnel noise test and flight noise test.

• Journal of the Korean Society for Nondestructive Testing
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• v.18 no.3
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• pp.191-204
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• 1998

One of the key issues in acoustic emission (AE) during hydroproof test of filament-wound composite rocket motor cases is the determination of the optimal component of elastic wave to be monitored. To solve this problem, broadband ultrasonic wave was generated into a symmetrically filament-wound composite motor case, and was received at 105 different locations after the propagation through the composite case with different distances and directions. By analysis of the received signals, characteristics of elastic wave propagation such as frequency components, the maximum propagating distance, and velocity surface were investigated. This analysis was performed for two different conditions of the motor case; air-filled and hydraulically pressurized. Based on these information, the effect of hydraulic pressure on the wave propagation characteristics was investigated and furthermore, the optimal component of elastic wave for AE during hydroproof test of the motor case was successfully determined.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.11a
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• pp.112-112
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• 2009

In this study, the effect of a long post-deposition thermal annealing(600 and 1000 $^{\circ}C$) on the surface acoustic wave (SAW) properties of polycrystalline (poly) aluminum-nitride (AlN) thin films grown on a 3C-SiC buffer layer was investigates. The poly-AlN thin films with a (0002) preferred orientation were deposited on the substrates by using a pulsed reactive magnetron sputtering system. Experimental results show that the texture degree of AlN thin film was reduced along the increase in annealing temperature, which caused the decrease in the electromechanical coupling coefficient ($k^2$). The SAW velocity also was decreased slightly by the increase in root mean square (RMS) roughness over annealing temperature. However, the residual stress in films almost was not affect by thermal annealing process due to small lattice mismatch different and similar coefficient temperature expansion (CTE) between AlN and 3C-SiC. After the AlN film annealed at 1000 $^{\circ}C$, the insertion loss of an $IDT/AlN/3C-SiC/SiO_2/Si$ structure (-16.44 dB) was reduced by 8.79 dB in comparison with that of the as-deposited film (-25.23 dB). The improvement in the insertion loss of the film was fined according to the decrease in the grain size. The characteristics of AlN thin films were also evaluated using Fourier transform-infrared spectroscopy (FT-IR) spectra and X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) images.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.14 no.9
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• pp.3639-3662
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• 2020

The network capability to accomplish its functions in a timely fashion under failures and attacks is known as survivability. Ad hoc routing protocols have been studied and extended to various domains, such as Intelligent Transport Systems (ITSs), Unmanned Aerial Vehicles (UAVs), underwater acoustic networks, and Internet of Things (IoT) focusing on different aspects, such as security, QoS, energy. The existing solutions proposed in this domain incur substantial overhead and eventually become burden on the network, especially when there are fewer attacks or no attack at all. There is a need that the effectiveness of these routing protocols be analyzed in the presence of Denial of Service (DoS) attacks without any intrusion detection or prevention system. This will enable us to establish and identify the inherently stable routing protocols that are capable to survive longer in the presence of these attacks. This work presents a DoS attack case study to perform theoretical analysis of survivability on node and network level in the presence of DoS attacks. We evaluate the performance of reactive and proactive routing protocols and analyse their survivability. For experimentation, we use NS-2 simulator without detection or prevention capabilities. Results show that proactive protocols perform better in terms of throughput, overhead and packet drop.

• Journal of Korean Academy of Nursing
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• v.46 no.4
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• pp.475-489
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• 2016

Purpose: This study was a systematic review and meta-analysis designed to investigate the effects of stimulation-oriented interventions for behavioral problems among people with dementia. Methods: Based on the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA), a literature search was conducted using seven electronic databases, gray literature, and other sources. Methodological quality was assessed using the Scottish Intercollegiate Guidelines Network (SIGN) for randomized controlled trials (RCTs). Data were analyzed using R with the 'meta' package and the Comprehensive Meta-Analysis (CMA 2.0) program. Results: Sixteen studies were included for meta-analysis to investigate the effect of stimulation-oriented interventions. The quality of individual studies was rated as '++' for eight studies and '+' for the rest. The effect sizes were analyzed according to three subgroups of interventions (light, music, and others); Hedges' g=0.04 (95% CI: -0.38~0.46), -0.23 (95% CI: -0.56~0.10), -0.34 (95% CI: -0.34~0.00), respectively. To explore the possible causes of heterogeneity ($I^2=62.8%$), meta-regression was conducted with covariates of sample size, number of sessions, and length of session (time). No moderating effects were found for sample size or number of sessions, but session time showed a significant effect (Z=1.96, 95% CI: 0.00~0.01). Finally, a funnel plot along with Egger's regression test was performed to check for publication bias, but no significant bias was detected. Conclusion: Based on these findings, stimulation-oriented interventions seem to have a small effect for behavioral problems among people with dementia. Further research is needed to identify optimum time of the interventions for behavioral problems among dementia pateints.

• The Journal of the Acoustical Society of Korea
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• v.9 no.2
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• pp.42-52
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• 1990

A method of measuring impulse response of acoustic system, two kinds of cross-correlation methods (the direct correlation method and the M-sequence modulation correlation method) were applied. According to the direct correlation method, by using stationary random noise source and by calculating the cross-correlation function between the sourece and the output signal, equivalent impulse response can be obtained not being influenced by the back ground noises. By applying this method, the measurement of echo-time patterns in rooms and oblique incident sound absorption characteristics of sound absorbing materials was carried out. In the case of the M-sequence modulation correlation method which was contrived by Aoshima, an intermittent random nosie modulated by M-sequence signal is used as the source signal, and the cross-correlation function between the M-sequence signal and the squared output signal is calculated. According to this method, equivalent energy impulse response (squared impulse response) of te propagation system can be obtained without being influenced by the back ground noises and the air fluctuation caused by wind. As the applcaition of this measuring method to the architectural acoustics, the meaurements of echo-time patterns, reverberation decays and sound pressure lev디 distributions in rooms and sound insulation efficiencies in buildings were carried out. From these experimental studies, it has been found that this M-sequence modulation correlation method is markedly useful especially for the field masurement of sound insulation under high back ground noise condition.

• Proceedings of the Acoustical Society of Korea Conference
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• spring
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• pp.237-240
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• 2000

본 연구에서는 한국 동해 대륙붕 해역에서의 주요 해양현상인 수직 수온구조의 단기변동, 수온전선, 그리고 내부파의 변동을 분석하고 이들 변화에 따른 음파의 음장변화를 고찰하였다. 한국 동해항 근해에서 수온의 수직적 변화는 계절적인 변화 이외에 약 2주간의 짧은 기간에도 매우 극적인 변화가 존재함이 실측자료를 통해 밝혀졌다. 1999년 관측된 CTD 자료를 바탕으로 음장 변화를 살펴본 결과 주파수 1kHz, 음원수심 $30{\cal}m$ 인 경우 수신기 수심에 따라 최소 3dB, 최대 10dB까지 차이를 가져올 수 있음을 알 수 있었다. 한국 동해에서 연안과 외해 사이에는 수온전선이 매우 자주 발달하며 여름에 가장 강한 것으로 알려져 있다. 동해항 근해에는 대표적인 수괴인 대마난류수와 북한한류수가 공존하며 이들의 상대적인 세력 변화 때문에 수은(음속)이 거리에 따라 급격하게 변하는 수온전선이 발달할 수 있다. 저주파수 대역 (200Hz)에 대한 간단한 시뮬레이션 결과는 수온전선이 정상적인 분포에 비해서 거리에 따라 7dB 정도의 큰 전파손실을 초래할 수 있음을 보인다. 한국 동해 연안에도 내부파가 존재한다는 사실이 최근 3년간의 연구 결과 밝혀졌다. 내부파는 외해에서 발생하여 대륙단을 거쳐 대륙붕으로 진행해 오면서 내부파 군 (Packets)으로 분산된다. 수직적 변화가 전체 수층의 $14\%$를 차지하는 간단한 형태의 내부파를 가정하여 음장변화를 시물레이션 한 결과 주파수 1 kHz, 음원수심 $20{\cal}m$인 경우 내부파는 수렴구역 형성을 현저하게 방해하여 최대 5dB까지의 차이를 유발하였다. 추후 이에 대한 연구는 내부파 전체의 시,공간적 분포 특성이 구체적으로 규명되면 보다 정확한 음장변화 추정이 이뤄져야 할 것으로 보인다. 또한 내부파와 음파의 상대적인 진행 방향에 따라 음장변화가 크게 다를 것이 예상되므로 이를 규명하기 위해서는 궁극적으로 3차원적인 음장분포 연구가 필요하다. 음향센서를 해저면에 매설할 경우 수충의 수온변화와 센서 주변의 수온변화 사이에는 어느 정도의 시간지연이 존재하게 되므로 이에 대한 영향을 규명하는 것도 센서의 성능예측을 위해서 필요하리라 사료된다.

• The Journal of the Acoustical Society of Korea
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• v.33 no.4
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• pp.232-237
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• 2014

A passive sonar of warship is composed of several directional or omni-directional sensors. In order to model the acoustic signal received into a warship sonar, the wave propagation modeling is usually required from arbitrary noise source to all sensors equipped to the sonar. However, the full calculation for all sensors is time-consuming and the performance of sonar simulator deteriorates. In this study, we suggest an asymptotic method to estimate the sonar signal arrived to sensors adjacent to the reference sensor, where it is assumed that all information of eigenrays is known. This method is developed using Taylor series for the time delay of eigenray and similar to Fraunhofer and Fresnel approximation for sonar aperture. To validate the proposed method, some numerical experiments are performed for the passive sonar. The approximation when the second-order term is kept is vastly superior. In addition, the error criterion for each approximation is provided with a practical example.

• The Journal of the Acoustical Society of Korea
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• v.32 no.4
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• pp.335-345
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• 2013

The present study investigates the effect of the inter-aural level difference(ILD) on the syllable articulation test in classrooms which can be occurred by the absorption of interior surfaces. In order to do this, the sound absorbing materials were installed in the classroom and sound pressure level(SPL) at each ear was measured using binaural recording systems. Also, syllable articulation tests were carried out at a classroom with and without sound absorption materials by 20 students who have normal hearing condition, in order to investigates the effect of the ILD on the speech intelligibility. As a result, it was found that the larger inter-aural level differences was occurred at the nearer positions to lateral walls after sound absorptions were applied to lateral walls in the classroom. At some places, the measured ILD was lager than JND of sound level (3dB). Also, it was shown that the correlation coefficient of inter-aural level difference with the score of syllable test has the significant result(-0.441). Thus, It is concluded that ILD can affect the subjective speech intelligibility in classrooms.