• The Journal of the Acoustical Society of Korea
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• v.9 no.6
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• pp.62-73
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• 1990

한국 포항 연해에서 Range-dependent 환경을 수용할 수 있는 Parabolic Equation에 기초한 수치 해석법을 도입하여 수층의 음속구조, 음원 깊이 및 음원의 주파수 변화가 전달손실에 미치는 영향을 규 명하였다. 수층의 음속 구조에 따라 전달손실 분포는 크게 달라지는데 혼합층 구조와 성층화된 구조 및 수온전선 구조간의 전달손실 변화는 25~40 dB의 큰 차이를 보인다. 성층화된 음속 구조에서는 음원 깊 이 변화에 관계없이 거리에 따라 큰 전달손실을 나타낸다. 그러나 같음 음속구조하에서 주파수를 변화 시켰을 때에는 전달손실 차이가 15~30dB에 이른다. 결국 포항 여해에서 전달손실 분포는 특정 주파수에 대해서 음속구조의 시공간적 변화에 따라 크게 영향을 받는다. 또한 주어진 음속구조하에서 주파수 변 화에 따라서도 전달손실 분포가 크게 변하는데, 이는 Range-dependent 환경하에서도 최적 전달 주파수 가 결정될 수 있는 가능성을 제시한다.

• Proceedings of the Acoustical Society of Korea Conference
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• 1998.06d
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• pp.29-34
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• 1998

Egg white의 gel화에 따른 음속과 흡수의 변화가 60와 75$^{\circ}C$에서 크게 나타난 것이 egg white의 어느 단백질 성분에 의한 것인가를 조사하기 위해 egg white의 주요한 단백질 성분인 obalbumin, conalbumin, ovomucoid protein에 대해 gel화에 따른 음속 및 흡수의 변화를 온도 10-95$^{\circ}C$의 범위에서 초음파pulse법을 사용하여 측정하였다. Ovalbumin는 7$0^{\circ}C$, conalbumin는 5$0^{\circ}C$에서 gel화가 시작되었고 ovomucoid는 측정온도범위내에서는 gel화가 진행되지 않았다. Gel화하는 이상의 온도에서 음속과 흡수에 대하여 aging측정을 행하여 gel화에 의한 dam속과 흡수의 변화를 관측하였다. 그 결과 conalbumin는 5$0^{\circ}C$, ovalbumin는 75$^{\circ}C$에서 음속과 흡수의변화가 많이 일어났다. Egg white의 60와 75$^{\circ}C$의 gel화에 의한 음속과 흡수의 큰 변화는 각각 conalbumin과 ovalbumin에 의한 것임을 알았고 Conalbumin과 ovalbumin는 aging 온도를 parameter로하여 이력현상이 관측되었다.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.13 no.3
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• pp.295-301
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• 2008

Bathymetric data collected using a multi-beam echo sounder during marine scientific survey is essential for geologic and oceanographic research works. Accurate measurment of sound velocity profile(SVP) in water-column is important for bathymetric data processing. SVP can vary at different locations during the survey undertaken for wide areas. In addition, an observational error can occur when different equipments(Sound Velocity Profiler, Conductivity Temperature Depth, eXpendable BathyThermograph) are used for measuring SVP at the same water column. In this study, we used an MB-system software to show changes in bathymetry caused by variation of SVP. The analyses showed that the sound velocity(SV) changes due to the depth and thickness of thermocline had more significant effects on the resulting bathymetric data than those of surface mixed layer. The observational errors between SVP measuring instruments did not cause much differneces in the processed bathymetric data. Bathymetric survey line is better to be established to the direction that the change of temperature can be minimize to reduce the variation of SVP during the data acquisition along the survey line.

• Journal of the Korean Society of Propulsion Engineers
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• v.4 no.4
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• pp.26-35
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• 2000

This paper depicts the computational results for the axisymmetric subsonic/sonic ejector systems with a second throat. The numerical simulations are based on a fully implicit finite volume scheme of the compressible Reynolds-Averaged Navier-Stokes equations in a domain that extends from the stagnation chamber to the ejector diffuser exit. In order to obtain practical design factors for the subsonic/sonic ejector systems which are applicable to industrial vacuum pumps, the ejector throat area, the mixing section configuration, and the ejector throat length are changed in computations. For the subsonic/sonic ejector systems operating in the range of low operation pressure ratios, the effects of the design factors on the vacuum performance of the secondary chamber are discussed.

• 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.

• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.5
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• pp.518-524
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• 2004

The feasibility of Y(z) curve method of scanning acoustic microscope using high frequency transducer was experimentally studied for assessment of the thermal degradation in Co-based superalloy. Thermal degradation was performed to simulate the microstructural changes in Co-based superalloy arising from long term exposure at high temperature. Longitudinal wave velocity measured by pulse echo method using 10MHz transducer and leaky surface acoustic wave (LSAW) velocity measured by V(z) curve method using 200MHE transducer were measured to investigate the effect on thermal degradation. Ultrasonic velocity decreased as the aging time increased in both ultrasonic waves. Moreover, the low frequency longitudinal wave velocity decreased a little. Otherwise, the high frequency LSAW velocity drastically decreased up to a maximum of 4.7% at the aging time of 4,000hours. A good correlation was found between LSAW and Vickers hardness. Consequently, V(z) curve method of SAM using high frequency transducer could be a potential tool for assessing thermal degradation.

• The Journal of the Acoustical Society of Korea
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• v.14 no.1E
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• pp.5-13
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• 1995

In order to estimate the variability of the wave propagation loss (PL) du e to hourly variations of the sound speed profiles (SSPs), we conducted oceanographic measurements every hour for 39 hours in October 1993 in the Korea Strait. Currents and meteorological data were measured simultaneously to examine the causes of the temporal variations of temperatures. During the experiment, the temporal variations of temperatures in the surface layer highly depend on the water mass transport from adjacent seas. The PL for low frequency (75-300 Hz) is calculated using the parabolic equation scheme and averaged over the whole water depth. The hourly variation of the SSP may cause a PL difference of up to 10 dB over a 30-50 km range. The variability of PL, represented by standard deviation for the 39 SSPs, is as large as 3 dB over a 50 km range.

• Journal of the Korean Society of Radiology
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• v.11 no.4
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• pp.221-225
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• 2017

To evaluate the performance of ultrasound imaging system, we investigated the change of spatial resolution according to changing sonic velocity change parameter provided by ultrasound equipment. Ultrasound phantom images were obtained using a 3.0 ~ 5.0 MHz convex transducer in an ultrasound diagnostic device used at a medical institution located at Iksan. N-365 multi-purpose ultrasound phantom was used to measure longitudinal distance measurement accuracy and longitudinal and transverse resolution. In the same manner, the sonic velocity of the ultrasound equipment was changed from 1580 m/sec to 1400 m/sec in six steps, and the full width at half maximum(FWHM) was measured using the image J program to determine whether the measured values were different. As a result, lateral resolution was measured from 1.91 mm to 5.3 mm according to the speed change, and the smallest FWHM was 1.91 mm at 1420 m/sec. The axial resolution was measured from 1.03 mm to 1.14 mm according to the speed change, and the smallest FWHM was 1.03 mm at 1400 m/sec. The slower the sound velocity of the ultrasound equipment, the shorter the length of longitudinal measurement.

• The Journal of the Acoustical Society of Korea
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• v.34 no.1
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• pp.20-35
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• 2015

Underwater acoustic channel impulse responses (CIR) are influenced by sound speed profile (SSP), and the variation of CIR has significant effects on the performance of underwater acoustic communication systems. A significant change of SSP can occur within a short period, which must be considered during the design of underwater acoustic modems. This paper statistically analyzes the effect of the variation of SSP on the long-range acoustic signal propagation in shallow-water with thermocline using numerical modeling based on the data acquired from JACE13 experiment near Jeju island. The analysis result shows that CIR changes variously according to the SSP and the depth of the transmitter and receiver. We also found that when the transmitter and receiver are deeper, the variation of sound wave propagation pattern is smaller and signal level becomes higher. All CIR obtained in this study show that a series of bottom reflections due to downward refraction and small bottom loss in the shallow water with thermocline can be very important factor for long-range signal transmission and the performance of underwater acoustic communication system in time varying ocean environment can be very sensitive to the variation of SSP even for a short period of time.

• The Journal of the Acoustical Society of Korea
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• v.22 no.7
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• pp.554-561
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• 2003

Results of the numerical simulation and experimental data analysis for identification of mode cutoff frequency and estimation of sound speed of seabed from the spectrum of acoustic signal received at fixed source-receiver range are presented. Model simulations for Pekeris waveguide show that the frequency-dependent propagation loss and interference pattern are closely related to mode cutoff frequencies and it could be possible to the identify them from the changes of interference pattern. The concept considered at numerical simulations is applied to signals acquired at sea test. Cutoff frequency and sound speed of seabed are estimated from the interference pattern of measured signal. Propagation loss predicted using the estimated sound speed of seabed as model input parameter shows similar estimation result compared to propagation loss derived from measured data.