• 한국전자통신학회논문지
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• 제7권4호
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• pp.855-859
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• 2012

본 연구에서는 선형시변 발진기 위상잡음 이론에 있어서 전력 스펙트럼 밀도식의 일반화된 형태를 유도한다. 유도된 전력 스펙트럼 밀도식을 바탕으로 선형시변 발진기 위상잡음 이론은 발진 신호의 전력 보존성을 예측할 수 있음을 본 연구에서 증명한다. 게다가, 유도된 전력 스펙트럼 밀도식은 선형시변 발진기 위상잡음 이론이 기본 주파수와 그 하모닉을 포함하는 전 주파수 영역에 걸친 전력 스펙트럼의 특성을 설명 할 수 있게 한다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 춘계학술대회
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• pp.1921-1926
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• 2004

The tone generation mechanism and aeroacoustic characteristics have been investigated for flow over open cavities using direct acoustic numerical simulations. Physically the tone generation mechanism of open cavity is more complicated when flow instabilities are excited by the correlation effects of flow parameters. From non-dimensional parameter studies in very low Mach number range, it is shown that characteristics of cavity resonance inherently involve typical acoustic pattern at each discrete tone frequency, and especially in laminar flow the fundamental tone frequency is determined within flow instability criterion of laminar shear layer as well as cavity geometry, length to depth ratio.

• Steel and Composite Structures
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• 제32권3호
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• pp.305-312
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• 2019

In this study, we investigated the effect of fatigue crack propagation of the beams which have a vital importance in engineering applications, on the natural frequency of the system. Beams which have a wide range of applications, are used as fundamental structural elements in engineering structures. Therefore, early detection of any damages in these structures is of vital importance for the prevention of possible destructive damages. One of the widely used methods of early detection of damages is the vibration analysis of the structure. Hence, it is of vital importance to detect and monitor any changes in the natural frequencies of the structure. From this standpoint, in this study we experimentally investigated the effect of fatigue crack propagation on beams produced from 4140 steel, of the natural frequency of the beam. A crack was opened on the $8{\times}16{\times}500mm$ beam using a 3 mm long and 0.25 mm wide wire erosion. The beam, then, underwent 3 point bending tests at 10 Hz with a dynamic fatigue device and its natural frequencies were measured in scheduled intervals and any changes taking place on the natural frequencies of the beam were measured. This data allowed us to identify and measure the crack occurring on the beam subjected to dynamic loading, during the propagation phase. This method produced experimental data. The experimental data showed that the natural frequency of the beam decreased with the propagation of the fatigue crack on the beam.

• 한국전자파학회논문지
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• 제19권7호
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• pp.713-718
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• 2008

본 논문에서는 두 개의 $\lambda$/4 단락 스터브를 사용하여 진폭과 위상 평형 특성을 개선한 링 하이브리드(ring hybrid) 회로를 제안하였다. 제안한 회로는 기존 링 하이브리드 결합기에 존재하는 위상과 진폭 비 평형 특성을 $\lambda$/4 단락 스터브를 통해 보정한 회로이다. 제안한 링 하이브리드 결합기의 성능을 검증하기 위하여 평형 다이오드 주파수 2 체배기 회로를 설계 및 측정하였다. 실험 결과 $1.6{\sim}2.35\;GHz$의 대역 내에서 최저 12.3 dB의 변환 손실과 30 dB 이상의 기본 주파수 격리도를 보였다.

• Journal of information and communication convergence engineering
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• 제13권2호
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• pp.139-143
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• 2015

A push-push voltage controlled dielectric resonator oscillator (VCDRO) with a modified frequency tuning structure using broadside couplers is investigated. The push-push VCDRO designed at 16 GHz is manufactured using a low temperature co-fired ceramic (LTCC) technology to reduce the circuit size. The frequency tuning structure using a broadside coupler is embedded in a layer of the A6 substrate by using the LTCC process. Experimental results show that the fundamental and third harmonics are suppressed above 15 dBc and 30 dBc, respectively, and the phase noise of push-push VCDRO is -97.5 dBc/Hz at an offset frequency of 100 kHz from the carrier. The proposed frequency tuning structure has a tuning range of 4.46 MHz over a control voltage of 1-11 V. This push-push VCDRO has a miniature size of 15 mm×15 mm. The proposed design and fabrication techniques for a push-push oscillator seem to be applicable in many space and commercial VCDRO products.

• 한국산학기술학회논문지
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• 제9권6호
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• pp.1500-1505
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• 2008

본 논문은 원자력 내진등급 IIA 글로브 밸브의 지진하중에 대한 구조 건전성을 평가하였다. 밸브 구조물을 3차원 모델링하여 유한요소법을 사용하여 모달 해석 및 등가 정적 응력해석을 수행하였다. 모달 해석 결과 밸브 구조물은 충분히 강건하여 등가 정적해석이 가능하였고 해석에서 구한 응력들을 조합하여 원자력 규격집에서 제시한 허용응력과 비교하여 밸브의 구조 건전성을 판단하였다. 모든 지진하중에 대한 밸의 구조물에서의 응력은 허용값 보다 작게 분포하므로 구조적 건전성을 유지한다고 평가할 수 있다.

• 말소리와 음성과학
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• 제15권3호
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• pp.53-58
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• 2023

갑상선 절제술 후 정상적인 성대 움직임을 보이는 경우의 29.7%가 주관적인 음성문제를 호소하며, 이는 의사소통과 관련된 삶의 질 저하로 이어질 수 있다. 본 연구의 목적은 신경학적 손상이 없는 갑상선 절제술 후 음성의 음도문제를 개선하기 위해 경부운동과 semi-occluded vocal tract exercise를 적용하여 고안한 음성치료법의 효과를 알아보는 것이다. 이를 위해 갑상선 절제술 후 음도문제를 보이는 여성 10명을 대상으로 무작위 배정에 의힌 음성치료를 갑상선 술 2주 후 1회 실시하였다. 술 전과 술 후, 음성치료 직후의 음성 비교하기 위해 음향학적 분석[fundamental frequency, jitter, shimmer, noise-to-harmonics ratio, min Voice Range Profile(VRP), max VRP, VRP]을 실시하였다. 연구결과, 술 전에 비해 술 후 유의한 감소를 보였던 max VRP, VRP가 음성치료 직후 유의한 증가를 보였다. 이와 같은 결과는 본 연구의 음성치료법이 갑상선 술 후 음성문제의 주요한 증상인 고음역대 주파수 저하를 개선하는데 효과적인 방법임을 시사한다. 추후에는 본 치료효과가 장기간 지속되는지에 대한 연구가 필요할 것이다.

• 수산해양기술연구
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• 제47권3호
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• pp.234-240
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• 2011

In order to obtain the fundamental data about the behavior of sharks by underwater audible sound, this experiment was carried out to investigate the auditory characteristics of tiger shark Scyliorhinus torazame which was caught in the coast of Jeju Island by heart rate conditioning method using pure tones coupled with a delayed electric shock. The audible range of tiger shark extended from 80Hz to 300Hz with a peak sensitivity at 80Hz including less sensitivity at 300Hz. The mean auditory thresholds of tiger shark at the frequencies of 80Hz, 100Hz, 200Hz and 300Hz were 90dB, 103dB, 94dB and 115dB, respectively. The positive response of tiger shark was not evident after the sound projection of over 300Hz. At the results, the sensitive frequency range of tiger shark is narrower than that of fish that has swim bladder. In addition, it is assumed that the most sensitive frequency in auditory thresholds of Chondrichthyes is lower than that of Osteichthyes. Critical ratios of tiger shark measured in the presence of masking noise in the spectrum level range of about 60-70dB (0dB re $1{\mu}Pa/\sqrt{Hz}$) increased from minimum 27dB to maximum 39dB at test frequencies of 80-200Hz. The noise spectrum level at the start of masking was distributed at the range of about 65dB within 80-200Hz.

• Journal of Biosystems Engineering
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• 제27권1호
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• pp.67-76
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• 2002

This study was conducted to investigate the effects of impact conditions on the acoustic characteristics of a watermelon. The study was crucial to develop a device for nondestructive internal quality evaluation of a watermelon by an acoustic impulse response method. An impact device was constructed with a pendulum to hit the watermelon, a microphone to detect the acoustic impulse responses, and a digital oscilloscope and computer to store and analyze the data. The selected samples were Guemcheon cultivar watermelons(Citrulus Vulgaris Schrad) harvested on Oct. 20,1998. Sixty watermelons were tested on flour different types of sample holders, with four kinds of ball made of different materials, at four bevels of the angular position of the pendulum and distance from the watermelon to the microphone. Since the magnitudes of frequencies obtained by hitting with the steel and rubber ball were relatively small at the bandwidths of above 500 Hz, it was shown that the steel and rubber ball were not suitable far a hitting ball in the pendulum to get informations on internal quality of the watermelon. In case of using broth of the wood and acryl ball, almost the same and good acoustic responses were shown on the wide range of frequency bandwidth. Therefore, it seemed that the acryl ball was more suitable to the test than the wood ball in considering its mechanical properties. The acoustic characteristics of the watermelon were not shown a significant difference between the types of sample holder. The amplitudes of the acoustic signals and the magnitudes of frequencies from the whole samples increased with increase of the angular position of pendulum and with decrease of the distance from the watermelon to the microphone. However, the resonance resonance of the sample were almost the same regardless of the angular positions and the distances.

• 대한기계학회논문집B
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• 제26권3호
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• pp.423-431
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• 2002

An experimental study is conducted to investigate the effect of two-frequency forcing on turbulent flow behind a backward-facing step at the Reynolds number of 27000 based on the step height. The forcing is provided from a thin slit located at the edge of the backward-facing step to increase mixing behind the backward-facing step and consequently to reduce the reattachment length. With single frequency forcing, the minimum reattachment length is obtained at the non-dimensional forcing frequency (F) of St$\_$h/ = 0.29. With two-frequency forcing, a subharmonic frequency (F/2) or biharmonic frequency (2F) is combined with the fundamental frequency (F), i.e. (F, F/2) or (F, 2F) forcing is applied. In the case of (F, F/2) forcing, the reattachment length is not much sensitive to the phase difference between F and F/2. However, the reattachment length significantly depends on the phase difference between F and 2F in the case of (F, 2F) forcing. At a certain range of the phase difference, the reattachment length becomes smaller than that of the single frequency forcing.