• 한국철도학회논문집
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• 제2권2호
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• pp.21-30
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• 1999

This study proposes the analysis and evaluation method of time series ultrasonic signal using the phase space-frequency domain. Features extracted from time series signal analyze quantitatively characteristics of weld defects. For this purpose, analysis objectives in this study are features of time domain and frequency domain. Trajectory changes in the attractor indicated a substantial difference in fractal characteristics resulting from distance shifts such as parts of head and flange even though the types of defects are identified. These differences in characteristics of weld defects enables the evaluation of unique characteristics of defects in the weld zone. In quantitative fractal feature extraction, feature values of 3.848 in the case of part of head(crack) and 4.102 in the case of part of web(side hole) and 3.711 in the case of part of flange(crack) were proposed on the basis of fractal dimension. Proposed phase space-frequency domain method in this study can integrity evaluation for defect signals of rail weld zone such as side hole and crack.

• 한국분무공학회지
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• 제10권3호
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• pp.29-37
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• 2005

The spray and acoustic characteristics of a gas/liquid swirl coaxial injector are studied experimentally. The self-pulsation is defined as a pressure and flow rate oscillations by a time-delayed feedback between liquid and gas phase. Self-pulsation has strong influences on atomization and mixing processes and accompanies painful screams. So. the spray and acoustic characteristics are investigated. Spray patterns are observed by shadow photography technique in order to determine the onset of self-pulsation. And self-pulsation boundary with Injection conditions and recess length is get. To measure the frequency of the spray oscillation. oscillation of the laser intensity which passes through spray is analyzed by Fast Fourier Transform. For acoustic tests, a PULSE System was used. Acoustic characteristics of a swirl coaxial injector are investigated according to the injection conditions. such as the pressure drop or the liquid and gas phase. and injector geometries. such as recess length and gap size between the inner and outer injector. Front the experimental results. the increase of recess length leads to the rapid increase of the sound pressure level. And as the pressure drop of the liquid phase increases. the frequency of the self?pulsation shifts to the higher frequency. The frequency of spray oscillations is the same as that of the acoustic fields by self-pulsation.

• IEIE Transactions on Smart Processing and Computing
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• 제3권3호
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• pp.142-152
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• 2014

Vehicular Ad-hoc Networks (VANETs) are comprised of wireless mobile nodes characterized by a randomly changing topology, high mobility, availability of geographic position, and fewer power constraints. Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidate for the physical layer of VANET because of the inherent characteristics of the spectral efficiency and robustness to channel impairments. The susceptibility of OFDM to Inter-Carrier Interference (ICI) is a challenging issue. The high mobility of nodes in VANET causes higher Doppler shifts, which results in ICI in the OFDM system. In this paper, a frequency domain com-btype channel estimation was used to cancel out ICI. The channel frequency response at the pilot tones was estimated using a Least Square (LS) estimator. An efficient interpolation technique is required to estimate the channel at the data tones with low interpolation error. This paper proposes a robust interpolation technique to estimate the channel frequency response at the data subcarriers. The channel induced noise tended to degrade the Bit Error Rate (BER) performance of the system. Parallel concatenated Convolutional codes were used for error correction. At the decoding end, different decoding algorithms were considered for the component decoders of the iterative Turbo decoder. A performance and complexity comparison among the various decoding algorithms was also carried out.

• 한국항공우주학회지
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• 제38권5호
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• pp.456-466
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• 2010

전형적인 하이브리드 모터의 연소 불안정 현상인 DC-Shift는 비선형적 특성을 갖는다. DC-Shift는 두 가지 형태로 나타나게 되는데, 압력이 급격하게 상승하는 양의 DC-shift와 연소 도중에 압력이 갑자기 감소하는 음의 DC-shift로 구분된다. 이 논문에서는 산화제 공급 유동 조건을 다양하게 변화시켜 음의 DC-shift 현상을 유도하고 산화제 공급 유동조건과 하이브리드 로켓이 갖고 있는 기본적인 주파수와 Helmholtz 주파수, 음향모드와의 공진을 유도하여 음의 DC-shift 현상의 발생 조건과 특징에 대하여 알아보았다. 실험 결과 연소 중 수백 Hz 대역의 압력 주파수 이동을 확인하였으며, 음의 DC-Shift가 연소로 발생하는 주파수와 유동의 흐름으로 인한 주파수간 위상이 상호 감쇠되어 발생한 것으로 나타났다. 향후 연소 중 나타나는 주파수의 변화에 따른 DC-Shift 현상에 대해서는 계속 연구되어야 한다.

• 한국조리학회지
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• 제19권1호
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• pp.215-229
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• 2013

본 연구는 테이크아웃 형태의 커피전문점에 주간과 야간으로 교대 근무를 실시하는 20대 여성 근로자 100명과 주간 근무를 실시하는 20대 여성 사무직 여성 근로자 100명의 식행동과 식품섭취 상태를 비교하였다. 본 연구의 결과를 요약하면 다음과 같다. 일반환경 요인은 월 평균 수입(p<0.001)과 재직기간(p<0.001)에서는 실험군이 유의적으로 낮게 나타났다. 생활습관 및 건강관련 요인은 실험군에서 최근 1년간 체중변화(p<0.001)과 위 장 관련 질환이 유의적(p<0.001)으로 높게 나타났다. 실험군에서 취침시간(p<0.001)은 늦게 자고, 수면시간(p<0.001)도 짧게 나타났다. 실험군은 규칙적인운동(p<0.001)은 낮고, 흡연(p<0.001)은 높게 나타났다. 식행동에서 식사동반자 부분에서 혼자 먹는 비율(p<0.001), 과식빈도(p<0.001), 아침결식빈도(p<0.001), 그리고 야식의 빈도(p<0.001)는 높게 나타났고, 규칙적인 식사(p<0.001)와 균형식의 비율은(p<0.001) 낮게 나타났다. 식품섭취빈도는 실험군에서 밥류(p<0.001), 육류(p<0.001), 생선(p<0.001), 난류(p<0.001), 두류(p<0.001), 김치(p<0.001), 채소류(p<0.001), 과일류(p<0.001)의 섭취빈도가 낮게 나타났고, 면류(p<0.001)와 빵류(p<0.001), 시리얼(p<0.01), 해조류(p<0.001), 우유(p<0.001)와 커피(p<0.001), 그리고 알콜(p<0.001)은 높게 섭취하고 있었다.

• 한국음향학회지
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• 제21권5호
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• pp.446-454
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• 2002

본 연구는 이동 표적체에서 주파수가 서로 다른 신호가 서로 다른 위치에서 발생할 때 도플러 주파수 편이량을 추정하여 이들의 상대적인 신호 발생 위치를 추적하는 도플러 스캐닝 기법에 관한 연구이다. 예를 들어 선박의 발전기와 프로펠러 등과 같은 탑재 기계장치들의 진동에 의해 야기되는 기계적 소음의 각 특징 주파수들의 도플러 주파수 편이는 최단근접거리 (CPA: Closest Point of Approach)에 따라 유일한 시간 정보를 갖고 각 소음원들의 위치에 관련되는 함수이다. 따라서 도플러 스캐닝 기법을 적용하면 이동 선박의 각 기계적 소음원들간의 공간적인 상대 위치 추정이 가능하다. 그러나 일반적으로 기계류 소음의 주파수는 저주파수대역이므로 도플러 주파수 편이량을 추정하기 위해서는 주파수 분해능이 높아야 하고 아울러 탑재장치의 공간분해능을 높이기 위해서는 동시에 시간 분해능이 높아야 한다. 따라서 상호 역비례 관계에 있는 이들 분해능을 동시에 높이기 위해 확장 칼만 필터 알고리즘을 적용하여 특징 신호들의 도플러 주파수 편이량을 추정하여 기계류의 상대적인 탑재 위치를 규명할 수 있음을 보인다. 먼저 수치모의 실험으로 그 가능성을 검증하고 자동차에 탑재된 스피커 음원을 사용한 실험 결과를 통해 그 성능을 확인하도록 한다.

• 대한청각학회지
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• 제25권2호
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• pp.80-88
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• 2021

Background and Objectives: Non-linear frequency compression (NLFC) technology compresses and shifts higher frequencies into a lower frequency area that has better residual hearing. Because consonants are uttered in the high-frequency area, NLFC could provide better speech understanding. The aim of this study was to investigate the clinical effectiveness of NLFC technology on the perception of speech and music in patients with high-frequency hearing loss. Subjects and Methods: Twelve participants with high-frequency hearing loss were tested in a counter-balanced order, and had two weeks of daily experience with NLFC set on/off prior to testing. Performance was repeatedly evaluated with consonant tests in quiet and noise environments, speech perception in noise, music perception and acceptableness of sound quality rating tasks. Additionally, two questionnaires (the Abbreviated Profile of Hearing Aid Benefit and the Korean version of the International Outcome Inventory-Hearing Aids) were administered. Results: Consonant and speech perception improved with hearing aids (NLFC on/off conditions), but there was no significant difference between NLFC on and off states. Music perception performances revealed no notable difference among unaided and NLFC on and off states. The benefits and satisfaction ratings between NLFC on and off conditions were also not significantly different, based on questionnaires, however great individual variability preferences were noted. Conclusions: Speech perception as well as music perception both in quiet and noise environments was similar between NLFC on and off states, indicating that real world benefits from NLFC technology may be limited in Korean adult hearing aid users.

• Journal of Audiology & Otology
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• 제25권2호
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• pp.80-88
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• 2021

Background and Objectives: Non-linear frequency compression (NLFC) technology compresses and shifts higher frequencies into a lower frequency area that has better residual hearing. Because consonants are uttered in the high-frequency area, NLFC could provide better speech understanding. The aim of this study was to investigate the clinical effectiveness of NLFC technology on the perception of speech and music in patients with high-frequency hearing loss. Subjects and Methods: Twelve participants with high-frequency hearing loss were tested in a counter-balanced order, and had two weeks of daily experience with NLFC set on/off prior to testing. Performance was repeatedly evaluated with consonant tests in quiet and noise environments, speech perception in noise, music perception and acceptableness of sound quality rating tasks. Additionally, two questionnaires (the Abbreviated Profile of Hearing Aid Benefit and the Korean version of the International Outcome Inventory-Hearing Aids) were administered. Results: Consonant and speech perception improved with hearing aids (NLFC on/off conditions), but there was no significant difference between NLFC on and off states. Music perception performances revealed no notable difference among unaided and NLFC on and off states. The benefits and satisfaction ratings between NLFC on and off conditions were also not significantly different, based on questionnaires, however great individual variability preferences were noted. Conclusions: Speech perception as well as music perception both in quiet and noise environments was similar between NLFC on and off states, indicating that real world benefits from NLFC technology may be limited in Korean adult hearing aid users.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.1122-1127
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• 2004

an isolated droplet combustion exposed to pressure perturbations in stagnant gaseous environment is numerically conducted. Governing equations are solved for flow parameters at gas and liquid phases separately and thermodynamic parameters at the interfacial boundary are matched for problem closure. For high-pressure effects, vapor-liquid interfacial thermodynamics is rigorously treated. A series of parametric calculations in terms of mean pressure level and wave frequencies are carried out employing a n-pentane droplet in stagnant gaseous air. Results show that the operating pressure and driving frequency have an important role in determining the amplitude and phase lag of a combustion response. Mass evaporation rate responding to pressure waves is amplified with increase in pressure due to substantial reduction in latent heat of vaporization. Phase difference between pressure and evaporation rate decreases due to the reduced thermal inertia at high pressure. In addition to this, augmentation of perturbation frequency also enhances amplification of vaporization rate because the time period for the pressure oscillation is much smaller than the liquid thermal inertia time. The phase of evaporation rate shifts backward due to the elevated thermal inertia at high acoustic frequency.

• 천문학회보
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• 제41권2호
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• pp.41.1-41.1
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• 2016

The Galactic center, Sagittarius A* (Sgr A*), is the closest supermassive black hole and emits synchrotron radiation. It provides great opportunity to study the origin of mm/sub-mm emission. Currently, two competing models have been suggested as a jet base and a radiatively inecient accretion flow (RIAF). To unveil the properties, the extremely high resolution(${\sim}10{\mu}as$) corresponding to the projected Schwarzschild radius of ~0.1AU is necessary. With KVN, a jet model can be tested by multi- frequency simultaneous observations because the optically thick surface in a jet (i.e. radio core) moves toward the center at a higher frequency. We conducted 8 observations with KVN at 43/86GHz in 2015, and found that the measured positional shift to the reference calibrator, J1744-3116, was ~0.3 mas to the south of Sgr A* using the source frequency phase referencing (SFPR) at Q/W bands for the first time. With the result, in the future, we will attempt to measure the variation of source position shifts that can constrain the direction of approaching jets and the variability of black hole activity of Sgr A*.