• The Journal of the Acoustical Society of Korea
• /
• 제17권4E호
• /
• pp.41-47
• /
• 1998

Microphone is the first link in the speech recognition system. Depending on its type and mounting position, the microphone can significantly distort the spectrum and affect the performance of the speech recognition system. In this paper, characteristics of the speech signal for different microphones and microphone distances are investigated both in time and frequency domains. In the time domain analysis, the average signal-to-noise ration is measure ration is measured for the database we collected depending on the microphones and microphone distances. Mel-frequency spectral coefficients and mel-frequency cepstrum are computed to examine the spectral characteristics. Analysis results are discussed with our findings, and the result of recognition experiments is given.

• 한국해양공학회지
• /
• 제13권2호통권32호
• /
• pp.90-98
• /
• 1999

Boiler high-temperature pipelines such as main steam pipe, header and steam drum in fossil power plants are degraded by creep damage due to severe operationg conditions like high temperature and high pressure for an extended period time. Such material degradation lead to various component faliures causing serious accidents at the plant. Conventional measurement techniques such as replica method, electric resistance method, and hardness test method have such disadvantages as complex preparation and measurement procedures, too many control parameters, and therefore, low practicality and they were applied only to component surfaces with good accessibility. In this study, both artificial creep degradation test using life prediction formula and frequency analysis by ultrasonic tests for their preparing creep degraded specimens have been carried out for the purpose of nondestructive evaluation for creep damage which can occur in high-temperature pipelline of fossil power plant. As a result of ultrasonic tests for crept specimens, we confirmed that the high frequency side spectra decrease and central frequency components shift to low frequency bans, and bandwiths decrease as increasing creep damage in backwall echoes.

• 한국전기전자재료학회논문지
• /
• 제24권7호
• /
• pp.605-608
• /
• 2011

In this study, OSA (the obstructive sleep apnea) periods were detected in patients with OSA during sleep because of the treatment was different according to the frequency and symptoms of obstructive sleep apnea. ANS (autonomic nervous activity) was changed by obstructive sleep apnea periods so we intended to detect the periods to care the obstructive sleep apnea patients. RR intervals, SDNN (standard deviation of normal to normal) and RMSSD (root mean square standard deviation) were calculated in time domain analysis and LF (low frequency), HF (high frequency), NHF (normalized high frequency), NLF (normalized low frequency) were calculated in frequency domain analysis of HRV (heart rate variability) with obstructive sleep apnea patients. In this paper, SDNN (standard deviation of normal to normal) of time domain analysis were decreased in the obstructive sleep apnea periods. And NLF and LF/HF ratio were increased and NHF (normalized high frequency) was decreased in the frequency domain analysis. The parameters can be used to treat obstructive sleep apnea patients by detecting the obstructive sleep apnea periods such as CPAP (continuous positive airway pressure).

• 소음진동
• /
• 제6권3호
• /
• pp.341-347
• /
• 1996

진동해석을 이용한 기계계통의 진단에는 시간영역(time domain)에서의 해석 과 FFT(Fast Fourier Transform)를 이용한 주파수영역(frequency domain)에서의 해석 을 생각할 수 있다. 이중 FFT 방법은 고속연산기의 출현과 물리적인 이해의 편이성 등오로 인하여 널리 사용되고 있으나, 시간 함수인 비정상 상태신호(nonstationary signal)의 경우는 주파수영역 해석만으로는 물리적 이해를 구하는데는 한계가 있다. 그래서, 최근 신호처리기법 분야에서는 주파수영역 해석과 시간영역 해석을 보완적 으로 표현할 수 있는 기간-주파수영역 해석기법에 많은 연구가 진행되고 있다. 그중 대표적인 신호처리 기법은 Wigner-Ville Distribution이며, 특히 본 Wigner-Ville Distribution은 많은 물리적 의미를 갖고 있어 주요 연구 대상이며, 많은 응용분야 를 갖고 있다. 그러나, 기계계통중 회전체의 진동신호을 분석하여 고장 진단 및 감시를 용이하기 위해서는 새로운 형태의 시간.주파수영역 해석기법이 필요하다. 본 논문에서는 회전체의 진동신호 분석이 용이하도록 물리적인 의미와 응용상에 중점을 둔 방향서 Wigner-Ville 분포라는 시간-주파수 분석기법을 제안하였고, 회전체를 이용한 실험을 실시하였다. 그 결과 제안된 방향성 Wigner-Ville Distribution은 회전체 진동신호를 시간-주파수 영역에서 잘 표현하고 있으며, 특히 회전체의 수직 및 수평방향 진동신호로 부터 얻어지는 방향성 Wigner-Ville Distribution은 각 주파수 성분의 방향성 정보를 갖고 있어 이를 회전체의 고장 진단 및 감시에 이용 하였다.

• Journal of information and communication convergence engineering
• /
• 제15권4호
• /
• pp.244-249
• /
• 2017

Wearable sensor-based gait analysis has been widely conducted to analyze various aspects of human ambulation abilities under the free-living condition. However, there have been few research efforts on using wearable sensors to analyze human walking on an unstable surface such as on a ship during a sea voyage. Since the motion of a ship on the unstable sea surface imposes significant differences in walking strategies, investigation is suggested to find better performing wearable sensor-based gait analysis algorithms on this unstable environment. This study aimed to compare two representative gait event algorithms including time domain and frequency domain analyses for detecting heel strike on an unstable platform. As results, although two methods did not miss any heel strike, the frequency domain analysis method perform better when comparing heel strike timing. The finding suggests that the frequency analysis is recommended to efficiently detect gait event in the unstable walking environment.

• Geomechanics and Engineering
• /
• 제7권2호
• /
• pp.149-164
• /
• 2014

One of the most advanced classes of techniques for ground response analysis is based on the use of Transfer Functions. They represent the ratio of Fourier spectrum of amplitude motion at the free surface to the corresponding spectrum of the bedrock motion and they are applied in frequency domain usually by FFT method. However, Fourier spectrum only shows the dominant frequency in each time step and is unable to represent all frequency contents in every time step and this drawback leads to inaccurate results. In this research, this process is optimized by decomposing the input motion into different frequency sub-bands using Wavelet Multi-level Decomposition. Each component is then processed with transfer Function relating to the corresponding component frequency. Taking inverse FFT from all components, the ground motion can be recovered by summing up the results. The nonlinear behavior is approximated using an iterative procedure with nonlinear soil properties. The results of this procedure show better accuracy with respect to field observations than does the Conventional method. The proposed method can also be applied to other engineering disciplines with similar procedure.

• Journal of Positioning, Navigation, and Timing
• /
• 제10권4호
• /
• pp.379-385
• /
• 2021

In order to synchronize a remote system time to the reference time like Coordinated Universal Time (UTC), it is required to compare the time difference between the two clocks. GNSS Precise Point Positioning (PPP) is one of the most general geodetic positioning methods and can be used for time and frequency transfer applications which require more precise time comparison performance than GNSS code. However, the PPP technique has a main drawback of day-boundary discontinuity which comes from the PPP model that the code measurements are applied to resolve the floating carrier-phase ambiguities. The Integer PPP (IPPP) technique is one of the methods which has been studied to compensate the day-boundary discontinuities exited in the conventional PPP. In this paper, we investigate the time and frequency capabilities of PPP and IPPP by using the measurement data obtained from two time transfer receivers which are closely located and using common reference 1 Pulse Per Second (PPS) and RF signals. From the experiment, it is investigated that the IPPP method can effectively compensate the day-boundary discontinuities without producing frequency offset. However, the PPP method can generating frequency offset which can severely degrade the time comparison performance with long-term period data.

• 소음진동
• /
• 제3권4호
• /
• pp.341-352
• /
• 1993

Although a frequency analysis by FFT algorithm has been widely used in the vibration community, this approach has somewhat limited features when an analysist want to see the details of frequency trends because FFT shows only energy contents along frequencies. So the concept of instantaneous frequency that represents the dominant frequency component at each time needs to be introduced. In this paper, to get the instantaneous frequency, two methods are used. Methods using Hilbert transform and evolutionary spectrum are those. One of the problems of estimating instantaneous frequency using Hilbert transform is that it is normally very sensitive to signal to noise ratio(SNR) because of the differentiation. Moving window is applied on the estimation of instantaneous frequency, and instantaneous frequency histogram are used to handle this problem and proved to be very effective. Computer simulations for various signals have been done to understand the characteristics of instantaneous frequency. The usefulness of signal analysis using instantaneous frequency was tested by three simple experiments, which were engine experiment, beam experiment, and car door experiment. The instantaneous frequency analysis is found to be a useful technique to analyze the signals that have time varying frequencies.

• 한국공작기계학회논문집
• /
• 제16권1호
• /
• pp.26-32
• /
• 2007

This study was carried out fusion-welded butt joints in SWS 490A high strength steel subjected to tensile test that load-deflection curve. The windowed or short-time Fourier transform(WFT or STFT) makes possible for the analysis of non-stationary or transient signals into a joint time-frequency domain and the wavelet transform(WT) is used to decompose the acoustic emission(AE) signal into various discrete series of sequences over different frequency bands. In this paper, for acoustic emission signal analysis to use a continuous wavelet transform, in which the Gabor wavelet base on a Gaussian window function is applied to the time-frequency domain. A wavelet transform is demonstrated and the plots are very powerful in the recognition of the acoustic emission features. As a result, the technique of acoustic emission is ideally suited to study variables which control time and stress dependent fracture or damage process in metallic materials.

• 한국공작기계학회:학술대회논문집
• /
• 한국공작기계학회 2005년도 춘계학술대회 논문집
• /
• pp.342-348
• /
• 2005

This study was carried out fusion-welded butt joints in SWS 490A high strength steel subjected to tensile test that load-deflection curve. The windowed or short-time Fourier transform (WFT or SIFT) makes possible for the analysis of non-stationary or transient signals into a joint time-frequency domain and the wavelet transform (WT) is used to decompose the acoustic emission (AE) signal into various discrete series of sequences over different frequency bands. In this paper, for acoustic emission signal analysis to use a continuous wavelet transform, in which the Gabor wavelet base on a Gaussian window function is applied to the time-frequency domain. A wavelet transform is demonstrated and the plots are very powerful in the recognition of the acoustic emission features. As a result, the technique of acoustic emission is ideally suited to study variables which control time and stress dependent fracture or damage process in metallic materials.