• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2007년도 춘계학술대회논문집
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• pp.127-131
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• 2007

A laboratory study on low frequency noise assessment was carried out to determine the acceptability limits in the frequency range between 20 and 200 Hz. The acceptability limits were tested in the anechoic chamber to minimize the background noise. A total of 30 test subjects, who were aged between 19 to 33 years, participated in this study. They were exposed to various stimuli for about 1 hour by supra-aural earphone. The experiment consisted of two listening sessions; hearing threshold and the acceptability limits session. The results showed that the trend of the acceptability limits curve was approximately equal to C-weighting curve which had been found to be superior to A-weighting curve in low frequency noise assessment.

• 한국정밀공학회지
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• 제32권10호
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• pp.845-850
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• 2015

This study was designed to investigate the feasibility of utilizing an adaptation for selective elicitation of tactile sensations by means of transcutaneous electrical stimulation. We conducted the first experiment to investigate how the stimulation frequency affected the adaptation. Twenty healthy subjects participated in the second experiment to confirm our proposal that the perception intensity of the low-frequency vibration can be enhanced after a high-frequency adaptation, and vice versa. It was found that (1) a low-frequency stimulation did not adapt the nerve afferents responsible for the high-frequency vibration, (2) a high-frequency stimulation affected the nerve afferents responsible for the low-frequency vibration, but adapted to the pressure sensation more intensely, and (3) more than 62% of the subjects reported a more clear selective sensation after the adaptation had lessened or depressed the unwanted sensation. The observations showed that adaptation of the nerve afferent could be utilized for selective elicitation of tactile sensations.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2008년도 추계학술대회 논문집 Vol.21
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• pp.193-193
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• 2008

Wearable and ubiquitous micro systems will be greatly growing and their related devices should be self-powered in order to avoid the replacement of finite power sources, for example, by scavenging energy from the environment. With ever reducing power requirements of both analog and digital circuits, power scavenging approaches are becoming increasingly realistic. One approach is to drive an electromechanical converter from ambient motion or vibration. Vibration-driven generators based on electromagnetic, electrostatic and piezoelectric technologies have been demonstrated. Among various generator types proposed so far, piezoelectric generator possesses considerable potential in micro system. To overcome low mechanical-to-electric energy conversion, the piezoelectric device should activate in resonance mode in response to external vibration. Normally, the external vibration excretes at low frequency ranging 0.1 to 200 Hz, whereas the resonant frequencies of the devices are fixed as constant. Therefore, keeping their resonant mode in varying external vibration can be one of important points in enhancing the conversion efficiency. We investigated the possibility of use of multi-bender type piezoelectric devices. To match the external vibration frequency with the device resonant frequency, the various devices with different resonant frequency were chosen.

• 한국소음진동공학회논문집
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• 제19권7호
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• pp.679-684
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• 2009

Low frequency exhaust noise of marine diesel engine is one of the most important noise sources in vessels. However, conventional absorptive silencers are ineffective to control exhaust noise because of low absorption in the low frequency range. In the paper, exhaust noise control of marine diesel engine was studied by using the hybrid silencer, which was composed of virtually divided array of concentric hole-cavity resonators and conventional absorptive silencer. A series of tests including field tests were performed to investigate the acoustic performance of the hybrid silencer. Consequently, its high performance of 5${\sim}$10 dB noise reduction in the low frequency range was confirmed and it is expected to be very helpful in reducing the exhaust noise of marine diesel engine.

• Journal of the Optical Society of Korea
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• 제20권6호
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• pp.762-769
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• 2016

The dynamic modulation transfer function (MTF) for image degradation caused by sinusoidal vibration is formulated based on a Bessel function of the first kind. The presented method makes it possible to obtain an analytical MTF expression derived for arbitrary frequency sinusoidal vibration. The error obtained by the use of finite order sum approximations instead of infinite sums is investigated in detail. Dynamic MTF exhibits a stronger random behavior for low frequency vibration than high frequency vibration. The calculated MTFs agree well with the measured MTFs with the slant edge method in imaging experiments. With the proposed formula, allowable amplitudes of any frequency vibration are easily calculated. This is practical for the analysis and design of the line-of-sight stabilization system in the remote sensing camera.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2003년도 춘계학술대회논문집
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• pp.393-396
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• 2003

Conventional method for designing and installing anechoic chambers is to utilize porous wedges for the sound absorbers. As cutoff frequency lowers down such as 63Hz or 50Hz, the corresponding long wedges diminish the free field area of the chamber. In this study, a new broadband compact absorber(BCA) is introduced which absorbs acoustic energy down to 50Hz. Most prominent is that it measures only 250mm thick. A freely vibrating panel between the non-fibrous absorbers allows tuned absorption at the low frequency region in addition to the high frequency absorption resulted from the conventional absorber installed at the front. Standing waves at low frequency range are suppressed as the BCA modules which are tuned to the corresponding modes absorb sound energy effectively, resulting in anechoic condition. Not only the low frequency performances, but the high frequency absorption is measured to meet adequate conditions for the anechoic chamber. Realized BCA chambers are presented.

• 한국정밀공학회지
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• 제21권2호
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• pp.67-73
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• 2004

Some of Spacecraft's structures are flexible so that a certain expected disturbance can easily excite a low frequency vibration on these structures, having very low natural damping. Such vibration will degrade the performance of the system, which should to be kept in a specific shape or attitude against the undesired vibration. In this paper, LQG/LTR controller is developed using an additional dynamic model to increase the performance of the frequency responses at low frequency area. This study presents that the LQG/LTR design was an effective controller for the flexible structure.

• Journal of Electrical Engineering and Technology
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• 제4권2호
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• pp.255-265
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• 2009

This paper presents the acoustic noise and mechanical vibration reduction of a coreless brushless DC motor with an air dynamic bearing used in a digital lightening processor. The coreless brushless DC motor does not have a stator yoke or stator slot to remove the unbalanced force caused by the interaction between the stator yoke and the rotor magnet. An unbalanced force makes slotless brushless DC motors vibrate and mechanically noisy, and the attractive force between the magnet and the stator yoke increases power consumption. Also, when a coreless brushless DC motor is driven by a $120^{\circ}$ conduction type inverter, high frequency acoustic noise occurs because of the peak components of the phase currents caused by small phase inductance and large phase resistance. In this paper, a core-less brushless DC motor with an air dynamic bearing to remove mechanical vibration and to reduce power consumption is applied to a digital lightening processor. A $180^{\circ}$ conduction type inverter drives it to reduce high frequency acoustic noise. The applied methods are simulated and tested using a manufactured prototype motor with an air dynamic bearing. The experimental results show that a coreless brushless DC motor has characteristics of low power consumption, low mechanical vibration, and low high frequency acoustic noise.

• 대한의용생체공학회:의공학회지
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• 제7권2호
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• pp.117-118
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• 1986

It is well known that nonlinear propagation characteristics of the wave in the tissue may give very useful information for the medical diagnoisis. In this paper, a new method to detect nonlinear propagation characteristics of the internal vibration in the tissue for the low frequency mechanical vibration by using bispectral analysis is proposed. In the method, low frequency vibration of f0( = 100Hz) is applied on the surface of the object, and the waveform of the internal vibration x (t) is measured from Doppler frequency modulation of silmultaneously transmitted probing ultrasonic waves. Then, the bispectra of the signal x (t) at the frequencies (f0, f0) and (f0, 2f0) are calculated to estimate the nonlinear propagation characteristics as their magnitude ratio, w here since bispectrum is free from the gaussian additive noise we can get the value with high S/N. Basic experimental system is constructed by using 3.0 MHz probing ultrasonic waves and the several experiments are carried out for some phantoms. Results show the superiority of the proposed method to the conventional method using power spectrum and also its usefulness for the tissue characterization.

• 센서학회지
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• 제18권5호
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• pp.372-376
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• 2009

This paper describes the characteristics of piezoelectric micro power generators by the ANSYS FEA(finite element analysis). The micro power generator was designed to convert ambient vibration energy to electrical power as a ZnO piezoelectric material. To find optimal model in low vibration ambient, the shape of power generator was changed with different membrane width, thickness, length, and proof mass size. Using the ANSYS modal analysis, bending mode and stress distribution of optimal model were analyzed. Moreover, the displacement with the frequency range was analyzed by harmonic analysis. From the simulation results, the resonance frequency of optimal model is about 373 Hz and investigate the possibility of ZnO micro power generator for ambient vibration applications.