• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2012.10a
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• pp.75-80
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• 2012

Helmholtz resonator is widely used acoustic instrument which has high absorption characteristics at its resonance frequency. Particularly it maintains good performance even in the low frequency region that is difficult to control by general porous absorptive materials. However, under severely high sound pressure level, the absorption characteristics are changed by increase of resistance due to nonlinear behavior of neck impedance. Because of this nonlinear behavior, it is difficult to obtain the expected absorption performance under high sound pressure environment. Thus, in order to prevent excessive rise of resistance, the resonator with neck having cross section dimension decrease away from the entry of the resonator cavity could be suggested. This paper introduces the experiment method and results about nonlinear characteristics of Helmholtz resonator with tapered neck and proposes the approximate nonlinear impedance model.

• Proceedings of the Korea Electromagnetic Engineering Society Conference
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• 2005.11a
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• pp.279-284
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• 2005

We investigated the impedance matching characteristics of UHF-band RFID tag antenna and tag chip for increased reading range. A voltage multiplier designed using 0.4 $\mu$m zero-$V_T$ MOSFET showed that DC output voltage of about 2 V can be obtained using standard CMOS process. The input impedance of the voltage multiplier was examined to achieve impedance matching to the RFID tag antenna using analytical and numerical approaches. The input impedance of the voltage multiplier could be varied in a wide range by selecting the size of MOSFET and the number of multiplying stages, and thus can be impedance matched to a tag antenna in presence of other tag circuit blocks. A meander line inductively-coupled RFID tag antenna operating at UHF band also shows the feasibility of impedance matching to tile RFID tag chip.

• The Journal of the Acoustical Society of Korea
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• v.22 no.1
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• pp.54-60
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• 2003

The characteristics of radiation impedance for piston source on a spherical baffle are analyzed by algorithms which consists of Finite Element Method (FEM) and Hybrid type Infinite Element Method (HIEM). The results of self-radiation impedance for radiation angle and mutual radiation impedance between piston sources coincided with other reports on the spherical rigid baffle. For the spherical non-rigid baffles, the variations of self-radiation impedance and mutual-radiation impedance are identified. Therefore, these results can be applied to design and radiation characteristics analysis of acoustic transducers.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• v.y2005m4
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• pp.291-298
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• 2005

Linear acoustic analysis has been performed to elucidate damping characteristics of acoustic cavities. Results have shown that resonant frequencies of acoustic cavity obtained by classical theoretic approach and present linear analysis are somewhat different from each other. This difference is due to the limitation of classical theory. To quantify the damping characteristics, acoustic impedance has been introduced and resultant absorption and conductance have been evaluated. Satisfactory agreement has been achieved with previous experiment. Finally the design procedure for optimal tuning of acoustic cavity has been established

• The Journal of the Acoustical Society of Korea
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• v.25 no.2E
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• pp.79-84
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• 2006

Acoustic characteristics of a sound absorbing material can be identified, if the characteristic impedance and propagation constants are known, which have generally been determined experimentally. One easy method determining these two essential parameters is to measure the one dimensional wave characteristics in the impedance tube. In th udy, the effects of backing conditions on the impedance tube measurement have been examined using several pairs of generally used end conditions. The results showed that the measured values are similar for most pairs of end conditions: however, it was observed that the measured characteristic impedance for different thickness did not agree well for some pairs. In this work, the multi termination method, using three or more known backing con ns, was suggested to reduce such random errors, which are mostly caused by the test procedure. Employing three terminations as a set, comprised of a rigid end, an end with porous material, and an end with a backing cavity, it was demonstrated that improved measured results could be obtained for an open cell PU foam varying widely with three different thicknesses.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.9
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• pp.897-902
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• 2011

Chemical reactions occurring in a PEMFC are dominated by the physical conditions and interface properties, and the reactions are expressed in terms of impedance. The performance of a PEMFC can be simply diagnosed by examining the impedance because impedance characteristics can be expressed by an equivalent electrical circuit. In this study, the characteristics of a PEMFC are assessed using the AC impedance and various equivalent circuits such as a simple equivalent circuit, equivalent circuit with a CPE, equivalent circuit with two RCs, and equivalent circuit with two CPEs. It was found in this study that the characteristics of a PEMFC could be assessed using impedance and an equivalent circuit, and the accuracy was highest for an equivalent circuit with two CPEs.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.48 no.11
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• pp.717-723
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• 1999

The transient characteristics of grounding systems play a major role in the protection of power equipments, electronic circuits and info-communication facilities against surges which arise from lightning or ground faults. Electronic devices are very weak against lightning surges injected from grounding systems and can be damaged. The malfunction and damage of electronic circuits bring about bad operation performances, a lot of economical losses, and etc. Therefore, in order to obtain the effective protection measure of electronic devices from overvoltages and lightning surges, the analysis of the transient grounding impedances in essential. One of this work is to examine the transient behaviors of grounding impedances under steplike currents for various grounding systems. And the other of this work is to evaluate the transient behaviors of a grid with rods under impulse currents and to investigate the effect of grounding lead wire. Transient grounding impedances of a grid with rods under impulse current waves have been measured as a parameter of the length of the grounding leads. Z-t, Z-i and V-i curves of transient grounding impedance under impulse current waveforms have been measured and analyzed. It was found that the grounding impedance gives the inductive, resistive and capacitive aspects under steplike current. Transient grounding impedance characteristics were very different with shapes, geometries of ground electrodes. Also, they were dependent on the waveform and magnitude of impulse current.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.18 no.10
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• pp.2345-2350
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• 2014

In this paper, measurement methods for the characteristics of antennas for indoor digital TV (DTV), having 75-ohm port impedance, are introduced. Reference impedance of typical radio frequency (or microwave) systems and measurement equipments is 50 ohm. However, the reference impedance of antennas for indoor DTV is 75 ohm, and the characteristics of the antennas cannot be measured directly using the measurement equipments. Measurement methods, using test equipments with reference impedance of 50 ohm, for the characteristics(impedance, radiation patterns, and gain) of antennas designed to fit for 75 ohm port are introduced.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2014.05a
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• pp.45-46
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• 2014

In this paper, measurement methods for the characteristics of antennas for indoor digital TV (DTV), having 75-ohm port impedance, are introduced. Reference impedance of typical radio frequency (or microwave) systems and measurement equipments is 50 ohm. However, the reference impedance of antennas for indoor DTV is 75 ohm, and the characteristics of the antennas cannot be measured directly using the measurement equipments. Measurement methods, using test equipments with reference impedance of 50 ohm, for the characteristics(impedance, radiation patterns, and gains) of antennas designed to fit for 75 ohm port are introduced.

• Bulletin of the Korean Chemical Society
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• v.20 no.11
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• pp.1340-1344
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• 1999

The electrochemical behavior of oxide layers on aluminium was studied using electrochemical impedance spectroscopy. Impedance spectra were taken at a compact and a porous oxide layer of Al. The anodic films on Al have a variable stoichiometry with gradual reduction of oxygen deficiency towards the oxide-electrolyte interface. Thus, the interpretation of impedance spectra for oxide layers is complicated, with the impedance of surface layers differing from those of ideal capacitors. This layer behavior with conductance gradients was caused by an inhomogeneous dielectric. The frequency response cannot be described by a single RC element. The oxide layers of Al are properly described by the Young model of dielectric constant with a vertical decay of conductivity.