• Journal of the Korean Society for Railway
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• v.19 no.2
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• pp.124-134
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• 2016

Research on low height noise barriers installed adjacent to railways to reduce the height of the noise barrier has actively progressed in many countries except Korea. The performance of low height noise barriers is evaluated to identify barrier acoustic characteristics using a scale model of the barrier in the present research. As shown in the experimental results, if it is considered the installation of 'ㄱ' type noise barrier, sound absorption material should be installed on both the top and the vertical surfaces of the barrier to improve insertion loss. Also, an analytical method such as the boundary element method, rather than a simple empirical equation, is required to evaluate the insertion loss of the barrier. In addition, noise level increase in passenger position is very small if a barrier with sound absorption material is installed. Finally, the two dimensional boundary element method is implemented to predict the acoustic characteristics of the low height barrier; the possibility of the application is confirmed from a comparison of the results of measurements and predictions.

• Wind and Structures
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• v.17 no.3
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• pp.299-315
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• 2013

Numerical research on four typical configurations of noise mitigation structures and their characteristics of wind loads are reported in this paper. The turbulence model as well the model parameters, the modeling of the equilibrium atmospheric boundary layer, the mesh discretization etc., were carefully considered in the numerical model to improve the numerical accuracy. Also a numerical validation of one configuration with the wind tunnel test data was made. Through detailed analyses of the wind load characteristics with the inclined part and the wind incidence angle, it was found that the addition of an inclined part to a noise mitigation structure at-grade would affect the mean nett pressure coefficients on the vertical part, and that the extent of this effect depends on the length of the inclined part itself. The magnitudes of the mean nett pressure coefficients for both the vertical part and the inclined part of noise mitigation structure at-grade tended to increase with length of inclined part. Finally, a comparison with the wind load code British/European Standard BS EN 1991-1-4:2005 was made and the envelope of the mean nett pressure coefficients of the noise mitigation structures was given for design purposes. The current research should be helpful to improve current wind codes by providing more reasonable wind pressure coefficients for different configurations of noise mitigation structures.

• Journal of the Korean Society of Radiology
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• v.17 no.5
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• pp.709-715
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• 2023

In this study, the performance evaluation of image quality for noise reduction was implemented using the U-net deep learning architecture in computed tomography (CT) images. In order to generate input data, the Gaussian noise was applied to ground truth (GT) data, and datasets were consisted of 8:1:1 ratio of train, validation, and test sets among 1300 CT images. The Adagrad, Adam, and AdamW were used as optimizer function, and 10, 50 and 100 times for number of epochs were applied. In addition, learning rates of 0.01, 0.001, and 0.0001 were applied using the U-net deep learning model to compare the output image quality. To analyze the quantitative values, the peak signal to noise ratio (PSNR) and coefficient of variation (COV) were calculated. Based on the results, deep learning model was useful for noise reduction. We suggested that optimized hyper parameters for noise reduction in CT images were AdamW optimizer function, 100 times number of epochs and 0.0001 learning rates.

• Explosives and Blasting
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• v.11 no.2
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• pp.60-68
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• 1993

The noise and vibration generated by the subway rolling stocks operated along the Seoul Subway Line No.1, 2, 3, and 4 lead to a controversy of pollution problem especially in residential areas. However, there is no data or guide to define the damage or provide adequate protection against such pollutions. The field measurements were made to characterize the noise attenuation due to distance, noise level distribution around the subway track of the aboveground and underground parts of each Line. The assessment criteria and methods are considered in addition to the practical available noise control methods. The noise level measured at Line No. 1 and 3 are less than 60 dB(A) with no pollution problem. Only a part of the aboveground section of Line No.2 and 4 indicates severe noise pollution. The effective boundary of these areas exposed to 70dB (A) noise are within 50m from the track centerline of No.2 line and 25m of No.4 line. The residents file a strong complaints whenever the noise level exceeds the 80dB (A) , and an occasional complaints between 70 to 80 dB(A). The distribution of high level noise of 80 dB(A) occurs within 25m from the track centerline of the overbridge, 12.5m of the short steel bridge, and about loom of the long steel bridge such as Dangsan Bridge. The intermediate noise level of 70 to 80 dB(A) is recorded within 50m from the overbridge, U-type retaining structure, and short steel structure, and 280m from the long steel bridge. The results presented in this paper can be used to understand the characteristics of the noise pollution along the Seoul Subway now In operation, and used as a guide to improve the existing noise pollution problems.

• The Journal of the Acoustical Society of Korea
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• v.43 no.5
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• pp.570-583
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• 2024

The supply of inverter-type air conditioners for cooling in summer and heating in winter is increasing. In addition, since the operating speed of the compressor has been continuously increased for higher efficiency and higher performance, the flow speed of the refrigerant has also increased. As a result, it results in the increase of the relative contribution of flow-born noise to total noise generated from outdoor unit, and this highlights the importance of designing for the noise reduction to addressing flow-borne noise and requires necessary to analyze noise generation mechanisms by flow borne noise. Therefore, in this paper, the noise generation mechanisms by flow borne noise from air conditioner outdoor unit was numerically investigated. The wall pressure field was predicted using Large Eddy Simulation(LES) for the refrigerant flow inside the pipe, and the vibration and radiated noise were predicted using structure and acoustic coupled scheme based Finite Element Method (FEM). In this step, the compressible/in-compressible pressure field were separated using Wavenumber-Frequency Analysis(WFA) for inner pipe wall, and this results were used in analyzing the noise source due to refrigerant flow.

• ETRI Journal
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• v.35 no.5
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• pp.827-837
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• 2013

In this paper, we present wideband common-mode (CM) noise suppression using a vertical stepped impedance electromagnetic bandgap (VSI-EBG) structure for high-speed differential signals in multilayer printed circuit boards. This technique is an original design that enables us to apply the VSI-EBG structure to differential signals without sacrificing the differential characteristics. In addition, the analytical dispersion equations for the bandgap prediction of the CM propagation in the VSIEBG structure are extracted, and the closed-form expressions for the bandgap cutoff frequencies are derived. Based on the dispersion equations, the effects of the impedance ratio, the EBG patch length, and via inductances on the bandgap of the VSI-EBG structure for differential signals are thoroughly examined. The proposed dispersion equations are verified through agreement with the full-wave simulation results. It is experimentally demonstrated that the proposed VSI-EBG structure for differential signaling suppresses the CM noise in the wideband frequency range without degrading the differential characteristics.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.7 no.5
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• pp.1131-1145
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• 2013

Energy detection is a widely used method for spectrum sensing in cognitive radios due to its simplicity and accuracy. However, it is severely affected by the noise uncertainty. To solve this problem, a blind multiband spectrum sensing scheme which is robust to noise uncertainty is proposed in this paper. The proposed scheme performs spectrum sensing over the total frequency channels simultaneously rather than a single channel each time. To improve the detection performance, the proposal jointly utilizes the likelihood function combined with Gerschgorin radii of unitary transformed covariance matrix. Unlike the conventional sensing methods, our scheme does not need any prior knowledge of noise power or PU signals, and thus is suitable for blind spectrum sensing. In addition, no subjective decision threshold setting is required in our scheme, making it robust to noise uncertainty. Finally, numerical results based on the probability of detection and false alarm versus SNR or the number of samples are presented to validate the performance of the proposed scheme.

• ETRI Journal
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• v.36 no.3
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• pp.510-513
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• 2014

A Q-band pHEMT image-rejection low-noise amplifier (IR-LNA) is presented using inter-stage tunable resonators. The inter-stage L-C resonators can maximize an image rejection by functioning as inter-stage matching circuits at an operating frequency ($F_{OP}$) and short circuits at an image frequency ($F_{IM}$). In addition, it also brings more wideband image rejection than conventional notch filters. Moreover, tunable varactors in L-C resonators not only compensate for the mismatch of an image frequency induced by the process variation or model error but can also change the image frequency according to a required RF frequency. The implemented pHEMT IR-LNA shows 54.3 dB maximum image rejection ratio (IRR). By changing the varactor bias, the image frequency shifts from 27 GHz to 37 GHz with over 40 dB IRR, a 19.1 dB to 17.6 dB peak gain, and 3.2 dB to 4.3 dB noise figure. To the best of the authors' knowledge, it shows the highest IRR and $F_{IM}/F_{OP}$ of the reported millimeter/quasi-millimeter wave IR-LNAs.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.21 no.1
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• pp.18-23
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• 2011

With hydraulic noise test facility, a variety of tests were performed to investigate the pulsation pressure generation mechanism and its transmission characteristics and to derive the noise control methodology. Many experiments were carried out by changing average pressure, flow rate, pump speed, hose length and MCV spool condition. From the test results, the correlations between pulsation pressure and other design parameters, such as static pressure, flow rate and MCV spool opening area and length of hose, were found out. And also each contribution factors were evaluated from the regression analysis. By changing hose length, the pulsation pressure resonance phenomenon was investigated. In order to find out the pulsation pressure reduction measures pulsation pressure analysis, such as pulsation pressure of hydraulic pump itself and pulsation pressure of hydraulic system, by using AMESim were studied. In addition hydraulic silencer was developed based on the Helmholtz resonator. And its performance was evaluated by installing the silencer at the excavator.

• The Journal of the Acoustical Society of Korea
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• v.19 no.2
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• pp.61-67
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• 2000

Recent progress in digital and VLSI technology has offered new possibility fer noticeable advance of hearing aids. Yet, environmental noise remains one of the major problems to hearing aid users. This paper describes results which speech recognition performance and speech discrimination performance was measured for listeners with sensorineural hearing loss, while listeners in speech-band noise. In addition, to ameliorate hearing-aided environments of hearing impaired listeners, environmental noise reduction using speech enhancement techniques are investigated as a front-end of conventional hearing aids. Speech enhancement techniques are implemented in a realtime system equipped with DSP board. The clinical test results suggest that the speech enhancement technique may work in synergy with gain functions fer the greater SNR improvement as the preprocessing algorithm of digital hearing aids.