• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.1355-1359
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• 2000

In this study, the insertion losses of various track-side railway noise barriers are analyzed by using a two-dimensional numerical boundary element method(BEM). The BEM is implemented using two out-of-phase monopole sources for each track to represent dipole source. All model for this study is based on actual size built in Test Line of KTX. And the results suggest that vertical absorptive barrier provide as effective screening as rigid barrier which is more 0.4 m than the height of those.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.9 s.180
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• pp.2302-2312
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• 2000

An active control of the transmission of noise through an aircraft fuselage is investigated numerically. A cylinder-cavity system was used as a model for this study. The fuselage is modeled as a fi nite, thin shel cylinder with constant thickness. The sound field generated by an exterior monopole source is transmitted into the cavity through the cylinder. Point force actuators on the cylinder are driven by error sensor that is placed in 3D cavity. Modal coupling theory is used to formulate the numerical models and describe the system behavior. Minimization of the acoustic potential energy in the fuselage is carried out as a performance index. Continuous parameter genetic algorithm is used to search the optimal actuator position and both results are compared.

• Journal of Advanced Marine Engineering and Technology
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• v.33 no.8
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• pp.1170-1179
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• 2009

The purpose of this work is to predict the low frequency aero-acoustic noise generated from the horizontal axis wind turbine, NREL Phase VI for the whole operating conditions of various wind speeds using large eddy simulation and Ffowcs-Williams and Hawkings model provided in the commercial code, FLUENT. Because there is no experimental data about wind turbine noise, we first of all compared aerodynamic performance such as shaft torque and power with experimentally measured value. Performance results show a good agreement with experimental data within about 0.8%. As the wind speed increases, the overall sound pressure level and the sound pressure level by the quadrupole and dipole source show a increasing tendency. Also, sound pressure level is proportional to $r^{-2}$ in the near field and $r^{-1}$ in the far field according to the increase of distance from the center of hub of wind turbine. According to 2 times increase of distance, sound pressure level is reduced by about 6dB.

• The Journal of the Acoustical Society of Korea
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• v.41 no.3
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• pp.268-277
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• 2022

In this study, each component of flow noise source of underwater propeller installed to the scale model of the KVLCC2 is investigated and the effect of each noise source on underwater-radiated noise is quantitatively analyzed. The computation domain is set to be the same as the test section of the large cavitation tunnel in the Korea Research Institute of Ship and Ocean Engineering. First, for the high-resolution computation of flow field which is noise source region, the incompressible multiphase Delayed Detached Eddy Simulation is performed. Based on flow simulation results, the Ffowcs Williams and Hawkings integral equation is used to predict underwater-radiated noise and its validity is confirmed through the comparison with the tunnel experiment result. For the quantitative comparison on the contribution of each noise source, the spectral levels of sound pressure and power levels predicted using propeller tip-vortex cavitation, blade surface and rudder surface as the integral region of noise sources are investigated. It is confirmed that the cavitation which is monopole noise source significantly contributed to the underwater-radiated noise than propeller blades and rudder which is dipole noise source, and the rudder have more contribution than propeller blades due to the influence of the propeller wake.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.4
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• pp.950-967
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• 1995

Acoustic holography makes it possible to reconstruct the acoustic field based on the measurement of the pressure distribution on the hologram surface. Because of the merit that one can obtain an entire three-dimensional wave field from the data recorded on a two-dimensional surface, the holographic method has been widely studied. Being an experimental method, holography has an unavoidable error which is generate by sampling in space and frequency domain and finite aperture size. Its magnitude is dependent on the space and frequency domain and finite aperture size. Its magnitude is dependent on the shape of hologram surface, acoustic holography may be classified into four types of holography : rectangular type planeholography, circular type plane holography, cylindrical holography and spherical holography. In this paper, four types of holography are studied by modal summation method. Numerical simulation is performed using a monopole source with varying parameters to find out effects to the estimation error in each holography. Experiments of circular type plane holography and cylindrical holography explain strong relation between the shape of hologram surface and the acoustic field.

• Geophysics and Geophysical Exploration
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• v.12 no.1
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• pp.143-153
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• 2009

Using a discrete wavenumber method, we examine the effects on Logging-While-Drilling (LWD) logs when a mismatch exists between the amplitudes or generating times of the signals from individual monopoles in a LWD multipole source. An amplitude-mismatched LWD dipole/quadrupole source produces non-dipole/non-quadrupole modes as well as flexural and screw modes. The strongest of non-dipole/non-quadrupole modes is the Stoneley mode, whose amplitude increases with increasing mismatch. However, we can recover the flexural mode signals by A-C processing, and the screw mode by A-B+C-D processing, respectively. The Stoneley mode, which has the same amplitude at the same radial distance from the borehole axis, is cancelled out by A-C and A-B+C-D processing as long as the tool is placed at the centre of the borehole. The responses from a time-mismatched LWD multipole source look like the summation of responses by two or four monopole sources off the borehole axis. However, we can avoid the misinterpretation of the formation velocities by referring to the computed dispersion curves, which are independent of the arrival times of the modes, on the frequency semblance plot.

• Progress in Medical Physics
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• v.11 no.1
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• pp.19-27
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• 2000

In order to predict the propagation of road traffic noise over level terrain, this paper presents the model of the simple monopole and dipole source. Excess attenuations by ground acoustic impedance with continuity are investigated by outdoor experiments. In this results, the propagation of road traffic noise is affected by the acoustic impedance properties of the ground surface over which the noise travels, particularly when the receiver position is close to the surface. These results are then used to derive sound pressure level contours for the attenuation with distance of the equivalent energy level received from passing vehicle. The percentage of deep sleep and the number of wakings are studied for continuous traffic noise. The hearing loss and sensitivity threshold shift is investigated by changing the various parameters such as the effects of aging and noise exposure. Evaluation of the response of human to traffic noise is discribed. It is shown that exposure to excessive noise can cause temporary loss of hearing that may become permanent if the exposure is prolonged or intense.

• Geophysics and Geophysical Exploration
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• v.7 no.1
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• pp.45-50
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• 2004

This paper describes a method of detecting formation boundaries, and permeable fractures, from frequency-domain Stoneley wave logs. Field data sets were collected between the depths of 330 and 360 m in well EE-4 in the Higashi-Hachimantai geothermal field, using a monopole acoustic logging tool with a source central frequency of 15 kHz. Stoneley wave amplitude spectra were calculated by performing a fast Fourier transform on the waveforms, and the spectra were then collected into a frequency-depth distribution of Stoneley wave amplitudes. The frequency-domain Stoneley wave log shows four main characteristic peaks at frequencies 6.5, 8.8, 12, and 13.3 kHz. The magnitudes of the Stoneley wave at these four frequencies are affected by formation properties. The Stoneley wave at higher frequencies (12 and 13.3 kHz) has higher amplitudes in hard formations than in soft formations, while the wave at lower frequencies (6.5 and 8.8 kHz) has higher amplitudes in soft formations than in hard formations. The correlation of the frequency-domain Stoneley wave log with the logs of lithology, degree of welding, and P-wave velocity is excellent, with all of them showing similar discontinuities at the depths of formation boundaries. It is obvious from these facts that the frequency-domain Stoneley wave log provides useful clues for detecting formation boundaries. The frequency-domain Stoneley wave logs are also applicable to the detection of a single permeable fracture. The procedure uses the Stoneley wave spectral amplitude logs at the four frequencies, and weighting functions. The optimally weighted sum of the four Stoneley wave spectral amplitudes becomes almost constant at all depths, except at the depth of a permeable fracture. The assumptions that underlie this procedure are that the energy of the Stoneley wave is conserved in continuous media, but that attenuation of the Stoneley wave may occur at a permeable fracture. This attenuation may take place at anyone of the four characteristic Stoneley wave frequencies. We think our multispectral approach is the only reliable method for the detection of permeable fractures.