• The Journal of the Acoustical Society of Korea
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• v.25 no.3
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• pp.107-112
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• 2006

The elastic waves in a plate are dispersive waves due to the characteristics of Lamb waves. However, S0 symmetric mode is less dispersive in the frequency region below the first cut-off frequency. The wave Propagation velocities vary with the direction in anisotropic plates such as Carbon Fiber Reinforced Plastic (CFRP) Plates. The wave vector direction and energy flow vector direction are same in isotropic plates. However, the wave vector direction same as the phase velocity direction is not in accordance with the energy flow direction same as the group velocity direction in anisotropic plates. In this study. the dispersion curves or the phase velocity from anti-symmetric and symmetric Lamb wave dispersion equation are calculated for unidirectional laminated composite plate. Slowness surface is sketched using phase velocity under the first cut-off frequency. The direction and magnitude of group velocity are corrected with this slowness surface. The measured group velocities are in good agreement with the corrected group velocity curve except near the fiber direction zone which is called the cusp region.

• The Journal of the Acoustical Society of Korea
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• v.36 no.4
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• pp.247-253
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• 2017

Generally, the vibration characteristics of refractory ceramics are identified by assuming them as isotropic materials. However, in practice, refractory ceramics exhibit anisotropic properties as they are manufactured by pressing ceramic powders along a particular direction. Therefore, in this research, the frequency responses of a refractory ceramic brick along its width, length, and height directions were acquired using finite element analysis by assuming that the ceramics had tetragonal symmetry in their material properties. The validity of the numerical analysis results was verified by comparing them with those from experimental measurements. Based on the frequency response, the thicknesses of the refractory brick along three different directions were estimated using the impact-echo technique. The maximum difference between the estimated and actual thicknesses was observed to be less than 5 %. This result confirms the effectiveness of the impact-echo technique along with anisotropic property characterization to evaluate the thickness of the refractory ceramic.

• Proceedings of the Acoustical Society of Korea Conference
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• spring
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• pp.365-368
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• 2000

자왜 재료는 자계 포화 이하에서 비선형 특성을 갖는다고 알려져 있다. 그러나 지금까지 비선형 특성을 표현하는 자왜 재료의 구조 방정식을 유도한 사례는 전무한 실정이다 본 연구에서는 자계 포화 이하에서 비선형 특성을 갖는 자왜 재료의 비선형 구조 관계식을 유도하였다. 나아가 유도된 구조 관계식을 이용하여 자왜 재료 내의 파동 방정식을 정식화하였다. 그리고 비선형 특성을 갖는 자왜 재료에서 평면파가 자계 방향을 따라 전파될 때 이방성 자왜 재료를 등방성 재료로 가정하여 종파와 횡파 속도를 구하였다.

• The Journal of the Acoustical Society of Korea
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• v.6 no.4
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• pp.55-63
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• 1987

Modified $PbTiO_3$ piezoelectric ceramics added with 0.2, 0.25, 0.3 mol of $CaCO_3$ and 0.04 mol of $(Co_{\frac{1}{2}}W_{\frac{1}{2}})$ and 0.05 mol of $MnO_2$ and NiO have been fabricated. These ceramics can be poled sufficiently within 10 minutes at $100^{\circ}C$ under about d.c. field of 40 kv/cm. Detailed measurement was performed on dielectric constants, cutie temperatures, elastic and piezoelectric properties and coupling factors for the fabricated ceramics. The most value of the piezoelectric coupling factors was coupling factor of thickness mode kt and its value for 0.25 mol of Ca was about $45\%$. Dielectric constants of $\varepsilon_{33}^T$ and $\varepsilon_{11}^T$ for 0.25 mol of Ca were 242 and 260, and coupling factor ratio (kt/Kp) and Qm were 6 and 357 respectively.

• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.5
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• pp.439-444
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• 2003

Acoustic emission(AE) ran be very effectively applied to locate the damaged area in large structures by detecting the elastic waves generated during the damage process within solids. Source location in the composite structures has been, however, extremely difficult due to the acoustic anisotropy with the velocity dependence on fiber orientations. In this study, it has been shown that a newly proposed method for 2-D source location of anisotropic structures is practically applicable to the real structure. The method employes wave velocities obtained with different velocities from $0^{\circ}\;to\;90^{\circ}$ for a filament wound composite pressure vessel under the air-filled and the water-filled conditions.

• The Journal of the Acoustical Society of Korea
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• v.11 no.2
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• pp.68-72
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• 1992

Recently, many researchers have studied the speaker recognition through the statistical processing method using Karhunen-Loeve Transform. However, the content of speaker's identity and the vocalization speed cause speaker recognition rate to be lowered. This parer studies the speaker recognition method using weighted orthogonal parameters which are weighted with eigen-values of speech so as to emphasize the speaker's identity, and optimal path which is made by DWP so as to normalize dynamic time feature of speech. To confirm this method, we compare the speaker recognition rate from this proposed method with that from the conventional statistical processing method. As a result, it is shown that this method is more excellent in speaker recognition rate than conventional method.

• Journal of the Korean Society for Nondestructive Testing
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• v.16 no.4
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• pp.215-224
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• 1997

A study has been made to investigate the acoustic emission(AE) behavior during the tensile and fracture tests of an Al-Li 8090 alloy, and to correlate it with the deformation mechanisms as a function of microstructure and plate orientation. Mechanical tests and AE measurements were conducted on the specimens with different microstructures (the ${\delta}'$ phase dominant microstructure and the $S'+{\delta}'$ microstructure) and with different orientations (L and ST). In the ${\delta}'$ structure, continuous emissions were produced in both the L and ST oriented specimens, while only the burst emissions were observed to occur in the $S'+{\delta}'$ structure. It was inferred from the above results that continuous type emissions were mainly attributed to the shearing of coherent ${\delta}'$ precipitates in the ${\delta}'$ structure, while the burst type emissions were produced due to the shearing or microcracking of incoherent S' phase in the $S'+{\delta}'$ structure. As to the effect of plate orientation, the ST oriented specimens showed more burst emissions than the LT oriented ones. A large number of burst emissions produced in the ST specimen were presumably due to the rapid crack propagation along the intergranular boundary located parallel to the crack propagation direction.

• The Journal of the Acoustical Society of Korea
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• v.14 no.4
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• pp.21-28
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• 1995

In this paper an intonation conversion method is presented which provides the basic study on converting the original speech into the artificially intoned one. This method employs the other speaker's excitation signals as intonation information and the original vocal tract spectra, which are warped with the other speaker's ones by using DTW. as vocal features, and intonation converted speech signals are synthesized through short-time inverse Fourier transform(STIFT) of their product. To evaluate the intonation converted speech by this method, we collect Korean single vowels and sentences spoken by 30 males and compare fundamental frequency contours spectrograms, distortion measures and MOS test between the original speech and the converted one. The result shows that this method can convert and speech into the intoned one of the other speaker's.

• Journal of the Korean Society of Marine Environment & Safety
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• v.23 no.7
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• pp.948-955
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• 2017

Currently, porous materials (e.g., mineral wool) are the core materials used in offshore plant panels, but in spite of their superb acoustic performance, these items must be replaced for environmental reasons. A honeycomb structure is widely used throughout the industry because of its high strength-to-weight ratio. However, research in terms of noise and vibration is minimal. An acoustic study should be conducted by taking advantage of honeycomb structures to replace porous materials. In this study, a simulation was performed assuming that a honeycomb panel is a superposition of symmetric mode and antisymmetric mode. Reliability was verified by comparing a simulation results based on a theory with a experimental results, and the possibility of the panel as a core material was evaluated by studying the sound insulation characteristics of a honeycomb. As the panel thickness increased, the coincidence frequency shifted to low frequency. As the angle between horizontal line and oblique wall and cell-size decreases, the sound insulation performance is improved. And as the cell-wall thickness increased, the sound insulation performance improved.

• Geophysics and Geophysical Exploration
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• v.5 no.3
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• pp.169-177
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• 2002

We develop the weighted-averaging finite-element method which uses four kinds of element sets. By constructing global stiffness and mass matrices for four kinds of element sets and then averaging them with weighting coefficients, we obtain a new global stiffness and mass matrix. With the optimal weighting coefficients minimizing grid dispersion and grid anisotropy, we can reduce the number of grid points required per wavelength to 4 for a $1\%$ upper limit of error. We confirm the accuracy of our weighted-averaging finite-element method through accuracy analyses for a homogeneous and a horizontal-layer model. By synthetic data example, we reconfirm that our method is more efficient for simulating a geological model than previous finite-element methods.