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Keunhwa Lee,Woojae Seong IEEE 2017 IEEE JOURNAL OF OCEANIC ENGINEERING - Vol.42 No.1
<P>In this paper, a ray-based noise model is developed to calculate the two-point spatial coherence function for ocean-surface-generated noise in a hydrophone triplet, which consists of three hydrophones arranged as an equilateral triangle with side length d. While existing ray-based models employ the semianalytic approach, this model is more generally formulated for the 3-D noise field with vertical and azimuthal directivity using a ray-tracing model. Moreover, we derive a Maclaurin series of the formulated spatial coherence function for the ratio of the hydrophone spacing and the acoustic wavelength λ. This series explicitly reveals the feature of spatial coherence for the noise field as a function of the orientation of the hydrophone pair and the ocean environmental coefficients. At small d/λ, the leading-order terms alone provide a good approximate solution for the spatial coherence function, which enables rapid computation of six coherence functions for the hydrophone triplet. Especially, for the ocean environment with weakly azimuth-dependent ocean bathymetry, the contribution from each term in the derived series is analyzed and a simpler formula for the spatial coherence function is suggested.</P>
Heuristic Physical Theory of Diffraction for Impedance Polygon
Lee, Keunhwa,Park, Sanghyun,Kim, Kookhyun,Seong, Woojae Korean Society of Ocean Engineers 2013 International journal of ocean system engineering Vol.3 No.1
A heuristic physical theory of diffraction (PTD) for an acoustic impedance wedge is proposed. This method is based on Ufimtsev's three-dimensional PTD, which is derived for an acoustic soft or hard wedge. We modify the original PTD according to the process of physical optics (or the Kirchhoff approximation) to obtain a 3D heuristic diffraction model for an impedance wedge. In principle, our result is equivalent to Luebbers' model presented in electromagnetism. Moreover, our approach provides a useful insight into the theoretical basis of the existing heuristic diffraction methods. The derived heuristic PTD is applied to an arbitrary impedance polygon, and a simple PTD formula is derived as a supplement to the physical optics formula.
Lee, Keunhwa,Park, Eungkyu,Seong, Woojae American Institute of Physics for the Acoustical S 2009 Journal of the Acoustical Society of America Vol.126 No.1
<P>Acoustic measurements of p-wave speed and attenuation were made for water-saturated granular medium, consisting of six kinds of glass-beads with mean grain size ranging from 90 to 875 microm, at frequency range between 400 kHz and 1.1 MHz. Sound speed and attenuation were obtained using the inter-receiver broadband estimation technique. The measured data exhibit various frequency dependencies for the different mean grain sizes, consistent with earlier measurements from other researches. These results reveal that the trend of dispersion relation for the sound speed and attenuation, in the high frequency region, is strongly dependent on the range of Rayleigh parameter kd.</P>
인수분해 된 분모를 갖는 두 변수 유리함수 근사에 기반한 3차원 음향 포물선 방정식 제곱근 연산자의 분할기법 제안
이근화,Lee, Keunhwa 한국음향학회 2017 韓國音響學會誌 Vol.36 No.1
본 연구에서는 두 변수 유리함수 근사법에 기반한 3차원 음향 포물선 방정식의 제곱근 연산자의 새로운 근사식을 제안한다. 이 근사식은 기존의 제곱근 연산자에 대한 근사 연구와 비교해서 두 가지의 장점을 가진다. 첫 번째는 광대역 각도 능력이다. 제안된 식은 방위각 $45^{\circ}$에서 3차원 음향 포물선 방정식의 거리 축으로부터 $62^{\circ}$까지 넓은 각도에 대해 정확도를 가지는데, 이 값은 기존에 연구된 3차원 음향 포물선 방정식 알고리즘의 각도 한계의 약 세 배이다. 두 번째로는 본 근사식의 분모는 수심과 횡 거리에 대한 연산자의 곱으로 표현된다는 점이다. 이러한 분할 형태는 3차원 포물선 방정식을 손쉽게 삼중대각행렬 방정식으로 변환할 수 있다는 점에서 수치해석에서 선호된다. 제안된 식의 성능을 검증하기 위해 위상 오차분석을 통해 타 근사법과의 비교 연구가 수행되었고, 제안된 방법은 가장 우수한 성능을 보였다. In this study, novel approximate form of the square-root operator of three dimensional acoustic Parabolic Equation (3D PE) is proposed using a rational approximant for two variables. This form has two advantages in comparison with existing approximation studies of the square-root operator. One is the wide-angle capability. The proposed form has wider angle accuracy to the inclination angle of ${\pm}62^{\circ}$ from the range axis of 3D PE at the bearing angle of $45^{\circ}$, which is approximately three times the angle limit of the existing 3D PE algorithm. Another is that the denominator of our approximate form can be expressed into the product of one-dimensional operators for depth and cross-range. Such a splitting form is very preferable in the numerical analysis in that the 3D PE can be easily transformed into the tridiagonal matrix equation. To confirm the capability of the proposed approximate form, comparative study of other approximation methods is conducted based on the phase error analysis, and the proposed method shows best performance.
Underwater Acoustic Research Trends with Machine Learning: Ocean Parameter Inversion Applications
Haesang Yang,Keunhwa Lee,Youngmin Choo,Kookhyun Kim 한국해양공학회 2020 韓國海洋工學會誌 Vol.34 No.5
Underwater acoustics, which is the study of the phenomena related to sound waves in water, has been applied mainly in research on the use of sound navigation and range (SONAR) systems for communication, target detection, investigation of marine resources and environments, and noise measurement and analysis. Underwater acoustics is mainly applied in the field of remote sensing, wherein information on a target object is acquired indirectly from acoustic data. Presently, machine learning, which has recently been applied successfully in a variety of research fields, is being utilized extensively in remote sensing to obtain and extract information. In the earlier parts of this work, we examined the research trends involving the machine learning techniques and theories that are mainly used in underwater acoustics, as well as their applications in active/passive SONAR systems (Yang et al., 2020a; Yang et al., 2020b; Yang et al., 2020c). As a follow-up, this paper reviews machine learning applications for the inversion of ocean parameters such as sound speed profiles and sediment geoacoustic parameters.
비스듬히 입사하는 음장에 대한 유한 길이의 탄성 원통 쉘의 음향 산란
이근화,변성훈,김시문,Lee, Keunhwa,Byun, Sung-Hoon,Kim, Sea-Moon 한국음향학회 2019 韓國音響學會誌 Vol.38 No.5
본 연구에서는 무한 유체에 놓여있는 유한 길이의 탄성 원통 쉘에 외부에서 비스듬히 평면파가 입사할 때 발생하는 음향 산란 현상을 이론적으로 연구했다. 유한 길이의 원통 쉘에서는 해석적인 산란 해가 존재하지 않기 때문에, Kirchhoff 가정을 적용한 Ye의 산란 기법[Z. Ye, J. Acoust. Soc. Am. 102, 877-884 (1997)]을 사용했다. 탄성 원통 쉘의 특성은 3차원 탄성파 이론을 적용하여 구현했으며 원통 쉘 내부의 유체를 고려했다. 유도된 해석 해를 이용하여 내부 유체가 산란 음장에 미치는 효과, Rayleigh 변수에 대한 산란 음장, 탄성 재질의 변화에 따른 먼 거리 산란 함수를 살펴보았다. In this study, we theoretically study the acoustic scattering of an obliquely incident plane wave from a finite elastic cylindrical shell. A heuristic scattering method of Ye [Z. Ye, J. Acoust. Soc. Am. 102, 877-884 (1997)] for a finite fluid cylinder is extended into a finite elastic cylindrical shell since no analytic solutions exist in the finite cylinder. The elastic cylindrical shell is modeled with the 3D elastic wave theory considering internal fluid. Using the derived analytic solution, we observe the effect of the internal fluid on the scattering field, the scattering field for the Rayleigh parameter, and the far-field scattering function for the elastic property of the cylindrical shell.