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D.N. Ghosh Roy, SFA Inc. (U.S.A.)
Luise S. Couchman, Naval Research Laboratory (U.S.A.)
Jeremy Warner, SFA Inc. (U.S.A.)
The results of direct and inverse scattering of plane acoustic waves from impenetrable and penetrable objects are reported here. It is assumed that the scatterer boundary is a superposition of an arbitrary deformation on an underlying simple geometry. The direct problem is solved via the Padé extrapolation of the boundary variations. This results in solving only certain algebraic recursion relations and requires neither Green's function nor integral representations. The inverse problem of recovering the obstacle's shape and material parameters from the far-field scattering data is solved by Gauss-Newton minimization. The calculation of the scattered field and its Jacobian involves no more than solving a series of Helmholtz scattering problems in the same domain, namely, exterior to the simple shape instead of the iteratively updated deformed surfaces leading thereby to substantial computational simplifications. Finally, several two-dimensional obstacles of various shapes are inverted for their boundaries as well as their material parameters of mass density and wavenumber.
sv970529.pdf (Scanned)
TOPAlex Zinoviev, The University of Adelaide (Australia)
A new numerical modelling method for sound wave scattering by elastic objects in an acoustic waveguide is proposed. The method is applied to the case of sound propagation and scattering in a plane 2-dimensional layer of liquid 80 metres deep containing a cylindrical air filled shell of finite wall thickness and an outer shell radius of 8 metres. The incident wave is considered to be the lowest order propagating mode of the waveguide. The reflection coefficient is calculated in the frequency range between 5 and 300 Hz for several values of wall thickness and distance between the shell and the waveguide bottom. Pictorial output shows that the amount of acoustic energy reflected strongly depends upon all variables. Maxima and minima in the reflection coefficient associated with cut-on frequencies of the waveguide modes and structural resonances of the shell are identified. The calculations show that the conventional definition of target strength is inappropriate.
sv970435.pdf (Scanned) TOPDmitri Azalinov, Institute for Problems in Mechanical Engineering (Russia)
We consider the following problem: There is a reservoir with a constant depth, covered by ice field. The depth is small in comparison with a wave length, The water is considered as an ideal liquid. The ice field is considered as a thin isotropic plate of a constant thickness. There are 2 half-infinite ice plates on the water. From the right to the left runs a plane traveling wave. As a result one obtained: 1) Dependence of transmission and reflection coefficients of the patch width of open water in ice, water depth and ice thickness, wave frequency; 2) Dispersion curves of wave velocities in different mediums (ice, water and ice on the water); 3) Energy flows through the reservoir profile, ice profile and profile of the system ice-water. The reflection and transmission coefficients depend periodically of the patch width; the distance between maximal values of reflection is equal to half-length of the gravitation wave on the water surface.
sv970427.pdf (Scanned) TOP
Noboru Watanabe, Waseda University (Japan)
Yoshio Yamasaki, Waseda University (Japan)
It is taken for granted that the walls of a room are rigid and stable when we evaluate the acoustic quality. But in real conditions, walls which are excited by low-frequency sound are sometimes vibrating and the high-frequency reflection sound from them is phase modulated; i.e. when the wall is moving inward, the pitch of the reflection sound will rise and vice versa. This modulation is a nonlinear phenomenon known as cross modulation. In highly reverberant space, there are many reflections which are reflected by the variable part of the room several times, and the amount of modulation is thought to be greater than single reflection sound. This distortion has been thought to be hazardous to the acoustic quality of an auditorium. The authors have examined the mathematical basics of this phenomenon and made a computer simulation program. The authors show that the amount of the modulation (i.e., the distortion ratio) is greater in small and highly reverberant space than in large and non-reverberant space.
sv970369.pdf (Scanned) TOP
Huinam Rhee, Korea Power Engineering Company (Korea)
Youngjin Park, Korea Advanced Institute of Science and Technology (Korea)
Elastic wave scattering from an acoustically rigid or soft cylinder is studied and compared with the acoustic wave scattering. The behavior of phases as well as magnitudes of partial waves and their total summation of scattered waves are numerically analyzed and discussed. The different phenomena between the acoustic and elastic wave scatterings are discussed. The effect of mode conversion, which occurs between longitudinal and transversal waves, on the magnitudes and phases of partial waves in elastic wave scattering is identified. Some parametric calculations are performed to study different behaviors of monopole and dipole modes between the acoustic and elastic wave scattering. Also, in this paper, summed partial waves for the elastic wave scattering are discussed and compared with those for the acoustic wave scattering, with two different scattering angles.
sv970202.pdf (Scanned) TOP