海洋音響学会誌

Characteristics of pulse propagation in Arctic Ocean were calculated by elastic Parabolic Equation (PE) method based on rotated Pade approximation. It was assumed that the transmitter was placed at 100 m of depth and that the frequency was 20 Hz. Propagation loss was estimated with the condition that the propagation path was 300 km when the thickness of the ice layer was changed. To estimate the arrival time of pulses, a simulation of eigen rays was used by ray theory. The sound pressure field in the vertical plane changed in proportion to the increment of the thickness of ice. It was predicted that the effect of an ice layer on the sea surface would be great because the last pulse incidents that occurred at a small grazing angle in the ice layer generated a shear wave. Therefore, the transmission loss was large compared to there being no ice. It is shown that we had to take account of the ice layer in sound propagation in Arctic Ocean.

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This theoretical report concerns the focusing of a plane progressive acoustic wave by a bicylindrically-curved lens. It was assumed that the lens was made of polymethylmethacrylate (PMMA) and was submerged in water. The analytical approach that is based on full-wave theory began with expanding an incident plane wave into cylindrical waves to satisfy appropriate boundary conditions at an interface between the water and the lens. The continuity conditions of displacement and stress at the interface may determine the generation of longitudinal and shear waves in the lens. Lens-aperture effects on focused field performance were taken into account by confining the incident wave to the region within the aperture. The Rayleigh integral associated with the particle velocity on the exit interface of the lens enabled us to evaluate the entire sound field behind the lens. Numerical examples demonstrated no significant influence on the fields near the focus, even when the shear wave generation was included in the theory. Wave diffraction due to a finite lens aperture, however, became a major effective factor in beam focusing.

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