Abstract
We investigate both theoretically and experimentally a parametric source where the major part of the difference frequency generation occurs within a solid cylinder of silicone rubber. The primary waves (∼1.5 Mhz) are confined to a narrow region around the axis of the cylinder and, owing to the relatively high absorption at these frequencies, are largely attenuated before reaching the end of the cylinder. The silicone rubber cylinder serves two purposes: (1) owing to the low sound speed (∼1000 m/sec) and high parameter of nonlinearity (B/A ∼8) of the silicon rubber, the virtual sources are more than three times as strong as they would be in water; (2) again, owing to its low sound speed, the silicon rubber cylinder acts as a slow waveguide antenna, so that the beamwidth is considerably narrower than that which would be produced by a conventional parametric array of the same effective length. The system is analyzed by numerically solving the system of equations obtained by coupling the surface Helmholtz integral equation for the interior of the silicon cylinder (which includes the virtual sources) with the integral equation for the region exterior to the cylinder. Theory and experiment compare favorably.
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