The surface acoustic wave (SAW) device on a ball is expected to provide high-performance sensors. Since we must deal with three-dimensional wave fields for precise design of ball SAW devices, numerical methods, such as the finite element method, will require too many elements. Therefore, we applied an analytical solution to a practical calculation of the full-field acoustic waves. For an IDT aperture of 6° and 80° with a wave number parameter of 45, the SAW has a divergent beam and a focusing beam, respectively. For an intermediate aperture of 30°, the SAW forms a collimating beam. This calculation confirmed the possibility of an extremely sensitive sensor with diffraction-free SAWs without disturbance by spurious bulk waves.
Using the laser ultrasonic method, we investigated the distribution of surface acoustic waves (SAWs) on a sphere excited by a line source. Experimental results revealed that the SAWs were confined within a narrow path due to the balance of the diffraction and convergence effects. Moreover, the distribution had unique divergent, collimating and focussing forms, which had been predicted theoretically. These results were generally in good agreement with the results calculated using the approximate Green's function. It was shown that the distribution of SAWs is well defined by the half-aperture angle of the source and by the product of the wavenumber and the radius of the sphere.
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