However the reports respected to the acoustic field of focusing spherical transducer of high frequency are scarce and also a standardized measurement method of diameter of fine focused beam does not exist at present. The present authors have already reported a fundamental study [1] on this problem by numerical computation and experiment and have proposed a measurement method of diameter of focused beam which is usable for small beam diameter.

In the present report, computations and experiments are expanded to wide range of transducer size and shape, and relation between diameter of focused beam and dimension of transducer are reported. Generally speaking, transducer dimension is so large compared to wavelength that the dimensionless approach using linear scale normalized by wavelength is possible. The wavelength and the aperture are the main factors which determine diameter of focused beam. It decreases with decrease of wavelength and increase of aperture. Theoretically, for example, diameter of about 20 micron is expected for a 100 MHZ transducer of aperture of 2 X 30 deg. However, designation of actual transducer is limited by the several condition. For example, a large aperture transducer cannot be made for high frequency because of limitation of capacitance. From a standpoint of actual application, the lower frequency is desirable because of the lower attenuation in the material. In order to obtain optimum conditions, consideration including modification of the shape of transducer is necessary. Some developments of new shape of transducer are reported. REFERENCE

1. J. Murai, T. Ida and T. Shiraiwa, "Acoustic Field of spherical focusing Transducer", Proceedings of the Eighth Asia - Pacific Conference on Nondestructive Testing, Taipei, Taiwan, pp. 599-605, 1995.