抄録
Lamb waves in a solid plate with water loading on its one surface, when their velocity exceeds that of sound in water, take a form of leaky wave which water, propagate with radiation of energy into water (see Fig. 1). It is necessary to know the propagation characteristics of such leaky waves in order to design a fluid-prism coupler (as in Fig. 2) for achieving efficient mode-conversion between the bulk and the Lamb waves. Furthermore, since the radiation angle θ_0 of the leaky Lamb waves depends on frequency (Eq. 1), and ultrasonic-beam steering can be achieved by frequency scanning. It may be applicable to ultrasonic viewing systems and broad-band acoustoopic deflectors. With interest in applications as mentioned above, the propagation and the far-field characteristics of the leaky modes designated by F_<1l>, L_<1l> and F_<2l>, corresponding to the modes of Lamb waves F_1, L_1 and F_2, respectively, are theoretically investigated. Fig. 3 shows the phase velocity v and the attenuation factor ε. The optimum beamwidth W_0 of incident bulk-wave to maximize the conversion efficiency is determined by using these values of ε(Eq. 8). Fig. 4 shows the particle velocity distribution in the modes F_<1l> and L_<1l>, as well as in the modes F_1 and L_1. It is found from Fig. 3 and 4 that the variations of ε are closely related to the velocity distribution. As the frequency is increased, the mode L_<1l> approaches the leaky Rayleigh wave, whereas the mode F_<1l> approaches the free Rayleigh wave. It is clarified by approximate analyses based on the coupled-mode theory that these phenomena are interpreted as result of strong coupling between the modes F_1 and L_1 by water loading. The approximate values of ε calculated by these analyses are also shown in Fig. 3(b). Fig. 5 shows the radiation angles θ_0 for different plate materials. The variation of θ_0 against frequency is larger as the bulk-shear wave velocity v_s of the plate materials is smaller. The far-field characteristics of a single leaky Lamb mode radiating from a finite aperture, such as the radiation pattern and the 3-dB beamwidth θ_B, are shown in Figs. 8 and 9. These indicate that there exists a frequency region in which the very sharp beam can be steered over a considerable wide range without large variation of θ_B. Several results of far-field measurements using the prototype "leaky-wave transducer" of Fig. 10 are shown in comparison with the theoretical ones (Figs. 11-13).
