ELECTROMAGNETIC ACOUSTIC-WAVE TRANSDUCER FOR THE GENERATION OF ACOUSTIC EVANESCENT WAVES ON MEMBRANES AND OPTICAL AND CAPACITOR WAVE-NUMBERSELECTIVE DETECTORS

A novel electromagnetic acoustic wave transducer (EMAT) capable of gen erating low-frequency tone bursts on metalized membranes is described. The bursts have a phase velocity that is much less than the speed of sound in the surrounding air so that they are accompanied by a plane e vanescent wave adjacent to the membrane. Unlike conventional EMATs tha t rely on an external magnetic field interacting with an eddy current distribution, this system makes use of an applied current pulse throug h the membrane itself. A spatially and temporally oscillating Lorentz force is generated on the membrane with the application of a tone burs t through a meander-line coil configuration simultaneous with a curren t pulse through the membrane. Studies were carried out on the frequenc y selectivity of the EMAT, the steady-state buildup of the tone bursts , and the dependence on gap spacing between the meander-line coil and the membrane. Optical and capacitive probes were used to detect the wa ve train and verify the wave shape characteristics. Wave-number select ive differential optical and capacitive transducers are also described . A property of these transducers is their insensitivity to long wavel engths on the membrane.