Electromagnetic acoustic wave transducer for the generation of acoustic evanescent waves on membranes and optical and capacitor wave-number selective detectors

Abstract

A novel electromagnetic acoustic wave transducer (EMAT) capable of generating 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 evanescent wave adjacent to the membrane. Unlike conventional EMATs that rely on an external magnetic field interacting with an eddy current distribution, this system makes use of an applied current pulse through the membrane itself. A spatially and temporally oscillating Lorentz force is generated on the membrane with the application of a tone burst through a meander-line coil configuration simultaneous with a current pulse through the membrane. Studies were carried out on the frequency 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 wave train and verify the wave shape characteristics. Wave-number selective differential optical and capacitive transducers are also described. A property of these transducers is their insensitivity to long wavelengths on the membrane.