NONCONTACT MEASUREMENT OF VIBRATION USING AIRBORNE ULTRASOUND

A noncontact ultrasonic method for measuring the surface normal vibrat ion of objects was studied. The instrument consists of a pair of 420 k Hz ultrasonic air transducers. One is used to emit ultrasounds toward the moving surface, and the other receives the ultrasound reflected fr om the object under test. Two effects induce a phase modulation on the received signal. The first effect results from the variation of the r ound trip time interval tau required for the wavefront to go from the emitter to the moving surface and back to the receiver. This is the Do ppler effect directly proportional to the surface displacement. The se cond effect results from the nonlinear parametric interactions of the ultrasonic beams (forward and backward) with the low frequency sound f ield emitted in the air by the vibrating surface. This latter phenomen on, which is a volume effect, is proportional to the velocity of the v ibrating surface and increases with the distance between the transduce rs and the surface under test. The relative contribution of the Dopple r and parametric effects are evaluated, and both have to be taken into account for ultrasonic interferometry in air.