Non contact measurement of vibration using airborne ultrasound

A non contact ultrasonic method for measuring the surface vibration of objects is studied. The instrument consists in a pair of 420 kHz air transducer. One is used to emit ultrasound toward the moving surface while the other receives the ultrasound reflected from the object under test. Two effects induce a phase modulation on the received signal. The first results from the variation of the round trip time interval τ required for the wavefront to go from the emitter to the moving surface and back to the receiver. This is the Doppler effect directly proportional to the surface displacement. The second one results from the nonlinear parametric interactions of the ultrasonic beams (forward and backward) with the low frequency sound field emitted in the air by the vibrating surface. This latter phenomenon, which is a volume effect, is proportional to the low frequency field and then to the surface velocity. The relative contribution of Doppler and parametric effects are evaluated, and it is shown that both have to be taken into account when sensing broadband vibrations in the air