Performances of the parametric acoustic vibrometer for vibration sensing

Ultrasonic vibration sensing is presently being developed as an alternative to optical techniques. It can be used in air or in liquid and is a highly sensitive method. A continuous ultrasonic beam is emitted towards the moving surface and is received after reflection from the object under test. Two different physical effects contribute to the phase modulation of the received signal: the Doppler effect and the nonlinear parametric interactions between the high frequency probing beams (forward and backward) and the low frequency sound field emitted by the vibrating surface. The following questions can then be addressed: What is the spatial resolution of a vibration measurement system based on the parametric effect? How does it compare to the resolution of the usual Doppler effect which is directly linked to the high frequency beam width? To answer that, the computation of the parametric field has been done by using a double decomposition: in Gaussian beams for the probing beam and in plane waves for the low frequency field.