Ultrasonic air-coupled wave phase conjugator for the low megahertz frequency range

Study of the possibility of wave phase conjugation (WPC) technology for ultrasonic testing of objects in gas medium and for velocimetry of air microflows is the objective of the present work. The main problem to design parametric phase conjugate wave (PCW) generator (conjugator) working in air is connected to the huge loss of power of the ultrasonic wave when passing the interface between air and the active element (magnetostrictive ferrite). It was found that the most suitable material for acoustical matching of transducers and active element of conjugator at low megahertz frequency range are porous membranes impregnated by oil with 6.3MHz resonance frequency. The resonator mode of the parametric generator arising from the increase of pumping duration is also under investigation. To record very weak signals we used for the first time in WPC the phase coding of signals by pseudo noise sequence. The results of velocimetry of an air jet using PCW are presented in this paper.