Comparison of acoustic fields radiated from piezoceramic and piezocomposite focused radiators

The acoustic field radiated from piezoelectric transducers is usually predicted supposing that the transducer vibrates in thickness mode. However, different reports have shown that not only thickness vibrations were excited, but also plate waves. These waves are responsible for discrepancy between the experimental acoustic fields and those predicted by the Rayleigh integral. It could be supposed that the plate waves are strongly attenuated in piezocomposite materials, as mechanical cross-talk between neighboring elements of the composite structure is fairly weak. A similar effect could be achieved in piezoceramic material by employing a heavy backing, which partially damps the plate waves. These opportunities of plate wave damping are investigated in the present paper. Three transducers are studied, which have identical geometrical characteristics, but are made from different materials. The plate waves in these transducers are indirectly compared by measuring corresponding ultrasound fields and comparing them with theoretically predicted field. It is shown that plate wave patterns are strongly material dependent and that it is only for piezocomposite sources (even when highly focused) that Rayleigh integral modeling can accurately predict the pressure field distribution. (C) 1999 Acoustical Society of America. [S0001-4966(99)04105-3].