4G-5 The Peculiarities of Propagation of Backward Acoustic Waves of Higher Orders in Piezoelectric Plates

The paper is devoted to theoretical investigation of the characteristics of Lamb waves of higher orders (A_n and S_n) propagating in piezoelectric plates in bandwidth including the cutoff frequency. Previously such modes were found and investigated only in isotropic materials. As materials under study we used lithium tantalate, lithium niobate, potassium niobate, langasite, and langanite. It has been found that these waves are characterized by negative values of energy transport velocity, which is directed oppositely to the phase velocity. Analysis has showed that the electromechanical coupling coefficient is always negative because electrical shorting of plate surface increases the phase velocity. Obtained results have the fundamental importance as well as practical use, for example, for determination of plate thickness or material constants