The energy density and power flow of acoustic waves propagating in piezoelectric materials

It is shown that, for a plane bulk acoustic wave propagating in arbitrary piezoelectric media, the densities of mechanoelectrical and electromechanical energies are always equal in absolute value and have opposite signs. However, in general, the mechanoelectrical and electromechanical power flows of such a wave calculated by the traditional expression for the Poynting vector do not compensate each other, although the total density of these energies is always equal to zero. A discovered discrepancy based on the dissymmetry of piezoelectric constants with respect to the electrical and mechanical indexes may cause difficulties for calculation of important parameters for practical applications such as energy transport velocity of acoustic waves in piezoelectric materials.