Sound in biased piezoelectric materials of general anisotropy

A theoretical model is presented that describes the propagation of sound in biased piezoelectric crystals of any kind of symmetry. The symmetry relations for the higher order material constants of trigonal 3m crystals, are calculated and listed in the appendix. The example of Lithium Niobate is highlighted, under influence of a bias pressure. The change of slowness (inverse velocity), because of this pressure, is calculated for every direction. Also the influence of stress on the acoustic polarization and the energy flow is outlined. Furthermore, the difference between the case where piezoelectricity is included and the case where it is omitted, is discussed. The description of changing slowness surfaces, because of a bias field, is not limited to homogeneous plane waves, but also inhomogeneous plane waves are taken into account.