  We reanalyze the problem of existence of longitudinal normals inside symmetry
planes of piezoelectric crystals belonging to the symmetry class mm2. The
equations determining components of longitudinal normals situated outside
symmetry planes for media of this symmetry are discussed. It is proven that
non-piezoelectric media of rhombic symmetry could have 4 or 8 distinct acoustic
axes. Examples of non-piezoelectric elastic media of monoclinic symmetry
without acoustic axes are given. The method of determination of components of
acoustic axes for piezoelectric media of arbitrary symmetry is presented. With
the help of this method we discuss the problem of acoustic axes for
piezoelectric media of the symmetry class mm2.

Other Condensed Matter