Specific propagation directions for media of rhombic symmetry
  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. 
Artur Duda