Calculation of acoustic axes in triclinic elastic anisotropy is considerably more complicated than for anisotropy of higher symmetry. While one polynomial equation of the 6th order is solved in monoclinic anisotropy, we have to solve two coupled polynomial equations of the 6th order in two variables in triclinic anisotropy. Furthermore, some solutions of the equations are spurious and must be discarded. In this way we obtain 16 isolated acoustic axes, which can run in real or complex directions. The real/complex acoustic axes describe the propagation of homogeneous/inhomogeneous plane waves and are associated with a linear/elliptical polarization of waves in their vicinity. The most frequent number of real acoustic axes is 8 for strong triclinic anisotropy and 4 to 6 for weak triclinic anisotropy. Examples of anisotropy with no or 16 real acoustic axes are presented.
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