Hi,

In general both the group and phase velocity of a wave depends on the material properties of the medium. Considering fibre reinforced polymeres the material properties are angle dependent and therefore the wave velocity is as well. If you plot the dependencies in a polar diagram you will not get a circle. The links below show you examples of what I am referring to:

http://www.sciencedirect.com/science/article/pii/S0022460X06009163

http://www.mae.ncsu.edu/ssml/publication/Journals/WangL_CST_2007.pdf

If you need something else please specify your question.

Best,

Sascha

Depending on the layout of panel, you may get quasi-isotropic laminates that are actually isotropic in terms of velocity for certain wave modes.