The global anisotropy of wood can be expressed by the ratio of acoustic invariants . For isotropic solids this ratio is 1. For wood of low density as spruce, the ratio is 0.35 and for wood of high density as pernambuco, the ratio is 0.6 . From acoustical point of view, this means that high density wood species, with a complex anatomic structure are less anisotropic than low density species, with a relatively simple anatomic structure .