# Dictionary:Isotropic media

A medium is isotropic if all directions of wave propagation are equivalent, and that case of isotropy has the most simple symmetry. In this case, the material has only two independent elastic moduli, called Lame's and shear modulus, and , which are related to the stiffness tensor by:
\begin{equation}
C_{ijkl}=[\lambda\delta_{ij}\delta_{kl}+G(\delta_{ij}\delta_{kl}+\delta_{ij}\delta_{kl})]
\end{equation}
where is the Kronecker delta function, for , and for , with *i,j,k,l=1,2,3*. The stiffness tensor of isotropic media is explicitly written as the stiffness matrix (in Voigt notation ) :
\begin{equation}
C_{IJ}^{iso}=
\begin{bmatrix}
C_{33} & C_{12} & C_{12} & 0 & 0 & 0\\
C_{12} & C_{33} & C_{12} & 0 & 0 &0\\
C_{12} & C_{12} & C_{33} & 0 & 0 & 0\\
0 &0 &0 &C_{44} &0 &0\\
0 &0 &0 &0 &C_{44} &0\\
0 &0 &0 &0 &0 &C_{44}\\
\end{bmatrix}
\equiv
\begin{bmatrix}
\lambda+2G & \lambda & \lambda & 0 & 0 & 0\\
\lambda & \lambda+2G & \lambda & 0 & 0 &0\\
\lambda & \lambda & \lambda+2G & 0 & 0 & 0\\
0 &0 &0 &G &0 &0\\
0 &0 &0 &0 &G &0\\
0 &0 &0 &0 &0 &G\\
\end{bmatrix}
\end{equation}
where *λ+2G = M = K +4G/3 = ρV _{P}^{2}* is the longitudinal modulus, and

*G = ρV*, with ρ the density, and K the bulk modulus.

_{S}^{2}