need help for implementing pml absorboing layer

hiii friends ,
i have written a code for 3d simulation of ultrasound .. now i want to apply pml layer at the boundaries ..
can anybody suggest me how to incorporate the absorbing conditions using matlab..
the generalised equations which i am using for simulation r given as

dv_x/dt = 1/ρ (〖dσ〗_xx/dx + 〖dσ〗_xy/dy+ 〖dσ〗_xz/dz)

dv_y/dt = 1/ρ (〖dσ〗_xy/dx + 〖dσ〗_yy/dy+ 〖dσ〗_yz/dz)

dv_z/dt = 1/ρ (〖dσ〗_xz/dx + 〖dσ〗_yz/dy+ 〖dσ〗_zz/dz)

〖dσ〗_xy/dt =μ (dv_x/dy + dv_y/dx)
〖dσ〗_xz/dt =μ (dv_x/dz + dv_z/dx)
〖dσ〗_yz/dt =μ (dv_y/dz + dv_z/dy)
〖dσ〗_xx/dt = (λ+2μ) dv_x/dx + λ (dv_y/dy + dv_z/dz)
〖dσ〗_zz/dt = λ (dv_x/dx + dv_y/dy) +(λ+2μ) dv_z/dz
〖dσ〗_zz/dt = λdv_x/dx + (λ+2μ) dv_y/dy + λdv_z/dz


thanks
rahul