In this paper, we are interested in separating waves in elastic and viscoelastic rods propagating in opposite direction. N strain and P velocity measurements are taken into account. This application of the likelihood method gives a solution in the frequency domain. Using the inverse Fourier transform, one can recover the strain, stress, displacement and velocity at any section of the rod. In experimental conditions, the results are stable against noise when N+P>2 and NP ≠ 0.