To test weak materials such as foams at high strain rates, the use of a Split Hopkinson Pressure Bar (SHPB) setup made of low impedance bars, which are mostly viscoelastic, is indispensable. In this paper a detailed study of the technical problems of such a viscoelastic setup related to the measurement and to the loading conditions is offered. On the basis of the three-dimensional (3D) Fourier stationary harmonic wave analysis, the wave shifting technique is developed. It is shown that the effect of geometrical dispersion in a viscoelastic setup is generally non-negligible. The case of loading with a viscoelastic projectile is also analysed, showing that the impact of a viscoelastic projectile creates an incident wave with a significant time extension.