The exact linear equations for propagation of a disturbance in a viscoelastic material are reviewed, together with the usual small‐loss approximation. For tan δ⩽0.165 the approximate relations are within about 1%. Many plastic materials, however, have loss tangents considerably greater than 0.165 and appreciable errors result if the exact expressions are not used. A table of corrections to the small‐loss approximate propagation constants is presented. The case of large‐loss materials (tan δ>>1) is also considered and approximate equation derived. For tan δ ⩾ 10 the large‐loss approximate relations are within 5%. An interesting result from the large‐loss approximation is that the product of the propagation velocity and attenuation is equal to 2 times the frequency (cα = ω). Some experimental data, both shear and longitudinal, for typical plastic material are given.