Far-forward and forward collective light scattering measurements of externally launched ion-acoustic waves in a low density argon plasma column have been performed with a c.w. CO₂ laser. The scattering processes taking place are described theoretically by the same general formalism. Wave dispersive and damping properties are studied consistently from 2 to 200 cm^-1. Plasma parameters are deduced and compared to electrostatic probe and antenna measurements. It is found that high frequency wave properties are erroneously measured with a grid antenna.