If in the expressionpe = Aei(ωt−kz)for the excess pressure in a plane progressive harmonic sound wave it is assumed thatkis complex and we writek = kr+iki, the solution corresponds to a damped wave with absorption coefficient α = −ki, measuring attenuation of pressure amplitude withdistance. This may be calledspatialattenuation. If, on the other hand, it is assumed that ω is complex, withω = ωr+iωi, the solution corresponds to a damped wave with absorption coefficient ωi, measuring attenuation of pressure amplitude withtime. This may be calledtemporalattenuation. It is not commonly emphasized that the associated dispersion is in general quite different in the two cases. Thus, in absorption in a fluid due to heat conduction and viscosity the phase velocity increases with frequency for spatial attenuation but decreases with frequency for temporal attenuation. At low pressures in gases the difference might be detectable. Typical dispersion and absorption curves are presented for the above‐mentioned cases as well as for thermal and structural relaxation mechanisms in which the situation is rather different.