General expressions have been derived relating the attenuation,
A, to the group and phase refractive indices μ′ and μ. This has been done by considering a perturbation about the case in which the collision frequency is zero. It is found that the expressions forμ′ –μ and the imaginary part of the refractive index are closely related in form so that the attenuation can be related to the difference in the group and phase paths, P′– P. Approximations of this form have been obtained in the past but only for situations in which the Q.L. and Q.T. approximations are valid. The expressions derived in this paper are not restricted in this way and numerical calculations show that the ranges of these Q.L. and Q.T. approximations are rather limited. Numerical calculations have enabled new approximations with much greater ranges of validity to be established for these cases.