Applying a perturbation technique to the correct form of the equation for the acoustic pressure field in a viscous turbulent atmosphere yields the power spectra for the amplitude and phase fluctuations of a propagating plane wave. The development includes the effects of absorption and removes the restrictions on wavenumber that previously prevented solution of the problem for other than ultrasonic frequencies. For a wide range of propagation parameters, the amplitude and phase spectra found by Tatarski for optical propagation are shown to be good approximations for acoustic waves. Complete expressions, without limitation on the assumed refractive-index spectra, determine the range of validity of this result and also provide an exact first-order solution.