The exact solution for first order nonlinear scattering of two plane waves of sound [J. Acoust. Soc. Amer. 29, 934 0957)] showed the scattered density to be ρs = c0−2E12+12ω1−1ω2−1(cosθ+12Λ)[sin−2(12θ)]∇2W12. The singularity at θ = 0, which occurs when the primary waves are collinear, may be removed by applying to this solution the operator {1 − exp[(|∂∂t||c0∇|−1−1)]r⋅∇}. The resulting solution is now valid for θ = 0 in addition to all other intersection angles. This new solution has been applied with the conservation of energy [P. J. Westervelt, in L. Cremer, Ed., Proc. Int. Cong. Acoust., 3rd (Elsevier, Amsterdam, 1960), p. 316], to determine the attenuation of one plane wave with wave vector k1, interacting with an isotropic distribution of waves having the energy density spectrum u(k). The general result for the pressure attenuation coefficient is α = (2ρ0c02)−1(1 + 12Λ)2π[ ∫ 0k1 ku(k)dk+2k1 ∫ k1∞u(k)dk+k12 ∫ 0k1k−1u(k)dk]. In the event k1 is much less than any component k of the interacting background, a great simplification results and α = (2ρ0c02)−1(1 + 12Λ)2πk1E, in which E is the energy density of the background radiation. This result is supported by experiments on the attenuation of sound in superfluid helium [H. J. Maris, Phys. Rev. Lett. 28, 277 (1972)]. [This work was supported by the U. S. Office of Naval Research.]