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We study the general infrared behavior of the power spectrum of a stochastic gravitational wave background produced by stress tensor in the form bilinear in certain dynamical degrees of freedom. We find ₆ₖk^3 for a very wide class of the sources which satisfy a set of reasonable conditions. Namely, the k^3 scaling is universally valid when the source term is bounded in both frequency and time, is effective in a radiation-dominated stage, and for k smaller than all the physical scales associated with the source, like the peak frequency, peak width, and time duration, etc. We also discuss possible violations of these conditions and their physical implications.
Cai et al. (Wed,) studied this question.