Certain nonlinear acoustic phenomena may be explained in terms of virtual or secondary sources. The parametric array is one classic example [P. J. Westervelt, J. Acoust. Soc. Am. 35, 535–537 (1963)]. It is argued here that there must also exist formally similar sources due to the thermoviscous heating of a fluid by a modulated sound wave. At moderate to high modulation frequencies the nonlinear mechanism dominates, but at sufficiently low modulation frequencies, the thermal phenomenon must become the more significant effect.