`  == All wave phenomena matching the conditions... ==  There was brief mention of ``analogous`` results for acoustics, but it would be nice to have an expansion of that. The most general result on the page has to do with Helmholtz' theorem and when combined with Green's identity, that applies to ALL wave phenomena (not just E&M;) which match the boundary conditions (linear water waves, gravity waves, acoustics, etc.). If I have time, I'll point that out. Unfortunately, it'll probably require radically expanding the whole article, and certainly renaming it to ``Reciprocity (wave phenomena)``.  If someone else can do the necessary editing, that'd be great, too.    `