= Mass-injection term in the acoustic analogy
{title2=$(\partial_t^2-c_0^2\Delta)\rho'=\partial_i\partial_jT_{ij}+\partial_tM-\partial_iS_i$}
If the <continuity equation> has mass source $M$ and the conservative momentum equation has momentum source $S_i$, elimination of momentum gives the displayed <wave equation>. With no imposed momentum source, its extra term is $\partial_tM$. A compact source of total mass rate $\mathcal M=\int M\,dV$ radiates the <acoustic monopole> $\rho'_M=\dot{\mathcal M}(t-r/c_0)/(4\pi c_0^2r)$. The momentum source supplies an additional <acoustic dipole>; specifying mass injection alone does not fix that dipole.
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