I'm specifically looking at the derivation of the 3D acoustic wave equation from Wikipedia. At the top of the image there are simplified equations for the conservation of momentum (where, in order to get to this form, the gradient of the density must have been assumed to be 0?), and the conservation of mass (where the gradient of the density appears to be finite). I can't quite wrap my head around the reason for this - could anyone offer any help?

Edit: Thanks to /u/rs6866, who pointed out that my original interpretation of the simplified momentum conservation equation was incorrect. The gradient of the density in the momentum equation does not need to be 0 in order to get the form of the equation I linked to above.