 Rotating acoustic point source

A point source moving at angular speed $\Omega$ on a circle of radius $a$ has observer-direction Mach number $m=\Omega a\sin \iota /c_0$. In the far field its retarded phase satisfies $\Theta +\theta -\pi /2=\Omega (t-r/c_0)+m\sin \Theta$. For $m<1$ there is one retarded root, and a constant source strength gives ${\rho }^{\prime }=q\dot{\Theta }/(4\pi c_0^2\Omega r)$. For $m>1$, multiple retarded roots can occur; their contributions require $|1-m\cos \Theta |$ and a sum over roots. Coalescing roots produce a caustic in the ideal point-source model.