Outgoing acoustic field of a pulsating sphere (source code)

= Outgoing acoustic field of a pulsating sphere

For $R(t)=a+\operatorname{Re}(\epsilon e^{i\omega t})$, put $k=\omega/c_0$. To first order in $\epsilon/a$, the exterior outgoing velocity-potential amplitude is
$$
\widehat\phi(r)
=-\frac{i\omega\epsilon a^2}{1+ika}
\frac{e^{-ik(r-a)}}r.
$$
It satisfies $\widehat u_r(a)=\widehat\phi'(a)=i\omega\epsilon$.