# Is this a valid change of variables?

We are solving for the sound wave caused by the motion of a spherical piston. In a spherically symmetric case, the classical wave equation reads: $$\phi_{tt}=a_0^2\Big(\phi_{rr}+\frac{2\phi_r}{r}\Big),\tag{1}$$ where $$\phi$$ is the velocity potential. The general solution for the forward moving wave is given by: $$\phi=\frac{1}{r}f(r-a_0t),$$ where $$a_0$$ is the constant speed of sound. The radial velocity $$u$$ is therefore: $$u=\frac{\partial\phi}{\partial r}=\frac{1}{r^2}f(r-a_0t) - \frac{1}{r}f'(r-a_0t).$$ If the piston motion is given by $$R=R(t)$$, then using the boundary condition for velocity at the piston $$u(R(t),t)=dR/dt$$, we get from the previous equation: $$\dot{R}=\frac{1}{R^2}f(R-a_0t) - \frac{1}{R}f'(R-a_0t),\tag{2}$$ which is a first-order ordinary differential equation that we can solve for $$f$$.

Well so it goes. However, the one thing I find annoying about this is the last equation. Namely, the term $$f'(R-a_0t)$$ is not the derivative of $$f$$ w.r.t $$(R-a_0t)$$, but rather the derivative of $$f$$ w.r.t $$(r-a_0t)$$, evaluated at $$r=R$$. Thus, can we solve this as an ODE even though the differentiation is not made w.r.t the same independent variable of the equation?

Edit: Replying to the comment by Mattos, $$\phi$$ represents the general form of a wave transported in the positive $$r$$ direction (i.e radially outwards), and it can be easily shown that it satisfies the wave equation, namely: $$\phi_t=-\frac{a_0}{r}f'(r-a_0t),$$ $$\phi_{tt}=\frac{a_0^2}{r}f''(r-a_0t),$$ $$\phi_r=\frac{1}{r}f'(r-a_0t) - \frac{1}{r^2}f(r-a_0t),$$ $$\phi_{rr}= \frac{1}{r}f''(r-a_0t)-\frac{2}{r^2}f'(r-a_0t)+\frac{2}{r^3}f(r-a_0t),$$ and substituting these values into equation (1) yields $$\frac{a_0^2}{r}f''(r-a_0t)=\frac{a_0^2}{r}f''(r-a_0t),$$ thus satisfying the equation.

As for the other question, since $$R(t)$$ is given, equation (2) can be solved for $$f$$ up to a constant.

• I'm not sure what you mean by 'the general solution for the forward moving wave is $\dots$', the solution $\phi$ doesn't satisfy your PDE. Also, I don't see how you can solve the equation for $f$. The form of $f$ should have been determined from the boundary and initial conditions. – mattos Nov 25 '19 at 14:53
• @mattos please see the edit – Tofi Nov 25 '19 at 15:26
• My apologies, ignore what I said. I totally missed the fact that the solution was $f/\color{red}r$, not $f$. This is why I don't do maths at 3am. I'll have a look at your question in the morning if no one has answered it already – mattos Nov 25 '19 at 15:41