While studying waves I read the fact that a sound wave gets shifted by $\pi$ as a result of reflection against a surface. But I am unable to prove that fact.
Assuming the interface to be a node I can prove that there is a phase shift of $\pi$ but speaking generally how do I deduce it? If we do not assume that the interface is a node I saw in some case it's not true.
Am I misunderstanding something here? Does the sound wave manipulate itself so as that the interface becomes node? How does it do that?