Just noticed you replied to this here!

I'm guessing that the effect is most pronounced when he opens the door quickly and when your door is nearly (but not quite) closed.
Well, first of all Lara comes home, then Alex half an hour later. Lara tends to open the door more slowly, so my door creaks open slower when she arrives. Alex burts in very quickly, and thus my door opens faster. I normally have my door slightly ajar.

I thought it was essentially a sound wave. My understanding is that a shock wave that is subsonic is a sound wave, while supersonic shock waves are pressure waves (like when that meteor exploded over Russia and blew out a shit load of windows). But this is through a gaseous medium. What happens to a solid that has a supersonic wave going through it? Can it happen?