A question about the simplest of things: Based on the physics of sound, how can you hear someone through a closed door? I'm quite confused because someone once told me its was because the sound passes through the door; since its causing the air molecules to vibrate, when this vibration hits the door, it similarly causes the door to vibrate and transfer the sound energy through to the other side. I've also read about diffraction as the cause so, what, is it a combination of both? Or just one. If its just diffraction, then if I was in a completely airtight room, would I then, technically, be unable to hear anything from outside?