Crossing boundaries with information intact

It is entirely possible for waves to cross boundaries, be distorted by them, and still say a lot about their source. A physicist would tell you this, so too could the door on my bedroom. I was awakened earlier than I would have liked one day by a curious rythmic thumping coming from the direction of my door. No one was knocking, in fact I think the door was just kind of thumping against its jamb. Given the general pattern of noise, and the fact that my roommate had an overnight guest I was quickly able to ascertain what they were up to. Even though they themselves weren't making much noise in their, er, activity, they must have been causing enough movement in the structure of the house to cause my door to bang gently against its jamb. Here we see it is possible for mechanical waves to migrate through a number of boundaries and have effects that are felt in other systems and their source can be reconstructed.