[Coda]

Well, maybe, maybe not. Acoustics is a very rich field, so I'm sure you can imagine that a lot of factors play into that perception.

So first off, being three-dimensional, that energy spreads out in a spherical wavefront. The farther from the source, the larger the sphere, so that energy is spread out over a greater area. This is, in fact, the primary reason why sounds get quieter the farther away you are. And since a sphere's surface is two-dimensional, you can observe that the loudness is inversely proportional to the square of the distance, that is, at twice the distance it's 4x quieter -- this is called the inverse square law and it comes up in a LOT of places. So if you're closer to the source of the sound than the other person, you might hear it louder.

Second, there's a loss of efficiency when the energy has to move between regions of different density, such as from a solid door into the gaseous air. This is because some of the energy gets reflected instead of transmitted -- just like light shining on a piece of glass; some bounces off instead of going through. This is why sound gets louder if you cup your hand around your ear (some of the sound bounces off of your skin instead of passing through, and it gets redirected into your ear) and why it gets muffled if there's a wall in the way. On the other hand, the sound might echo inside the room, allowing more of the energy to come to the other person's ear, but the sound on the outside will just disperse out into the air.

That effect also applies to the act of knocking itself. Some sound goes into the door; some sound reflects. Depending on how the door is constructed, it might transmit the sound well, or it might muffle it, and it might reflect the sound well (so you hear it better) or it might not (so it sounds a lot duller to you than on the inside).

There are other things, but I think that's enough of an answer for now. :P