If air is made up real little particles, then why would the waves made up of these particles not bounce off? Here's my attempt at an explanation for the principle of superposition for soundwaves. The wave is transferred by the air molecules hitting the ones adjacent to it. But because air is of low density, some of the original wave's particles do not collide with the next packet of air because there is a lot of empty space. For a reflected soundwave interacting with the original wave, although the waves are of higher density compared to the surrounding air, it's still not dense enough such that some of the particles from each wave just pass each other by. The passing of the gas molecules would seem to explain the super-position principle. We can also say that gas molecules are moving too fast to be appreciably affected by the passing-by of another gas molecule. But this will explain why collisions of soundwaves appear differently than collisions of solids because every molecule in a solid is bound to a neighbor molecule, and pushing one solid molecule causes that to pull against the solid neighbor molecule while collisions in a soundwave are made of some collisions at the meeting point, but also a lot of passing-bys. From experimentation we know that two sound waves 180 degrees out of phase but of same amplitude will cancel each other out, so that means probably most of the air molecules in a wave are going to pass-by the oncoming wave and preserve the amount of amplitude needed to cancel out. Otherwise if say 50% molecules collided and 50% passed-by, we'd only get 50% net reduction even if the soundwaves were of same initial amplitude. Then I guess you could come up with a formula describing the probability of a certain air molecule colliding or passing by. What do you think?