The two are related :) I chose displacement because it's a simpler thing to explain visually, and didn't want to cover both. In the bit with the air molecule g

The two are related :) I chose displacement because it's a simpler thing to explain visually, and didn't want to cover both. In the bit with the air molecule grid, the areas where the molecules are clustered together are high-pressure, while the areas with large spaces between them are low. This is caused by the staggered nature of the displacement vibrations. (I'm sure there's a more sophisticated way to explain this, but that's as I understand it)