Let's start with a look at a very simple acoustic resonator: a tube with one end open and the other end closed. You can make such a resonator out of any kind of tube, and you can make it "ring" or resonate by slapping the open end with your hand. For this discussion, let's assume you've made it resonate somehow, and think about what's going on inside the tube.
The air at the open end of the tube bounces in and out, but it doesn't get squeezed (because it's open to the outside air). The air at the closed end of the tube gets squeezed and stretched, but it doesn't move (because the closed end stops it). Physicists say that the open end is a velocity maximum and a pressure minimum, and the closed end is a pressure maximum and a velocity minimum.
Figure 1. The dots represent parcels of air. At the closed end,
they get hot (red) when squeezed, and cold (blue) when stretched.
This page maintained by Wil Howitt