The movie below shows a standing sound wave in an open pipe.
- Notice the dots running off the edge of the picture at each end. That's showing the air moving in and out of the open ends of the pipe.
- The pressure antinodes show up as places where the dots clump up periodically. The compression (clumping) is easier to see than the rarefaction.
- If you look carefully, you should be able to find at least one dot at each pressure antinode that doesn't move. That shows that pressure antinodes are displacement nodes.
- Half-way between pressure anti-nodes you should see the dots moving a lot, but the density remaining fairly constant. (I say fairly, because there are few enough dots that small fluctuations in their distribution may lead to small but noticeable changes in density.)
- The red line is the displacement (the vertical axis shows how far - horizontally - an air molecule at each position in the pipe has moved from its equilibrium position).
- The green line is velocity (the vertical axis shows how fast an air molecule at each position in the pipe is moving).
- The blue line is pressure (the vertical axis shows the pressure at each position in the pipe).
- The lines disappear when they become completely horizontal. This is probably related to the fuzziness, but I haven't found a cure for either yet.
You should be able to step through the movie frame by frame with the controls showing at the bottom of the window. At the left should be a button that looks like ||. Clicking that will pause the movie. Then the arrows on the right can be used to step through the frames.