Video: Sound is a pressure wave

Here is a 50 cm long tube filled with air. It is open at one end and, at the other end, there is a speaker that emits a pure sound with a frequency of 680 Hz.  

This frequency corresponds to the resonant frequency of the second  harmonic. We see here the stationary (or standing) wave characteristic of this mode.

Some  layers of air never move. They are located at the nodes of vibration. (…)

The layers of air that vibrate with the greatest amplitude are located at the antinodes of the vibration.

You can see this very well on the curve s(x,t) which measures the horizontal displacement of a layer of air with abscissa x.

Let’s place a microphone in the tube. It is not sensitive to displacement of air molecules, but to pressure.

We see that a node of vibration corresponds to an antinode of pressure, and vice versa. An antinode of vibration is a node of pressure.

We always have to specify which wave we are considering.