Kundt's pipe

Since the sound wave is reflected at the closed end of the tube, it is superimposed on the incoming wave. If the adjusted tone has the correct wavelength, both waves form a so-called standing wave. This is called a standing wave because its so-called wave bellies and nodes are always at the same point. Where the wave has knots, the polystyrene balls remain in place because this is where the movement of the air is lowest. In the bellies of the wave, on the other hand, the polystyrene balls are pushed away because here the vibration of the air is strongest. This is how the observed pattern is created.

If you change the pitch with the controller, the wavelength of the sound wave inside the pipe also changes. Their bellies and knots shift, the polystyrene balls rearrange themselves and form a different pattern.