The incident acoustic wave interferes with the reflected one, creating the stationary waves. The polystyrene balls visualizes nodes and bellies, so making wavelength measurement possible. Now, knowing the frequency, you can measure the acoustic waves’ speed in the air. The item is supplied with tube, stands and bases, piston and the polystyrene balls spreader. It must be used with a loudspeaker code 3017 and an oscillation generator code 5718 sold separately.