The waves from the sound source at the end of the tube react with the waves reflected by the moveable piston creating stationary waves. The cork powder creates patterns making wavelength’s measurement possible. Now, knowing the frequency, you can measure the acoustic waves speed in the air. The item comes supplied with the tube, stands and bases, piston and cork powder spreader. It needs a loudspeaker, code PH137176 and a Signal Generator, code PH137168 sold separately.