Now you hear it, now you don't. Show how two sounds cancel each other out when added together. Two Y-shaped glass tubes are connected with rubber tubing in such a way that one path is about one-half wavelength longer than the other. When a tuning fork is held near one end and the path difference is about 65 cm, you'll hear no sound at the other end. When either tube is pinched shut, however, the tuning fork comes through loud and clear. With instructions. You need 256 or 512 Hz tuning fork, not included.