Ruben's tube: Standing sound waves in flames along a large pipe.

Standing Longitudinal Waves in a gas: Reuben's Tube. A 12' steel tube has holes along the top. A speaker is at one end, and a 'tuning' plunger is at the other. The plunger is pushed in to the black mark, tuning the tube to multiples of about 70 Hz. Propane is introduced at full force into the tube, and the gas is lit above the holes. A signal generator causes standing waves in the gas, causing the flames to be high in the areas of compression and low in the areas of rarifaction. To find a standing wave, start with low volume, and vary the frequency across some multiple of 70 Hz. Slowly bring up the volume and fine adjust the frequency until the wave front is displayed on the flames. NOTE: the Reuben's tube in 72 LeC. resonates at multiples of about 70. The one at Pimentel is at multiples of about 80 Hz.