I was on board as soon as one of my group mates described the project as a "tube of fire." A perforated 5 ft PVP pipe is pumped full of propane and hooked up at one end to a speaker. The sound waves move the propane through the tube, creating areas of high and low pressure, which change the amount of propane pushed out of the tiny holes along the top.
The end result is essentially a musical gas grill, and one with some pretty interesting physical applications. It converts sound from a transverse wave into a longitudinal wave using flame height. This allows the wavelength of sound to be measured directly.
It also looks really really cool.
This is a constant 400hz being played through the tube. Of note is the end of the video, when the frequency stops playing and the flame levels immediately equalize. Using this image, our group was able to measure the wavelength of the sound waves. Knowing both this and the frequency, we could calculate the speed of sound in propane. We found this value to be about half the speed of sound in air. This value was in line with other calculations done through the use of these tubes.
Here are some pictures of it in the daylight, to get a better idea of our setup:
The top of the tube was covered in double layered aluminum tape to prevent the fire from spreading to the PVP pipe. The holes were approximately 0.25 centimeters and drilled through both the pipe and the tape. As the day went on, the fire started to melt the adhesive in the tape and clog the holes. The heat from the flames also warped the tube. This rendered the project unusable after the first day of testing was over. Some of the holes produced no flame at all, while others were disproportionately large. A better design, and one I am likely to try again in the future, is to use copper piping instead of PVC.
After the scientific aspect of the project was over, we blasted various songs through the speaker to see the effect. The result can be seen in the concluding video (if possible, turn your sound up):
