The Rubens' Tube: Soundwaves in Fire!


Step 4: Nerdy stuff - measuring sound.

The speed of sound is roughly 340 meters per second at sea level, but this is when air is the medium through which the sound waves travel. But because propane is of a different density than air, the velocity of sound is also different, and like all gases, the density changes with heat or pressure changes. For our purposes, we can work with the a velocity of 257 meters per  second.

As mentioned in the last step, sound is a vibration, we measure the frequency of this vibration in hertz (Hz), which is the number of cycles of the vibration per second. Wikipedia tells us that "The frequency (f )is equal to the speed (v) of the wave divided by the wavelength (lambda) of the wave".

So in other words - frequency = speed / wavelength or:
f = v / lambda

To find the wavelength, we use basic algebra - multiply by lambda and divide by f to get.
lambda = v / f

To test this we can take the sound wave used to demonstrate the device in the video as an example (360Hz), and use or rough speed of sound for v.

lambda = 257(m/s) / 360Hz
This gives us a value for lambda of about 0.71 meters. Which should be close to the distance between the peaks of the flames. Though the actual measured value may differ from what is calculated given the above mentioned scenarios.

Note - for some reason the lambda symbol keeps turning into this when I save "�»". So I've replaced the symbol with the word "lambda". I apologize for any confusion.

Special thanks to user cposparks, who found an error on this page when it was originally published, I've since made best efforts to correct it.
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to pennyroyal69 , i think the problem is with the size of the holes in your ruben's tube !! Not the size ! You see larger holes mean lower pressur and the fire gets into the tube but make the holes smaller and te length of the flames will increase. Hope it workes :-)
is it possible to do this rubens tube by using lpg...pls send me wetr it can b done by using lpg to my email id
cposparks3 years ago
Were your experimental measurements similar to your theoretical measurements? I ask this because the speed of sound is 340 m/s in air but the sound waves are being created in a medium that is composed of propane. The speed of sound in propane is ~247 m/s. This would significantly change the theoretical wavelength for an arbitrary frequency.
yourtvlies (author)  cposparks3 years ago
I recall things being pretty close -- though now that you mention it, considering the peaks and troughs are fairly subtle, particularly at the longer wavelengths where the discrepancies would be more noticeable, I very well may have been off in my measurements. There's also the possibility of an air/propane mixture in the tube, which would put the density of the medium somewhere between propane and air.

It's been three years (to the day!) since this Instructable was published, and unfortunately, I no longer have the device, so am unable to go back and check for confirmation, one way or the other. However, your observation's very astute, and I've started to do some research on the issue, though have found conflicting reports. As soon as I've got things figured out one way or another, I'll update the Instructable to reflect that - though until then I'll put a disclaimer on the top of this page.

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