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 "Ã�Â»". 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.