# Finding the air cavity resonance of a propane tank for a drum.

1. Nov 30, 2012

### great_sushi

I am going to build a drum out of a propane tank. I think it will be handy to find the resonant frequency of the tank and choose the tuning accordingly so not to pick frequencies that are extremely damped.

I have a cavity resonance formula found on hyper physics

I am not going to build a drum with only one note, there will be approximately 7. 7 notes in most scales.
If you look at the image you'll see the notes, im going to refer to them as tongues.

Once I find the resonant frequency I can decide on the tuning. Another problem is choosing the dimensions of the tongues at the top. I've been trying to find another formula so that I can determine the size of the tongues, so a larger tongue results in a slower oscillation producing a lower note and vice versa. But I will need to relate the tongue dimensions to the resonant frequency of the tank.

Im curious to know how to calculate the resonant frequency of something steel with particular dimensions.

Once I have that relation I can move forward. Also, fine tuning will be done by either bending the tongue downward/upward permanently as to result in a lower/higher frequency respectively.

Thankyou, If all goes well, my drummer sister will have another present for christmas.

2. Dec 3, 2012

### Staff: Mentor

Do you have an oscilloscope on which to measure frequency, if you were to use a microphone? (You might also need an amplifier.) If you held a microphone at the mouth of the cavity, and dropped a marble inside, you'd record a decaying echo which would be the frequency of resonance. I don't quite understand how something like this could simultaneously have 7 unrelated resonant frequencies, but maybe that is possible. I don't claim to understand much about the mysteries of music.

3. Dec 3, 2012

### great_sushi

I don't have a microphone but I could introduce a speaker into the cavity and do a frequency sweep. Noting the frequencies that 'stand out'. I imagine that there will be one natural frequency and perhaps 'n' integer values of the wavelength.
Say having one wave with (i think its nodes where you hear maximum intensity) nodes at both ends, (only two). Then one whole wavelength with 3 nodes, then 4 nodes, then 5.
I might not be explaining it very well and I may be wrong.

It would be great if someone could explain it properly

4. Dec 4, 2012

### Staff: Mentor

On further reflection, I guess the air cavity could have a resonant frequency that is independent of the mechanical resonance of the steel shell. I expect drumming effect would be most impressive when the design sees these two frequencies coincide.

You certainly could try sweeping the speaker frequency and listening for a peak, but when I tried this using a column of glass tube as a class demo, the peak was as good as imperceptible.

An idea that could hold promise involves introducing both a microphone and a speaker into the cavity, the speaker delivering an amplified version of what the mic picks up. As the gain is turned up, feedback should see the system begin to oscillate and show feedback squeal. I expect that frequency of oscillation would be the resonant frequency of the cavity. You might be able to press into service the mic and earpiece from a discarded cell phone for this. This isn't going to reveal multiple resonant frequencies, though.

Last edited: Dec 4, 2012