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Holiday Physics: Resonance

Chris Creagh's picture

There is a lecture demo in the resources section called "Resonance in a Rotating Tube" but...

Interesting observation: If I take a cardboard tube from the seasonal wrapping paper and wave it about I do not get resonance in the same way. Why is it so? Why does it work for the pool pipe and not the cardboard tube?

I can hear the resonant frequency of the tube if I put my ear to one open end and point the other end at a source of white noise (the airconditioner). I know it is resonating because if I put my hand over the end of the tube nearest the airconditioner, the fundamental frequency of the sound I hear drops considerably even though it is still somewhat "white noise" like. By recalling the formula it must in fact be half the original frequency because there is now a quarter wavelength in the tube as opposed to half a wavelength.

I checked my assumptions about using the noise from the airconditioner as a source of "white noise" by putting the open end of a mug against my ear and as I bring the mug closer to my ear I hear the white noise of the airconditioner but it seems lower than the ambient sound so the mug must be eliminating some frequencies in the "white noise" and not others. My mother use to call this effect the "sound of the sea" when she would put a shell against my ear.

Anyway why does the pool pipe resonate but the cardboard one does not? My partner says it must be the "Q" factor (or quality factor). An object with a high Q resonates in a narrow band of frequency with high amplitude. This is dangerous if it is a bridge and why bridges have dampers on them. So my cardboard tube must have a low Q i.e. each time the sound wave traverses the tube it loses so much energy that audable resonance can not be sustained i.e. the cardboard walls are too soft and squishy. So now I am looking for a cardboard tube with solid walls to test this new theory.

Found one! And yes there is possibly resonance there when I wave it around but not as much as with the plastic pool pipe.

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