Monday, 28 December 2009

Twang on a can

video
Here, as unwisely promised yesterday, is a clip of me twanging the ring-pull of an aluminium fizzy drink can whilst simultaneously filling it with water. What do you think happens to the pitch of the note?

My own, subjective and non-scientific, opinion is that the pitch stays the same. The note seems to get higher because the upper harmonic partials are increasingly favoured as the can fills. If I were to blow the can like a flute then I would hear the air inside vibrating. Less volume of air, as the water replaces it, means a gradual rise in pitch. When I twang the can it is the can I hear resonating. The water dampens (no pun intended - 'deadens' might be a better word) the sound of the lower partials.

Today I went for a long walk by the sea so no sonic experiments. But tomorrow I shall continue to explore the seasonal debris and make some more rubbish instruments. If you would like to join in then hang on to all sizes of bottles, both glass and plastic, tin cans of various types, wrapping paper and anything else you think might produce a noise.

3 comments:

  1. Jonathan - that was strangely enjoyable and yep - I think your physics is pretty sound there.

    I was tempted to say something (far too obvious)about needing to get out more but I culd list a set of projects that have happened round this house that make your experiment seem incredibly meaningful.

    Who was your assistant? Nicely poured.

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  2. It sounds to me like the pitch drops by a semitone or so before rising again. At the end it does seem to favour the upper partials to the exclusion of the lower ones.

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  3. Hmm. There does seem to be some pitch variation. I don't know why that should be. Perhaps the air inside the can is being agitated as the water from the jug hits the water already inside? This would impact on the sound of the resnoating can but I don't think it really explains the phenomenon.

    I have two cans of Fosters that I discovered after a party here last summer. Next time a scientist calls I'll open them in the interests of, well, science. (And with any luck, said scientist will drink them both.)

    Alas, I had no assistant, Molly. It was all done too early in the morning.

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