looking for an unknown driver

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Hi everyone .
some time ago on this forum I saw an acoustic transducer. I can't find the right link again, I don't know what it is and I don't know in which section I've seen it. it consisted of two empty cardboard cylinders, between them near and parallel, and presumably a third behind them, inside these cylinders there was something long and thin in coincidence with the axis of the paper cylinders, perhaps a electromagnet, from the appearance I deduce that it is self-made.
do you know what it is?
 
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