I was half-way through this when I saw Robert's post, which is also in the same vein.
One caution is that all of the "playing card and bike spoke" type of demonstrations typically include two contributions: the pulse of the card at the frequency of the spokes and the "slap" of the spoke against the card. Even when the spoke hits once per second, there is definitely some audible sound produced - presumably a transient, broadband pulse. At high frequencies, you "hear" the overall frequency of the spokes; at low frequencies you hear the slaps. Some of the increase in audibility is simply that the faster the wheel turns, the harder the spokes are hitting - an effect that can mask some of the effect you are trying to produce.
Tim Folkerts
Instructor of Physics
Barton County Community College
Great Bend, KS
620-792-9320
"The only thing necessary for the triumph of evil is for good men to do nothing." Edmund Burke
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