[Physltest] [Phys-L] Re: 'Seeing' frequencies above 30-40Hz (fwd)

[John Denker <jsd@AV8N.COM>]

Robert Cohen wrote:
> Since it is the back and forth motion of the card and not the tapping of
> the spokes on the card* that make the pitch go higher, I find it more
> illuminating to apply a signal generator to a speaker cone (or to a
> solenoid with a bar magnet placed inside).  That requires special
> equipment but nothing more special than you'd find in a university lab.
>
> *With the wheel and card, I get students who think they can hear sounds
> below 30-40 Hz.

Excellent points.

Here's a low-tech way to solve the problems:

Rig up something approaching the classic "chopped air" siren.
   http://www.airraidsirens.com/tech_howtheywork.html
   http://www.airraidsirens.com/projects/tbolt/page1.html
   http://www.victorysiren.com/x/articles/HW10/HW10.htm
   etc.

As a first approximation, an old squirrel-cage fan plus a
stethoscope (held near the blades) will get you in the ballpark.

Once you see the principle of the thing, you can improvise
all sorts of examples, some of which are reeeally loud.
And the frequency depends on the rotation rate, without
the artifacts associated with the playing-card scheme.
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