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



Scott Goelzer wrote:

Tuning fork had a bit of mirror mounted on the top of one fork and was
set vertically on rotating turntable. A laser aimed at the mirror
reflected a nice sine wave on the wall as the fork rotated and
vibrated.

Ooooh, I like that.

Was impressive.

I'm impressed just imagining it. I've been thinking about it for
the last couple of hours, with a big smile on my face.

==============================

Here's a scheme for making it more general.

Key idea: If you have a rotating mirror, none of the other
stuff needs to rotate.

For example:

(laser) ----->---- \ (audio thing, with vibrating mirror)
|
|
|
|
(rotating mirror) \ ------->---- (screen)


The advantage here is that the audio thing can be something
other than a tuning fork; it could be a loudspeaker, which
has too many wires and too much mass to be conveniently
rotated.

The tuning fork gives you a nice sine wave at a single
fixed frequency. The loudspeaker gives you almost anything.

One tiny problem to be overcome is that you want the
vibrating mirror to change its _angle_, but most of
a typical speaker changes only its position.
1) One way to overcome this is to mount the mirror
somewhere on the suspension of the cone, not on
the main part of the cone.
2) There are other ways, involving lenses and such,
but I think they're harder than way (1).
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