Re: Resonance Demo

[Leigh Palmer <palmer@SFU.CA>]

> I need help in getting printed plans for making the following resonance
> demo, which I saw at one of the conferences/workshops I attended in the
> last two years (I can't remember exactly where or when):
>
> The apparatus consists of a wooden box about the size of two shoe boxes
> laid end-to-end. There is a speaker (possibly two) inside and the box is
> divided into (3 or 4 ?) chambers. There is an opening or two facing
> upward. There are two supports on which is placed a piece of (plate) glass
> (about 2 feet long x 4-6 inches wide). The supports hold the glass about
> 1-2 inches above the box. As the frequency of the sound is adjusted, a
> standing wave can clearly be seen to develop in the glass, and at the
> right frequency the glass shatters.
>
> This is more dramatic than the usual approach with speakers and a beaker
> or wine glass. It is also easier to setup--no microphone and oscilloscope
> are needed to tune the frequency. The sound does not have to be so loud as
> to hurt the ears of those in the front row of the class.
>
> If you have plans for this (including what kind of glass to use, which
> is critical) or can refer me to someone who might be able to help, please
> let me know.

Sounds like a demo I made using two tweeters pointed upwards under a
frame. I suspended aluminum plates about 1/16" thick in the shapes of
polygons and a circle about 15" in diameter above the speakers. I
mounted them on upturned, sharpened rubber stoppers placed at what I
expected to be the nodes. I anodized the aluminum and dyed it black.
Sprinkling salt and/or lycopodium on the alumninum, I drove the
speakers (with a polarity reversing switch so they could be driven in
antiphase if desired) and tuned an audio VFO through many, many
resonances. The patterns of the salt at the nodes were very sharp and
symmetrical, and the lycopodium migrates to the antinodes! You can
use this setup as an unmixer.

The rubber stoppers worked *much* better than the standard Chladni
mount for this purpose (a sodlily bolted mount) and it is much easier
to change plates, too. Boeing is more elegant, of course, but I saw
many more modes with this rig than Chladni could have seen.

Leigh