Re: Chladni figures...

[palmer@sfu.ca (Leigh Palmer)]

> Greetings!  I was wondering if anyone has ever tried making chladni
> figures as a lab or demo?  If so, could you give me some advice on doing
> so?  What materials did you use?  What type of patterns did you get?
> I've tried using a metal disk (galvanized sheet metal) supported at the
> center and various "bows" to produce the sound vibrations.  I've been
> unsuccessful and was wondering if there is anything else I could use that
> is readily available.  Any help would be greatly appreciated.

Boundary conditions are *very* important to the successful production of
Chladni figures. There are two ways to go, so I will describe both. The
first (and more common method, used by Chladni) is to clamp the plate as
firmly as possible at its center or at some point which must then become
a node of the vibratory motion. The clamp must also be as small as is
practical consistent with the requirement for solidity. Ideally one would
set a threaded steel bar vertically in the upper surface of a massive
concrete structure and clamp the plate tightly between two nuts. It is
then easy to excite the plate with a rosined violin bow. The modes you
will see with this method are strictly not the modes of the plate. They
are the modes of a plate clamped firmly at the point you picked. This is
the method used at science museums (like the Exploratorium) and students
can use it without supervision. I prefer another method, however.

I have produced very symmetrical figures on accurately machined circles
of aluminum 12" in diameter supported by resting them horizontally on
sharpened rubber stoppers  which I adjusted to produce the "best" results.
These patterns were excited by using two tweeters and a signal generator.
The boundary condition is the same; the supports must be placed at the
vibration nodes. However I avoid the necessity for firm clamping and the
distortion that is inherent in the large size of the contact necessitated
by the requirement for solidity. The result is really quite beautiful. I
used it mostly for demonstrations in a physics of music class. Many modes
can be produced with a bit of care, and the use of two speakers (located
underneath the plate) together with a phase reversing switch illustrates
the relative phase relations.

Leigh