Re: chladni plates

["Prof. John P. Ertel (wizard)" <jpe@nadn.navy.mil>]

On Fri, 14 Mar 1997, Dwight K. Souder wrote:

> 	I was wondering if someone could assist me.  We are currently
> covering waves and I've been trying to get a chlandi plate to work.  I've
> put the plate above a speaker, which is connected to a wave generator.  I
> can get some vibrations in the plate, but nothing to great.  I've tried a
>  wide variety of plates and wavelengths, but I'm still getting poor to no
> results.  I know many times this demo is done with violin bow strings,
> but I've also heard of using speakers.  My guess is that maybe my
> amplitude is to low.

The most likely problem is a poor coupling between the speaker and the
plates.  I have never used a "speaker" in this demo.

However, for years, we have used an acoustical driver (essentially a high
powered speakear w/o a cove or external frame --- no don't try to make one
by stripping a standard speaker).  This will work well but will not excite
all of the modes seen through direct edge excitation using a bow. 

Another way to "drive" the plate is to attach a nickle-chrome rod to its
center, bias a driving coil which is wrapped around the rod with enough DC
current to place the rod-coil system way up its B-H curve near saturation,
then add in the AC signal with which you wish to drive the plate.  This
has proven to be the best system that we have ever tried in terms of
sensitivity, repeatability of patterns, etc.  (One caveat: if you forget
the DC bias, it will hardly work at all!) 

One last way to drive a plate is to simply use a PASCO "Vibrating Driver"
which is readily available and moderately priced.  It is almost fool-proof
but is sometimes barely stable when coupled to a relatively high inertia
plate.  The patterns may vary in appearance and clarity.

> 	Also, I showed my students a picture of the "quiet room", the
> room where there is very little sound reflection from the wedge shaped
> material that is all over the room.  Does anyone know what type of
> material they are using?  Any chance I can get some for my underclassmen
> study hall?  Or better yet, a large enough speaker to generate a complete
> destructive interference for the entire room.  :^)
>
> Dwight
> dsouder@juno.com

ERTEL SENDS.         _____________________
                    / Prof. John P. Ertel \
                   /   jpe@nadn.navy.mil   \    
       +================================================+
       | Physics Department, 9C   Office : 410-293-6657 |
       |  U. S. Naval Academy        FAX : 410-293-3729 |
       | Annapolis, MD    21402     Home : 410-757-6618 |
       +================================================+