What it shows:
Bell plates are polygonal-shaped flat pieces of sheet metal which, when held in the hand and struck with a beater, produce a pleasant, sustained, slightly bell-like tone. Compare this to any arbitrary shaped piece of metal which produces a "clunk" when struck. The sound of the bell plate depends strongly on its shape and even the most modest change in the symmetry (like snipping off a corner) or proportions will make it go clunk when struck.
How it works:
Why does a particular shape ring so well, while slightly different shapes do not? The short answer is that the nodal lines of one of the lowest modes of vibration merge in the handle to produce a region that exerts neither vibrational force nor torque on the hand, and therefore does not transfer vibrational energy to the hand. Our bell plates are designed from a paper1 by D. Lavan, S. Hogg, and J. Wolfe, "Why Do Bell Plates Ring?," Acoustics Australia 31, August 2003, No. 2.
Setting it up:
The bell plates are cut from 1/16" thick aluminum sheet. You need not use the dimensions shown in the drawing, but the ratios of a, b, and c are important. Smaller plates will sound at higher frequencies and larger ones lower. You can also use thicker or thinner stock material, but stick with aluminum for easy cutting.
Hold the plate by the tang (the little handle) and strike it with with a beater somewhere between the middle and top of the pate. It should ring beautifully. With a pair of tin snips, cut off a corner of the plate. Now the plate goes "clunk" when struck with the beater!
Footnotes:
1. The paper may be downloaded from the University of South Wales, Australia website http://newt.phys.unsw.edu.au/jw/basics.html, a site well worth visiting for a wealth of information on acoustics.