If you get a message "need java2 for this applet" then you
can use the Java 1 version of this applet
(but you won't get sound). If you don't have Java 2 I recommend
you go get the Java plug-in.
This java applet is a simulation of waves in a circular membrane (like
a drum head), showing
its various vibrational modes. To get started, double-click on one
of the grid squares to select a mode (the fundamental mode is in the
upper left). You can select any mode, or you can click once on multiple
squares to combine modes.
