A circular membrane (drum head) vibrates with a variety of interesting patterns and shapes, each at their own frequency. In this demonstration I took a 6-inch square of latex rubber dental dam (courtesy of my dentist), stretched it over a short length of 4-inch diameter PVC pipe, and held it in place with rubber bands. I drew some lines and circles on the membrane to help identify vibration shapes. A frequency generator (you could use a computer) provides a sine wave to a loudspeaker. A strobe light was used to "slow down" the vibration so that it is easily visible. The flickering in this video is the result of mistiming between the strobe light and the video camera frame rate.