By comparing the frequencies of the first four modes of an ideal circular vibrating membrane to those of a drum made from a dentists rubber dental dam and PVC pipe, the influences air-loading, membrane thickness and stiffness can be demonstrated.
Circular Membrane (drum head) Vibration
Video courtesy of Dr. Dan Russell
The above chart shows that the frequencies and subsequent ratios to the other modes of the latex rubber dental dam membrane differ from those of an ideal vibrating circular membrane if both had a fundamental frequency of 82.2 Hz. Since the ideal membrane exists in theory only, the frequencies of the dental dam membrane have been affected by the thickness and stiffness of the membrane itself and to some degree, by the interaction between the membrane and the air inside of the PVC pipe and the air above the membrane. This interaction is referred to as air loading and significantly affects how larger membrane vibrates. Because of the small footprint, the membrane on the four inch PVC pipe is not affected by air loading to the same degree that a large timpani head would be, but it should be take into consideration.
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