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Using a dual electronic speckle-pattern interferometer to study coupled vibrations in drumheads

Worland, Randy and Boe, Benjamin 

A dual electronic speckle-pattern interferometer (ESPI) system is described that allows images of operational deflection shapes of two sides of a vibrating object to be viewed simultaneously and recorded. Experimental details of the system are discussed and applied to the study of musical drums with two heads at either end of a cylindrical shell. In particular, methods for determining the degree of coupling and the phase relations between the two oscillating heads are shown. A variable length drum has been constructed and used to investigate the coupling of membrane vibrational patterns as a function of the distance between the heads. It is shown that the coupling vs. length data depend strongly on the shapes of the vibrational patterns. These coupling trends are illustrated and further interpreted with the use of a finite element model of the drum, which shows the role of the enclosed air motion within the shell.