Measurements of coupled drumhead vibrations using electronic speckle-pattern interferometry

Many musical drums such as snare drums, tom toms, and bass drums consist of two membranes at opposite ends of a cylindrical shell. Striking one head causes both to vibrate, as their motions are coupled due to the enclosed air as well as the shell itself. An optical system consisting of two electronic speckle-pattern interferometers was constructed allowing operating deflection shapes of both heads to be viewed simultaneously while the drum is driven acoustically at a resonant frequency. This system allows the determination of the relative phases, orientations, and amplitudes of the vibrational patterns on the two heads. Previously reported results for coupled drumheads were verified and extended to include the effects of degenerate single membrane mode pairs that are split due to non-uniform tension in the heads. Examples of higher frequency coupled patterns are also shown. Parameters influencing the degree of coupling are described briefly and compared qualitatively with results from a finite element model.