The acoustics of a Karen bronze drum
Nickerson, Laura Margaret
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Bronze drums are important to the culture of the Karen people, who live mainly in Burma and the mountainous region between Burma and Thailand. A Karen bronze drum, which is cast in one piece, consists of an overlapping tympanum that may range from nine to thirty inches in diameter and a cylinder that is slightly longer than the diameter. Bronze drums have a variety of use, both musical and nonmusical. The acoustical properties of this drum, which have never previously been studied, are presented. Both the sound spectrum and the vibration spectrum of the Karen drum show a large number of modes of vibration, especially in the frequency range up to 2 kHz. Although there is no single dominant strike tone, prominent partials can be heard around 330 Hz, 277 Hz, and 247 Hz plus a low hum tone around 55 Hz. With the exception of the hum tone, these are associated with identifiable modes of vibration or mode combinations. The many modes observed with TV holography can be classified as 1) primarily modes of the cylinder, 2) primarily modes of the tympanum, or 3) mode mixtures. When arranged into families according to the number of nodal circles n, the cylinder mode frequencies for n = 1 and n - 2 families appear to have a minimum at some value of m, as predicted by the theory of vibrations in a capped cylindrical shell. The tympanum mode frequencies are in better agreement with those calculated for a flat brass plate with a simply supported (hinged) edge as compared to a clamped edge. The observed mode frequencies are consistently less than the calculated frequencies, however, because of increased mass due to the lead impurity and to the mass of the portion of the tympanum overhanging the cylinder.