Inharmonic Partials of the Resonator Guitar

Abstract

Resonator guitars are rich in inharmonic partials, i.e., strong frequency components of a plucked note that are not integer multiples of the pitch frequency. A simple experiment demonstrates that their physical origin is overwhelmingly the single thing all resonator guitars have in common. To be louder than ordinary acoustic guitars, the strings drive a relatively light member to achieve a much greater amplitude of motion than occurs with ordinary wood soundboards. But a pluck’s sudden attack sets all normal modes of the instrument in motion. The unplucked strings stand out as getting significant amplitudes and are very efficient at producing sound that is “inharmonic” relative to the plucked string. Previous experiments established inharmonic partials as the salient distinction between resonator guitars and more common wood-topped acoustic guitars. The sound of those partials on their own is distinctly metallic or gong-like. All this supports the idea that basic resonator design is the physical origin of “twang.” In fact, as demonstrated previously, the job of the cone can be performed by something neither cone shaped nor thin aluminum, and the body need not be steel, brass, or metal at all.

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