High Frequency Formants from Banjo Bridge Design

Abstract

Eight bridges, matched for weight and height, reveal some of their secrets. Seven are ebony-topped maple; one is solid poplar. Five have radically different shapes. Two are similarly shaped three-foot bridges with obvious differences in grain. And three differ by the tiniest of wood removal (i.e., less than 0.04 gm). Below 2 kHz, the bridges perform nearly identically — in accord with the notion that bridge weight and break angle determine the bridge's impact in that frequency region. But at higher frequencies, they impart individual voices. The results are consistent with the idea that the flexing of the bridge plays an essential role in producing the differences. With flexing the key, the same bridge does different things to the sounds of different banjos because the operative flex footprint has to move in conjunction with motion of the head. The crucial additional variables are string and head materials, tensions, and break angles. The three dimensional aspect of flexing is why wood species matters for a given weight and 2D shape. The examples contradict the relevance of a "path" taken by the sound from the strings to the head; that is also dubious from the perspective of basic wave physics.

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