  Spring-like forces on the bridge are key to a banjo's characteristic voice.
These are due to the tension in strings and head. Conceptually distinct from
the forces of waves in the strings and head that encode the underlying music,
the spring-like forces impact the timbre of how those waves are converted to
sound. This note presents a simplified model that allows the head contribution
to be calculated (mostly) with paper and pencil. The key simplification is
placing a circular bridge at the center of the head. The resulting formulae
show how design elements and player's adjustments can effect the sound. The
results also provide estimates of the magnitudes of the effects when evaluated
with numerical values for Young's moduli and typical banjo set-up specs. For
steel strings and tight mylar head, the head contribution is about three times
as large as that of the strings.
