The shell is not the head, but it is not silent either.
It has its own modes, fixed by its mass and stiffness.
When the heads land near them, the shell joins in.
A shell has shapes too.A shell has shapes too.
Tap a bare shell and it rings. Like the head, it has modes: it goes oval, then three-lobed, then four-lobed, each higher than the last.
Those frequencies come from the shell itself: wall thickness, stiffness, mass and diameter. Tuning the heads does not move them.
Fig. 3
Tune the heads past the shell.Tune the heads past the shell.
The heads' modes move every time you turn a lug. The shell's stay put, so as you tune, head modes slide past shell modes.
When one lands close to a shell mode, the two trade energy. The shell rings along, that part of the sound gets more body, and the head mode often dies a little sooner because it is sharing its energy.
Fig. 3 / Heads move, the shell stays putSimplified ring model · 14 × 6.5 maple
Fundamental
220 Hz
Closest head mode
(0,2) 505 Hz
Nearest shell mode
n=4 515 Hz
Gap
1.9%
220 Hz
Shell mode
Listen to
Orange lines are the head's modes for a typical hit. Dashed lines are the shell's own modes for a plain 10-ply maple shell, 7.2 mm thick. Tuning the heads slides every orange line, but the shell's modes stay where its mass and stiffness put them. When a head mode lands in a shaded band, the two trade energy and the shell joins the sound. Shell mode frequencies here come from a thin-ring model, so read them as the pattern rather than exact figures.
That is why drums have sweet spots.That is why drums have sweet spots.
Most drummers know a drum that comes alive in one tuning range and goes flat in another. Part of that is the heads meeting the shell's modes. Part is the two heads meeting each other through the air.
A shell does not have one sound. It has a set of places where it agrees with the heads.
Damping decides how long the shell holds on.Damping decides how long the shell holds on.
Wood, glue lines, finish and hardware all soak up vibration. A heavily damped shell joins in briefly and lets go. A lively one keeps ringing with the heads.
Neither is right. They are different instruments.
Our view.Our view.
A very stiff, heavy shell holds the heads and mostly stays out of the way.
A lighter, more compliant shell moves its modes down toward the range where the heads are working, and joins the sound. We design for that: a shell that takes part on purpose, in places we chose.
Bibliography
Selected sources.Selected sources.
Primary papers and references used throughout the essay.
