The head only touches the drum at the bearing edge.
If the edge is round and true, tension spreads evenly. If it is not, you can hear it.
This is where precision stops being a spec and becomes a sound.
Even tension is the goal.Even tension is the goal.
A head is pulled down at every lug but rests on the drum only at the bearing edge. When that edge is round and flat, tension spreads evenly across the head.
When it is not, some parts of the head end up tighter than others, however carefully the lugs are set.
Many of a head's modes come in pairs: the same shape, turned 90°. With even tension, each pair rings at one frequency.
Make the head tighter along one axis and each pair splits into two close frequencies. You hear the difference as a beat: the note swells and fades a few times a second.
Fig. 5 / Uneven tension splits a mode(1,1) mode pair · slowed, split exaggerated on the head
(1,1) mode
351 Hz
Split
4.4 Hz
Beats every
0.23 s
You hear
Clear warble
±5.0%
Presets
The (1,1) mode is really two identical shapes at right angles. With even tension they share one frequency and ring as one. If the shell is out of round, or the bearing edge has high spots, the head is tighter along one axis than the other. The pair splits into two close frequencies, the nodal line starts to turn, and the sound swells and fades at the difference between them. Lugs can tune much of it out, but a round shell and a true edge mean there is less to tune out.
You can tune around it, up to a point.You can tune around it, up to a point.
Drummers chase that warble lug by lug. On a round shell with a true edge it settles quickly.
On an out-of-round shell it keeps moving as you tune, because the problem is in the shell, not the lugs.
Where machining helps.Where machining helps.
CNC turns a drawing into the same cut every time. Roundness, edge profile and edge height come from the program rather than from the day.
That does not make a drum good on its own. It means the drum you hear is the drum we designed.
Repeatability is a design feature.
Tolerance you can hear is tolerance worth holding.
Bibliography
Selected sources.Selected sources.
Primary papers and references used throughout the essay.
