A stick hits a head. Within a few milliseconds the whole drum is moving.
This chapter follows that one hit through the head, the air, the second head, the shell and the room.
Every figure here is live. Hit it, tune it, listen.
Fig. 1
Watch one hit.Watch one hit.
Slow a drum down enough and the sequence is easy to follow.
The stick hands the batter head a short burst of energy. The head bows inward, springs back past flat and keeps going. Each swing squeezes and stretches the air sealed inside the shell.
That air acts like a spring. It pushes on the resonant head, which starts to move too, and then pushes back on the batter head.
Fig. 1 / One hit, slowed downSlowed about 298×
01 / StrikeThe stick touches the batter head for a few thousandths of a second and hands it energy.
02 / HeadThe head bows in and springs back, over and over. That motion is most of what you hear.
03 / AirEach push squeezes the air inside the shell. The air pushes back on both heads.
04 / Second headThe resonant head answers through that air, a beat behind. The two trade energy back and forth.
05 / Shell and roomThe shell flexes a little with every squeeze. Both heads push waves into the room.
Batter head
220 Hz
Resonant head
202 Hz
Together
210 / 272 Hz
Head tension
2,925 N/m
220 Hz
Shell
A 14″ drum with both heads on, in cross-section. Orange marks air being squeezed, pale marks air being stretched. Coupled through the air, the two heads stop ringing at their own pitches and ring together at two new ones. A vent lets some of that air escape, so the heads pull on each other less.
Two heads ring as a pair.Two heads ring as a pair.
On its own, each head has a fundamental set by its tension, its size and its weight.
Put both heads on a shell and the air joins them. They stop ringing at their own pitches and ring together at two new ones: one with the heads moving together, one with them moving apart.
That is why changing the resonant head changes the note you hear from the batter side, and why the same heads sound different on a deeper or shallower shell.
Coupled heads split one note into two.
Stronger coupling pushes the two further apart.
A vent is a leak on purpose.A vent is a leak on purpose.
A vent lets some of the squeezed air escape instead of pushing on the other head.
The heads pull on each other less, the low end loses some push, and the decay changes shape. Size and placement decide how much.
Run Fig. 1 sealed, then vented, and watch the pressure inside.
The shell is listening.The shell is listening.
Every squeeze of the air also pushes outward on the shell. It flexes a little, and it has shapes of its own that it prefers to move in.
How much the shell joins in depends on where those shapes sit compared with the heads. Chapters three and four are about exactly that.
Most of the sound leaves through the heads.Most of the sound leaves through the heads.
The heads are large, light and moving a lot, so they push most of the sound into the room. The shell's share is smaller and shows up as color: which overtones last and which die early.
Then the room takes over. Walls, floor and microphone decide what the hit sounds like by the time it reaches you.
Bibliography
Selected sources.Selected sources.
Primary papers and references used throughout the essay.
