Engines · Piston · Demo
An engine’s voice is its firing intervals — hear the crank’s arithmetic.
Audio only starts when you press the button — then the crank runs on the audio clock itself, so every flash lands exactly on its pulse.
The shared I4 firing order 1-3-4-2 — a pulse every 180°, dead even.
Idle to redline for this engine. Pitch climbs with rpm because the pulse rate does — the character (even vs uneven) never changes.
Equal gaps: every pulse is interchangeable, so the fundamental you hear IS the pulse rate — one clean note that rises with rpm.
In plain terms
Every power stroke shoves a slug of hot gas out of the exhaust — one pulse. A four-stroke cylinder does that once per 720° of crank, so an engine is just a drum machine: n drum hits spread around a 720° loop, played faster as the revs rise. Space the hits evenly and you hear one clean pitch. Space them unevenly — a 90° V-twin can’t help it, and each bank of a cross-plane V8 hears it in its own pipe — and you hear a rhythm riding on the pitch: the lope, the burble. Same physics, different arithmetic.
A cylinder fires once every two crank turns, so a four at 3 000 rpm fires 100 times a second — a 100 Hz buzz. That’s why engine pitch climbs with rpm and why, at the same revs, a six sits 1.5× higher than a four: more pulses per cycle, more pulses per second.
When every gap is identical the pulses blur into a single steady tone. The 90° V-twin’s shared crankpin forces gaps of 450° then 270°: two bangs close together, a long pause, repeat. Your ear locks onto the repeating PATTERN, so the perceived note drops to the pattern rate — the loping gallop.
Cross-plane and flat-plane V8s both fire dead-even every 90°. The difference is which BANK each pulse leaves by. Flat-plane alternates L-R-L-R, so each manifold hears an even 180° — the hard exotic wail. Cross-plane hops the banks irregularly, so each manifold hears 180-270-180-90 — the muscle-car burble, even though the engine as a whole is smooth.
Formally, the exhaust is a pulse train with period 720° of crank: fundamental f₀ = rpm⁄120 Hz, with energy at harmonics n·f₀. Even firing puts all the energy on every N-th harmonic (N pulses/cycle), which reads as a single tone at N·f₀; uneven firing spreads energy across the in-between harmonics — sidebands your ear hears as roughness. Real engine voices add intake honk, mechanical noise and pipe resonances on top; this demo synthesises the firing-interval skeleton those sit on.