Procedural Engine/Rotor Loop Synthesis

A code-only 7DTD mod can synthesize a looping motor/rotor sound entirely in C# — no .wav, no asset bundle — and drive its pitch/volume live. This is how the FPV mod's drone motor works (FPV/src/FPVDroneAudio.cs), not a sampled clip. Same runtime AudioClip path as [[Runtime WAV Loading (AudioClip from disk)]], just filled by a synth instead of a file.

Recipe (see BuildRacingLoopClip / BuildTinyLoopClip)

1. Pick a loop length L seconds; count = ceil(sampleRate * L).
2. Seamless-by-construction tonal bed: snap every tonal frequency to an integer number of cycles over the loop, i.e. to the grid 1/L Hz (round(freq / grid) * grid). A grid-snapped sine wraps with zero discontinuity, so no crossfade is needed on the tonal layer.
3. Per motor (4 for a quad), sum a harmonic series Σ harm[h]·sin(2π·f0·(h+1)·t). Detune the four motors a few percent so their tones beat against each other (the "angry swarm" shimmer). Add a once-per-rev wobble + low hum via the rotation-rate sine.
4. Prop wash: differentiated white noise (crude high-pass) → two-pole lowpass → amplitude-chop at the blade-pass rate. The noise layer is NOT grid-aligned, so crossfade its head into its tail (~1024 samples) to loop it.
5. Mix SoftClip(tonal·a + wash·b) (tanh soft clip), Normalize to ~0.9 peak, AudioClip.Create(name, count, 1, sampleRate, false) + SetData.
6. Live pitch = rpm model. Render the clip at a reference RPM (RenderRpm), then at runtime set source.pitch = rpmNow / RenderRpm where rpmNow chases throttle (smoothed). Unity clamps pitch to ±3, so keep the full-send multiplier under 3. Volume tracks sqrt(rpm).

Timbre knobs (what makes it read big vs small)

• Fundamental / base freq: higher = smaller airframe. A 5" quad's blade pass ≈ RenderRpm/30; the tiny whoop uses ≈ RenderRpm/11.5 (~2.6x higher).
• Harmonic rolloff: fundamental-dominant + fast rolloff = fat buzz; near-flat rolloff with hot upper partials = thin, screechy whine. Hot upper harmonics + tanh drive is also what makes a loop read as "raspy".
• Prop wash: stronger lowpass = beaten-air whoosh; weaker lowpass (brighter)
  • louder mix = thin hiss. Small props are hissier/screechier.
• Watch aliasing: cap harmonic count so the top harmonic × max pitch stays under Nyquist (22050 Hz). Tiny loop uses 6 harmonics for this reason.

Per-airframe acoustic profiles

Same drone code, different voice by size: FPVDroneAudio.SetAirframeScale(visibleSize) sets _tinyAirframe = visibleSize <= 0.35m, chosen from the item's VisibleSize at Attach time (before the startup chirp). The bomb drone (0.65m) keeps the 5" buzz; the Tiny Drone (0.1625m) gets BuildTinyLoopClip plus higher-pitched arming chirps (_beepSource.pitch 1.5x) and frame knocks. Clips are cached in static fields, so each profile's loop is built once and reused across launches.

Applies when

Any mod that needs a controllable engine/rotor/vehicle drone loop and wants to avoid shipping/looping a recorded clip, or wants several sizes of the same machine to sound distinct without new assets.