# Playing on a 3D system: the rules in short

What a DJ needs to know to make movement in the room work for a crowd. Each rule points to where it
comes from: [motion perception](motion-perception.md) (the research, part A/B), the
[MJ playbook](mj-playbook.md) (rules checked against the movement lab's simulation), or club practice.
The simulation is a model, not a listening test; the listening test is still to be run.

## The one fact

**The crowd does not study the motion, it notices it.** A move that has to be listened for is a move
that did not happen. Everything below follows from that.

## The room

- **The crowd is not in the sweet spot.** Off centre, transients are pulled to the nearest speaker
  (the precedence effect): most of the floor hears a moving hi-hat as "which speaker near me gets
  louder", not as an angle. Higher-order Ambisonics (A³ uses 3rd order) widens the area where
  direction holds to almost the whole array. [research A1]
- **Direction errors grow to the sides**: about 5° straight ahead, more than 30° at ±90°. Design a
  bend that must be heard for **≥ 30°** as seen from the floor. [research A1, B2]
- **Height is heard worse than azimuth**, and only with high frequencies. Fly the big paths low (just
  above ear height); keep elevation moves for bright material. [research B3; playbook rule 28]
- **Know where front is.** Azimuth 0 is front; the performer needs it marked before anyone drags a
  blob.
- **A visible gesture helps.** In a hall, a performer's visible, synchronous gesture partly makes up
  for listeners being off centre. [research A1, Marentakis 2008]

## What to move

- **The bass holds the floor.** Below about 150 Hz nothing localises in a club (room modes, shared
  subs); practice keeps it steady. Never move anything under ~250 Hz as a path. [research A2;
  4DSOUND; L-ISA at the Rex Club]
- **Move the top of the mix.** Hats, claps, air, vocal presence: a band of 6–8 kHz at 2 octaves, or
  3 kHz at 4 octaves. [playbook rule 6]
- **Never narrower than one octave** for a move that should be followed; width matters more than
  centre frequency. [research A2; playbook rule 7]
- **A narrow high band sits at its own height**, whatever the system sends. Don't try to move it
  vertically. [research A2, Middlebrooks 1992]
- With **stems**, pick the stem that has the top end (hats, vocals) for the clearest paths, and leave
  the bass stem steady.

## How fast

Speeds in laps per bar, because tempo is the unit a DJ thinks in (at 120 BPM, one lap per bar is
180 °/s):

| Lap | Reading |
|---|---|
| 1 per 8 bars or slower | a drift, not motion — "the room drifts" |
| **1 per 4 bars** | **the cruise: a clear path, readable from anywhere in the crowd** |
| 1 per 2 bars | energetic; the cruise below about 110 BPM |
| 1 per bar | the ceiling for a path, and only with a wide band above 3 kHz |
| 1 per 2 beats or faster | an effect (swirl, Leslie, tremolo), 1–2 bars at most |
| ~2.5 laps per second | the hearing limit: direction is lost, it becomes texture |

- **A speed change must be ×2 to be heard as faster** (×½ as slower). Double or halve; never nudge.
  [research A1, B1]
- **Laps are powers of two in bars**, like loop lengths on a deck.

## How much at once

- **Two movers; the rest wait.** "Less becomes far more" (4DSOUND). Two moving sounds beat one and
  beat four in every variant of the simulation. In a DJ set usually only two decks play anyway.
  [playbook rule 13]
- **Never turn several sounds the same way at the same speed** — a rotating field can make people feel
  they are turning (auditory vection). [research A3]
- **At most one audible event per mover every two beats.** [playbook rule 15, a hypothesis]

## Keep it alive: the breath

- **A steady orbit is heard and then tuned out** after a few laps. [playbook rule 19]
- **The breath:** every moving sound stands still for **beat 4 of every bar** and restarts on the
  one. One whole beat (it must outlast the ~336 ms the ear integrates over), once a bar (twice is a
  stutter). The most robust result of the movement lab. With two movers, call and response: one
  breathes on beat 4, the other on beat 2. [playbook rule 18]
- **The beat link is a move on the one**, not a wobble on every beat: a per-beat pulse under 20–30° is
  not heard. [playbook rule 17; research B5]
- **No spatial kicks on the one** — they add fatigue faster than interest. [playbook rule 21]
- **Contrast in time beats contrast in space**: stops and doublings on bar lines work; jolts and
  throws are seasoning. [lab log, round 6]

## The 3d control

On a stereo channel, 3d decides how much of the channel is in the room:
- **0** — plain stereo, like any mixer;
- **0.5** — the filtered band in 3D and the rest steady (the sum is the original track);
- **1** — only the filtered band, in 3D.

With 3d at zero a moving blob moves nothing you can hear. A first-time user who presses play, watches
the blob fly and hears the track sitting still decides the box is broken: "turn up 3d" is step one of
any quick start.

## Before trusting any of this

The numbers are from the literature and a simulation. The [listening test](motion-perception.md#c-listening-test-one-evening-at-the-system)
on a real system decides the speed caps, the band defaults and how many movers a floor enjoys.