audio-design

The full skill.

Original instructions from the publisher’s SKILL.md

# Audio design

Game audio is a **mixing graph plus a music system**. Route every sound through a
small set of buses so you can balance and process groups; make music *react* to
play through layering and re-sequencing rather than looping one track. This skill
teaches the portable practice; bind it to `godot-audio`, Unity's AudioMixer, or
middleware (FMOD/Wwise) for concrete APIs.

## When to use

- Use to design a bus/mixer layout, set group volumes, and apply effects (reverb,
  compression, EQ) to groups of sounds.
- Use to duck music/ambience under dialogue or impacts (sidechain).
- Use to build adaptive music that responds to combat/exploration intensity.
- Use to add SFX variation (pitch/sample randomization) and sync events to a beat.

**When *not* to use:** for the engine's concrete audio nodes/streams, use
`godot-audio` or the engine's audio skill. Loading/streaming and asset import are
engine concerns. For UI sliders that drive bus volume, see the engine UI skill.

## Core workflow

1. **Lay out buses, not per-sound volume.** A typical tree: `Master ← {Music,
   SFX, Ambience, UI, Voice}`. Everything plays into a bus; the player's settings
   sliders map to bus volumes. Never set hundreds of clip volumes by hand.
2. **Work in decibels, not linear.** Perceived loudness is logarithmic. Volume
   controls and automation should operate in dB; convert only at the edges.
3. **Leave headroom.** Mix so the Master peaks below 0 dBFS (aim for a target
   loudness, e.g. around -14 to -16 LUFS for many games) to avoid clipping.
4. **Duck competing sources** with a sidechain compressor (or volume automation):
   when voice/important SFX plays, the music bus dips, then recovers.
5. **Make music adaptive** via *vertical* layering (stems faded in/out) and/or
   *horizontal* re-sequencing (swap segments at musical boundaries). See the
   reference.
6. **Vary repeated SFX** with small random pitch/volume offsets and sample pools
   so footsteps and hits don't sound robotic.
7. **Verify on real output.** Listen on headphones and speakers; check that the
   mix balances, ducking is audible but not pumping, and music transitions land
   on the beat — never assume from the editor meters alone.

## Patterns

### 1. Bus routing and dB gain

```gdscript
# Route sounds to named buses; control GROUPS, not individual clips.
sfx_player.bus = "SFX"
music_player.bus = "Music"

# Map a 0..1 settings slider to decibels (linear_to_db), the perceptual unit.
func set_bus_volume(bus_name: String, slider01: float) -> void:
    var idx := AudioServer.get_bus_index(bus_name)
    var db := linear_to_db(clamp(slider01, 0.0001, 1.0))   # 0 -> silence, 1 -> 0 dB
    AudioServer.set_bus_volume_db(idx, db)
# RIGHT: slider -> dB via linear_to_db. WRONG: assigning slider01 straight as dB
# (a "0.5" would be only +0.5 dB — almost no change — and 0 would be 0 dB, full).
```

### 2. Ducking via sidechain (music dips under voice)

```gdscript
# A compressor on the MUSIC bus, keyed by the VOICE bus, lowers music while
# dialogue plays, then releases. This is "sidechain ducking".
# Setup (engine-specific): add a compressor effect to the Music bus and set its
# sidechain to the Voice bus. Then tune:
#   threshold: level on Voice that triggers ducking (e.g. -30 dB)
#   ratio:     how hard to duck (e.g. 8:1 for a clear dip)
#   attack:    fast (~10 ms) so music gets out of the way promptly
#   release:   slow (~300-500 ms) so it recovers smoothly, not pumping
# No-middleware alternative: tween the Music bus volume down on voice start and
# back up on voice end.
func duck_music(active: bool) -> void:
    var target_db := -12.0 if active else 0.0
    create_tween().tween_method(
        func(v): set_bus_volume_db("Music", v), current_music_db, target_db, 0.25)
```

### 3. SFX variation (kill the "machine gun" repeat)

```gdscript
# Randomize pitch slightly and pick from a sample pool so repeats feel organic.
func play_varied(samples: Array, bus := "SFX") -> void:
    var p := AudioStreamPlayer.new()
    p.stream = samples[randi() % samples.size()]   # rotate through several takes
    p.bus = bus
    p.pitch_scale = randf_range(0.94, 1.06)         # +/- ~6% pitch wobble
    add_child(p); p.play()
    p.finished.connect(p.queue_free)                # clean up one-shots
```

### 4. Beat-synced events (quantize to the music grid)

```gdscript
# Schedule gameplay/visuals on musical time, not frame time, so they land on beat.
const BPM := 120.0
var seconds_per_beat := 60.0 / BPM

func current_beat(playback_position_sec: float) -> int:
    return int(playback_position_sec / seconds_per_beat)

# Quantize an action to the NEXT beat boundary instead of firing immediately.
func time_until_next_beat(pos: float) -> float:
    return seconds_per_beat - fmod(pos, seconds_per_beat)
# Drive timing from the audio playback clock, which is steadier than frame delta.
```

## Pitfalls

- **Treating slider values as dB.** Volume is logarithmic; map `0..1` through
  `linear_to_db` (and back with `db_to_linear`). A linear slider on raw amplitude
  feels like it does nothing until the very bottom.
- **Per-clip volume instead of buses** makes a global balance pass impossible and
  bloats save/settings. Mix on buses.
- **Clipping the master.** Summed sounds exceed 0 dBFS and distort. Leave
  headroom; put a limiter on Master as a safety net, not as the mixer.
- **Pumping ducking**: too-fast release or too-high ratio makes music audibly
  breathe. Lengthen release; lower ratio.
- **Looping a single music track** for the whole game feels flat. Use layers or
  segments that respond to state (see the reference).
- **Beat sync off frame time.** `delta` drifts; read the **audio playback
  position** for musical timing, and account for output latency.
- **Unbounded one-shot players**: spawning AudioStreamPlayers without freeing
  them leaks. Free on `finished`, or use a small pool.

## References

- `references/adaptive-music.md` — vertical layering vs horizontal re-sequencing,
  transition timing (bars/quantize), stingers, intensity mapping, and crossfades.

## Related skills

- `godot-audio` — buses, `AudioStreamPlayer`, effects, and sync-to-beat in Godot.
- `input-systems` — trigger audio from input actions.
- `physics-tuning` — collision events that drive impact SFX.
- `platformer`, `roguelike` — genres whose feel leans on audio feedback.