# Psychoacoustics Reference

## Brainwave Entrainment

### Mechanism
External rhythmic stimuli synchronize neural oscillations. The brain's "frequency following response" (FFR) matches internal oscillation to external periodic input.

### Methods
| Method | How It Works | Requires Headphones | Effectiveness |
|--------|-------------|-------------------|---------------|
| Binaural beats | Different freq in L/R ear, brain perceives difference | Yes | Moderate; also stimulates hemisphere sync |
| Isochronic tones | Single tone pulsed on/off at target freq | No | Strong; clean entrainment signal |
| Monaural beats | Two tones mixed before reaching ear | No | Moderate; natural beating |
| Amplitude modulation | Volume of carrier oscillates at target freq | No | Strong; works on any sound source |
| Rhythmic entrainment | Periodic musical events (kicks, hats) | No | Very strong; the basis of dance music |

### Brainwave Bands
| Band | Frequency | State | Musical Correlation |
|------|-----------|-------|-------------------|
| Delta | 0.5-4 Hz | Deep sleep, unconscious | Slow ambient drones |
| Theta | 4-8 Hz | Meditation, creativity, trance | Psytrance sweet spot via subdivision |
| Alpha | 8-13 Hz | Relaxed awareness, flow | Groovy tempos, liquid DnB |
| Beta | 13-30 Hz | Active thinking, alertness | Fast hihat patterns, complex rhythms |
| Gamma | 30-100 Hz | Peak awareness, insight | Granular textures, high-freq content |

### Embedding Entrainment in Production
- **Kick patterns**: At 145 BPM, the kick hits 2.42 times/sec → alpha range. This is NOT coincidence — psytrance tempos evolved to match trance-inducing frequencies
- **Stereo pairs**: Place two oscillators slightly detuned (e.g., 200 Hz L, 206 Hz R) in pad layers → 6 Hz theta binaural beat, subliminal
- **Tremolo/gate**: LFO on volume at 4-8 Hz on pads → isochronic entrainment
- **Hi-hat rolls**: 16th notes at 140 BPM = 9.3 Hz → alpha band

## Fletcher-Munson Curves (Equal Loudness Contours)

Human hearing is NOT flat. Sensitivity changes with volume:
- At low volumes: poor bass and treble perception → mixes sound thin
- At high volumes: flatter response → everything sounds bigger
- Peak sensitivity: 2-5 kHz (voice intelligibility range, evolutionary advantage)

### Production implications
- Mix at moderate levels (~85 dB SPL) for balanced decisions
- Check mixes at low volume: if bass disappears, it's EQ not level
- The 2-5 kHz zone is where ear fatigue happens fastest → careful with harsh synths
- Sub-bass below 40 Hz is felt more than heard → use distortion/harmonics to make it "audible" on small speakers

## Masking

### Frequency masking
A louder sound makes nearby quieter sounds inaudible. 
- A loud bass note at 100 Hz can mask everything from ~50-200 Hz
- Critical bandwidth: ~1/3 octave at most frequencies
- Implication: carve EQ space for each element. Two sounds fighting = neither heard

### Temporal masking
- **Pre-masking**: A loud sound masks quiet sounds up to 20ms BEFORE it (brain processes retroactively)
- **Post-masking**: Masking continues 100-200ms after loud sound stops
- **Implication for buildups**: As layers accumulate, details disappear. The drop feels massive partly because post-masking from the buildup "clears" and suddenly everything is audible again

## Spatial Perception

### Haas Effect (Precedence)
- A delayed copy (1-30ms) of a sound is perceived as the same source from the leading direction
- Used in production: short delays for width without losing center image
- Forest psy trick: subtle delays on organic textures create "living" spatial field

### Interaural Time Difference (ITD) & Level Difference (ILD)
- Below ~700 Hz: brain uses timing differences between ears for localization
- Above ~1.5 kHz: brain uses volume differences
- Mid/Side processing exploits this: M = center (both ears equal), S = sides (differences)

### Phantom Center
- Identical signal in both speakers → perceived as center (phantom image)
- Slight detuning → image widens and destabilizes
- Heavy detuning → two distinct sources → width

### Distance Perception
- More reverb = farther away (early reflections are key)
- Less high frequency = farther (air absorption)
- Less transient definition = farther
- Quiet = far, loud = close
- Layer these cues deliberately to create front-to-back depth

## Harmonic Perception

### Combination Tones
When two tones sound together, the brain can perceive a third tone at the difference frequency.
- 440 Hz + 554 Hz → ghost tone at 114 Hz
- This is how psytrance "acid" lines create phantom bass: rapid note patterns generate perceived sub-frequencies

### Roughness & Beating
- Two tones within ~15 Hz of each other → slow beating (pleasant, chorus-like)
- Two tones within ~15-40 Hz → roughness (unpleasant, tense)
- Beyond ~40 Hz apart → two distinct tones
- Wave/phonk uses roughness deliberately for its characteristic "dirty" aesthetic

### Consonance and Dissonance
- Perfect intervals (octave, fifth) → stable, resolved
- Minor seconds, tritones → tension, unease
- Moving from dissonance to consonance = the most fundamental emotional arc in music
- Forest psy uses extreme dissonance for its alien, unsettling character