audio-engineering

Audio Engineering Skill

Built on research from 30 Music cluster projects, 360 PNW musicians deep-dived (S36/SPS), Ableton Live research (ABL), Deep Audio (DAA), Dead Frequencies (DFQ), and High Fidelity amplifier analysis (HFR/HFE).

Expert-level audio engineering covering mastering, mixing, loudness standards, synthesis, podcast production, music theory, and spectrum analysis. Works alongside the ffmpeg-media skill for codec/format operations.

Loudness Standards

Target Levels by Platform

| Platform | Target LUFS | True Peak | Standard | |----------|------------|-----------|----------| | Spotify | -14 LUFS | -1 dBTP | AES streaming | | Apple Music | -16 LUFS | -1 dBTP | Sound Check | | YouTube | -14 LUFS | -1 dBTP | ITU-R BS.1770 | | Podcast (Apple) | -16 LUFS | -1 dBTP | Apple spec | | Podcast (Spotify) | -14 LUFS | -1 dBTP | Spotify spec | | Broadcast TV | -24 LUFS | -2 dBTP | EBU R128 | | Broadcast US | -24 LKFS | -2 dBTP | ATSC A/85 | | CD master | -9 to -12 LUFS | -0.3 dBTP | Red Book | | Film/Cinema | -24 LUFS | -1 dBTP | SMPTE RP 200 |

Measurement Commands

# Measure integrated loudness (LUFS) with ffmpeg
ffmpeg -i input.wav -af loudnorm=print_format=json -f null - 2>&1 | grep -A20 "Parsed_loudnorm"

# Full EBU R128 scan
ffmpeg -i input.wav -af ebur128=peak=true -f null - 2>&1 | tail -20

# Loudness normalization to -14 LUFS (two-pass for accuracy)
# Pass 1: measure
ffmpeg -i input.wav -af loudnorm=I=-14:LRA=11:TP=-1:print_format=json -f null - 2>&1 > /tmp/loudnorm.json
# Pass 2: apply (use measured values from pass 1)
ffmpeg -i input.wav -af loudnorm=I=-14:LRA=11:TP=-1:measured_I=-18.5:measured_LRA=9.2:measured_TP=-0.5:measured_thresh=-28.3 output.wav

With sox

# Normalize peak to -1 dBFS
sox input.wav output.wav gain -n -1

# Compressor (threshold -20dB, ratio 4:1, attack 5ms, release 50ms)
sox input.wav output.wav compand 0.005,0.05 -20,-20,-10,-10,0,-6

# 3-band EQ (low shelf +3dB at 200Hz, mid cut -2dB at 2kHz, high shelf +1dB at 8kHz)
sox input.wav output.wav bass +3 200 equalizer 2000 1q -2 treble +1 8000

# Noise reduction (profile then reduce)
sox noisy.wav -n noiseprof /tmp/noise.prof
sox noisy.wav clean.wav noisered /tmp/noise.prof 0.21

# Generate tone (440Hz sine, 3 seconds)
sox -n -r 44100 -c 1 tone.wav synth 3 sine 440

# Spectrum analysis (generate spectrogram PNG)
sox input.wav -n spectrogram -o spectrum.png

Mastering Chain

Standard Mastering Signal Flow

Input → EQ (corrective) → Compression → EQ (tonal) → Stereo Width → Limiting → Dithering → Output

With ffmpeg Filters

# Full mastering chain: EQ → compression → limiting → loudness normalization
ffmpeg -i mix.wav -af "\
  equalizer=f=80:t=h:w=100:g=2,\
  equalizer=f=3000:t=h:w=1000:g=-1.5,\
  equalizer=f=12000:t=h:w=2000:g=1,\
  acompressor=threshold=-18dB:ratio=3:attack=10:release=100:knee=6,\
  alimiter=limit=-1dBFS:level=false,\
  loudnorm=I=-14:LRA=11:TP=-1\
" -ar 44100 -sample_fmt s16 mastered.wav

# Dithering (16-bit with triangular dither for CD)
ffmpeg -i master_24bit.wav -af "dither=method=triangular" -sample_fmt s16 -ar 44100 cd_master.wav

EQ Reference

Frequency Bands and Characteristics

| Band | Range | Character | Common Uses | |------|-------|-----------|-------------| | Sub-bass | 20-60 Hz | Felt, not heard | Kick fundamental, sub bass | | Bass | 60-250 Hz | Warmth, body | Bass guitar, kick punch, vocal warmth | | Low-mid | 250-500 Hz | Muddiness zone | Cut here to clean up mixes | | Mid | 500-2000 Hz | Body, presence | Vocal clarity, guitar body | | Upper-mid | 2-4 kHz | Presence, bite | Vocal intelligibility, guitar attack | | Presence | 4-6 kHz | Definition, edge | Consonant clarity, string attack | | Brilliance | 6-12 kHz | Air, shimmer | Cymbals, vocal air, acoustic sparkle | | Ultra-high | 12-20 kHz | Air, sparkle | Subtle sheen (careful: sibilance) |

Common Problem Frequencies

• 200-300 Hz — boominess in vocals, acoustic guitar
• 400-600 Hz — cardboard/boxy sound
• 1-2 kHz — nasal, telephone quality
• 3-5 kHz — harshness, listening fatigue
• 6-8 kHz — sibilance (de-ess here)

Compression Reference

Settings by Source

| Source | Threshold | Ratio | Attack | Release | Knee | |--------|-----------|-------|--------|---------|------| | Vocals | -18 to -12 dB | 2:1 to 4:1 | 5-15 ms | 40-80 ms | Soft | | Drums (bus) | -15 to -10 dB | 3:1 to 6:1 | 10-30 ms | 50-100 ms | Hard | | Bass | -15 to -8 dB | 3:1 to 8:1 | 10-30 ms | 100-200 ms | Hard | | Acoustic guitar | -20 to -12 dB | 2:1 to 4:1 | 10-25 ms | 100-150 ms | Soft | | Mix bus | -20 to -15 dB | 1.5:1 to 2:1 | 10-30 ms | 100-300 ms | Soft | | Podcast | -20 to -15 dB | 3:1 to 5:1 | 5-10 ms | 50-100 ms | Soft |

Compression Types

• VCA — fast, transparent, precise (SSL, dbx 160)
• Optical — smooth, musical, slow (LA-2A, CL 1B)
• FET — aggressive, colorful, fast (1176, Distressor)
• Variable-mu — warm, glue, gentle (Fairchild 670, Manley Vari-Mu)

Synthesis Reference

Synthesis Types

| Type | How It Works | Character | Classic Synths | |------|-------------|-----------|----------------| | Subtractive | Oscillator → Filter → Amplifier | Warm, analog, rich | Minimoog, Prophet-5, Juno-106 | | FM | Operators modulating each other's frequency | Metallic, bell-like, bright | DX7, FM8 | | Wavetable | Morphing between stored waveforms | Evolving, complex, modern | PPG Wave, Serum, Vital | | Granular | Tiny audio grains layered and scattered | Atmospheric, textural, ambient | Granulator, Pigments | | Additive | Sum of individual sine wave partials | Precise, organ-like | Kawai K5, Razor | | Physical modeling | Mathematical model of physical instrument | Realistic, expressive | Chromaphone, Pianoteq | | Sample-based | Recorded audio, pitch-shifted and layered | Realistic, natural | Kontakt, Sampler |

ADSR Envelope Quick Reference

• Pad: A=500ms, D=200ms, S=0.8, R=1000ms
• Pluck: A=1ms, D=200ms, S=0, R=100ms
• Bass: A=5ms, D=100ms, S=0.6, R=50ms
• Lead: A=10ms, D=50ms, S=0.7, R=200ms
• Kick drum: A=0ms, D=150ms, S=0, R=50ms

Music Theory Quick Reference

Circle of Fifths (Major Keys)

        C
    F       G
  Bb          D
    Eb      A
       Ab/E

Common Chord Progressions

| Name | Numerals | Example in C | Use | |------|----------|-------------|-----| | Pop | I-V-vi-IV | C-G-Am-F | 80% of pop music | | Blues | I-IV-V | C-F-G | Blues, rock | | Jazz ii-V-I | ii-V-I | Dm7-G7-Cmaj7 | Jazz standard | | Andalusian | i-VII-VI-V | Am-G-F-E | Flamenco, dramatic | | Canon | I-V-vi-iii-IV-I-IV-V | C-G-Am-Em-F-C-F-G | Pachelbel, ballads | | Minor blues | i-iv-V | Am-Dm-E | Minor blues |

Scales

• Major (Ionian): W-W-H-W-W-W-H
• Natural Minor (Aeolian): W-H-W-W-H-W-W
• Pentatonic Major: 1-2-3-5-6
• Pentatonic Minor: 1-b3-4-5-b7
• Blues: 1-b3-4-#4-5-b7
• Dorian: W-H-W-W-W-H-W (minor with raised 6th — jazz, funk)
• Mixolydian: W-W-H-W-W-H-W (major with flat 7th — blues rock)

Podcast Production Workflow

Recording

# Record from default mic (sox)
sox -d -r 44100 -c 1 -b 16 recording.wav

# Record with ffmpeg (specify ALSA device on Linux)
ffmpeg -f alsa -i default -ar 44100 -ac 1 recording.wav

Processing Chain

# 1. Noise reduction
sox recording.wav -n trim 0 0.5 noiseprof /tmp/noise.prof
sox recording.wav clean.wav noisered /tmp/noise.prof 0.21

# 2. Normalize + compress + EQ for voice
ffmpeg -i clean.wav -af "\
  highpass=f=80,\
  lowpass=f=12000,\
  equalizer=f=3000:t=h:w=1000:g=2,\
  acompressor=threshold=-20dB:ratio=4:attack=5:release=50,\
  loudnorm=I=-16:TP=-1\
" -ar 44100 podcast_ready.wav

# 3. Export MP3 for distribution
ffmpeg -i podcast_ready.wav -c:a libmp3lame -b:a 128k \
  -metadata title="Episode Title" \
  -metadata artist="Show Name" \
  -metadata album="Podcast Name" \
  -metadata genre="Podcast" \
  episode.mp3

# 4. Generate waveform for show notes
ffmpeg -i