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

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

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

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

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

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 episode.mp3 -filter_complex "showwavespic=s=1920x200:colors=0x1a1a2e" -frames:v 1 waveform.png

ID3 Tags

# Set all metadata
ffmpeg -i episode.mp3 -c copy \
  -metadata title="EP 42: The Memory Architecture" \
  -metadata artist="GSD Podcast" \
  -metadata album="Getting Shit Done" \
  -metadata track="42" \
  -metadata date="2026" \
  -metadata comment="LOD-tiered memory system deep dive" \
  tagged.mp3

BPM and Key Detection

With ffmpeg/aubio

# Install aubio for beat/pitch detection
# apt install aubio-tools

# BPM detection
aubiotempo input.wav

# Pitch/key detection
aubiopitch -i input.wav -p yinfft

# Onset detection (transient markers)
aubioonset input.wav

With sox

# Generate stats (includes RMS, peak, DC offset)
sox input.wav -n stats 2>&1

Sample Rate / Bit Depth Reference

Conversion

# Downsample from 96kHz/24-bit to 44.1kHz/16-bit with dither
sox input_96_24.wav -r 44100 -b 16 output_441_16.wav dither -s

# Same with ffmpeg
ffmpeg -i input_96_24.wav -ar 44100 -sample_fmt s16 -af "dither=method=triangular" output.wav

Related Skills & Agents

• ffmpeg-media — codec/format operations, video+audio conversion
• ffmpeg-processor agent — media processing specialist
• gource-visualizer — repository visualization with audio sync capability
• Audio research: ABL, DAA, DFQ, HFR, HFE, S36/SPS (360 musicians)

When This Skill Activates

• Audio mastering, mixing, EQ, compression
• Loudness measurement and normalization (LUFS, EBU R128)
• Podcast recording, editing, production
• Music theory questions (chords, scales, progressions)
• Synthesis design (FM, subtractive, granular, wavetable)
• Spectrum analysis and audio visualization
• Sample rate/bit depth conversion
• Noise reduction and audio cleanup
• BPM/key detection