openverse-sfx

openverse-sfx: Free Licensed Sound Design for Video, Verified by Measurement

You cannot hear. Every decision in this skill is made by measurement (librosa, ffmpeg ebur128, waveform plots) and verified on the exported file - never by feel, and never tell the user it sounds good. Hand them a short list of timestamps to check with headphones instead.

All audio comes from Openverse (https://api.openverse.org): 5.2M free audio files, no API key, $0. Never synthesize music or sound effects in code - no sine-wave beeps, no noise bursts. Use real licensed recordings only.

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Critical Rules

1. License filter is mandatory. Only use Openverse results with license=cc0,by,pdm and source=freesound,jamendo. scripts/sfx_search.py enforces this - do not bypass it.
2. One search per kind of sound. Anonymous Openverse use is limited to ~200 searches/day. Search once per sound kind ("whoosh", "ui pop", "upbeat loop") and pick from the shortlist. Do not re-search with rephrased queries.
3. credits.txt is not optional. CC BY audio is only legal to use with credit. Every downloaded file's attribution field goes into credits.txt verbatim. scripts/sfx_fetch.py does this automatically. CC0/PDM need nothing, but log them anyway.
4. Prerequisites first. If missing, install before starting: ffmpeg, and pip install librosa matplotlib numpy scipy.
5. Measure the exported MP4, never the pre-export WAV. AAC compression and mono phone playback change the result; only the shipped file counts.

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Workflow

Step 1 - Music

Use a track the user gave you, or search Openverse for one (python3 scripts/sfx_search.py --query "upbeat loop" --min-ms 15000). Note the source and license. Download it with scripts/sfx_fetch.py into sfx/ so its attribution lands in credits.txt.

Step 2 - Beat Grid

Run python3 scripts/beat_grid.py sfx/<music_file>. It plots the sub-120 Hz waveform (where the kick drum lives) against the detected beat grid, detects off-beat lock numerically, and writes beats.json. Open beatgrid.png: red lines must sit ON the kick spikes. If they don't, shift the whole grid by half a beat interval (the script checks this numerically and shifts automatically when the half-beat alignment scores >5% higher).

Step 3 - Put Big Moves on the Beat

Scene cuts, titles appearing, big animations: place them on beats from beats.json. If you can change the animation, snap the cut to the nearest beat. If you can't, shift where the music starts so the existing cuts land on beats (-itsoffset / adelay on the music input).

Step 4 - One Ready-Made SFX per Cut and Big Move

Use the project's existing sound library first if it has one; ask the user if a needed sound is missing. Otherwise, run one sfx_search.py call per kind of sound: whoosh for movement, pop for things appearing, click for clicks, typing sounds for typing. Pick the best fit for each shot's action from the shortlist, and download each pick with sfx_fetch.py.

Step 5 - Mix with the Right Hierarchy

• Voiceover is the loudest layer; music drops while someone talks (sidechain duck or manual volume automation).
• Sound effects sit above the music, and source files arrive at wildly different loudness - normalize every layer by measurement, never fixed volume multipliers:
  • SFX one-shots peak-normalized to -3 dBTP with scripts/sfx_normalize.py (one-shots cut through by transient, so peak not LUFS; and NEVER run ffmpeg loudnorm on a short SFX - its 3-second buffer destroys short files).
  • Music bed normalized to -20 LUFS.
  • Exact commands are in references/ffmpeg_recipes.md.
• Target -16 LUFS integrated, true peak below -4 dBFS on the shipped MP4, so nothing clips after AAC compression or on a phone speaker in mono. Master to TP=-4.5: the AAC encode adds ~0.3-0.6 dB of overshoot and a -4.0 master comes back off the mux failing the gate.
• Read references/ffmpeg_recipes.md for the exact ffmpeg commands for ducking, SFX placement at beat times, and two-pass loudnorm.

Step 6 - Verify the Export, Then Hand Over Check Points

Run python3 scripts/verify_mix.py final.mp4 --cue <t1> --cue <t2> ... with every cue time. It measures the exported MP4 with ebur128, grabs one frame before and after every cue from the MP4, and plots the exported waveform with cue lines. Confirm loudness and peak pass, and that each cue sits on a waveform spike and its frames show the cut/title. Then give the user 5 timestamps to check with headphones and the path to credits.txt. Do not claim it sounds good.

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Scripts

Script	Purpose
scripts/sfx_search.py	Openverse search, license/source filters enforced (cc0,by,pdm from freesound,jamendo), duration filtering, JSON shortlist with attribution fields
scripts/sfx_fetch.py	Downloads the chosen file into sfx/ via Python stdlib (urllib.request) and appends its verbatim attribution to credits.txt
scripts/sfx_normalize.py	ebur128 measure + gain + re-measure; peak mode (-3 dBTP) for SFX one-shots, LUFS mode for beds - safe where loudnorm destroys short files
scripts/beat_grid.py	librosa beat tracking + numerical off-beat lock check + sub-120 Hz kick-band/grid plot (beatgrid.png) + beats.json
scripts/verify_mix.py	ebur128 loudness + true peak on the exported MP4, before/after cue frame grabs, waveform plot (verify/waveform.png), PASS / NEEDS FIX verdict

References

• references/ffmpeg_recipes.md - exact ffmpeg commands for beat-shifting music, placing SFX at cue times, voiceover sidechain ducking, and two-pass loudnorm to -16 LUFS / -4 dBTP. Read it before writing any ffmpeg command.