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motion lab

07 Sound

Half of motion is audio. Find the beats, in frames, and cut on them.

Floor
07 of 09
Demos
Beat grid
Skills
video-shotcraftRemotion

Half of motion is audio

Turn the sound off on a good launch film and it falls flat. Music sets the pace, sound effects give weight to impacts and transitions, and cuts that land on the beat feel inevitable instead of arbitrary.

To cut on the beat in code, convert tempo into frames: frames per beat = fps × 60 ÷ BPM. At 120 BPM and 30 fps that is exactly 15 frames, so every beat lands on a frame. At 128 BPM it is 14.06 frames, and each cut rounds to the nearest whole frame.

Beat grid

Set a tempo and a frame rate to find the frames every cut should land on. Press play with sound on, then nudge the picture early or late until it feels off.

120 BPM
Frame rate
2 × 4 beats
In sync
Frames per beat
15.000
Frames per bar
60.00
Beat length
500 ms
Worst rounding
0.0 ms

Every beat lands exactly on a frame. Cuts can be frame-perfect.

f0Downbeats in red · 120 frames
// 120 BPM at 30 fps: one beat every 15.000 frames
// Cut points (rounded to whole frames), first 8 beats:
f0, f15, f30, f45, f60, f75, f90, f105

Sync, and which way it can slip

We are much more forgiving of sound that arrives late than sound that arrives early, because in the real world light reaches us before sound does. The ITU’s broadcast guidance (BT.1359) puts the point where viewers notice at about 45 ms of sound leading the picture, but about 125 ms of sound lagging behind it.

In practice: never let the picture land after its sound. If a hit is going to be off by a frame, make the picture early, not late. Use the offset slider to feel the difference.

Rules of thumb

  • Pick the music first, then cut the picture to it. Re-timing a finished animation to a track is painful.
  • Choose tempo and frame rate together: 120 BPM at 24, 30 or 60 fps gives whole-frame beats.
  • Cut on downbeats for big changes, on off-beats for small ones.
  • Every impact on screen wants a sound. Every sound wants something on screen.
  • Use licensed music only. video-shotcraft, for one, organizes sound effects by category so they fit the shot.