Why Do This in Your Browser?
Slowing audio down to transcribe an interview, or speeding up an audiobook for the commute, used to mean opening a media player with playback rate controls — only to discover the chipmunk effect when the pitch dropped or rose with the speed. The fix is decades old: time-stretch the waveform without resampling, so the audio plays back at a new tempo while every note and every vowel sound stays at its original pitch.
FFmpeg's `atempo` filter ships this algorithm. It runs natively in the wasm build, no extra dependencies, and produces clean output across a 0.5× to 2.0× range. For ranges outside that, the filter chains itself — `atempo=2.0,atempo=2.0` applies a 4× tempo change with high quality. This tool exposes the slider; the engine handles the math.
How It Works
Drop an audio file. Pick a tempo as either a percentage (50% to 200%) or by tap-tempo — tap the spacebar at the original BPM and the tool calculates the ratio needed to hit a target BPM. Preview the slider position before committing to the export.
FFmpeg applies `atempo=R` where R is the ratio (0.5 for half speed, 2.0 for double speed). For ratios outside that range, the tool chains multiple `atempo` filters because each `atempo` instance is limited to 0.5–2.0 to preserve quality. Output duration scales accordingly: half-tempo audio is twice as long, double-tempo is half as long.
Tip: For changing pitch without affecting speed (the opposite operation), use the Audio Pitch Shifter. To trim before slowing down a long interview for transcription, the Audio Trimmer handles the cut.
Common Use Cases
How We Compare
Honest read on free, paid, and self-hosted options for this kind of job: