Change speed
What Change speed actually does
A speed change has two numbers people care about: how much faster it plays, and whether the voice or instrument still sits at its original pitch. This tool uses the ffmpeg atempo filter, which stretches time without moving pitch - verified here by running a 440 Hz sine through it at half, double and three-quarter speeds and measuring the result back at 440 Hz every time. Speeds beyond the range a single filter instance covers are handled by chaining instances, so 0.25x is two halvings rather than a degraded approximation, and the whole 0.1x to 10x range is reachable. After the render the page does not just hand over a file: it decodes the result, states the measured duration beside the duration your setting predicts, draws the source and the result on one shared amplitude scale so a level change cannot hide as a drawing artifact, and checks the produced bytes carry the format signature their name claims. The classic effect where pitch rides up with speed belongs to the pitch tool, which changes both together on purpose.
How to use it
- Drop one audio file or a batch. Each stays in this tab; the engine runs locally after a one-time download.
- Set the speed multiplier between 0.1 and 10. A value outside that band is refused by name before anything runs.
- Pick an output format, then run. Chained filter stages cover the full range automatically.
- Drag the multiplier and the preview lane redraws the result's shape and length immediately; press Audition to hear an approximate stretch before spending a render.
- Read the receipt: measured duration next to predicted, waveforms stacked on one divisor, and a tempo check comparing the measured ratio against the setting within two percent.
- Play source and result through one transport with an A/B flip, then download exactly the bytes shown.
Useful for
- Turn a lecture or podcast into a faster listen without the chipmunk effect.
- Slow a fast speaker or a solo down to transcription speed with the voice still natural.
- Fit a fixed-length slot: compute the multiplier from the durations, render, and read back how close the file landed.
- Batch a folder of recordings to one common speed and take the results as a zip.
Limits worth knowing
- Pitch is preserved but extreme stretches can sound rougher than a studio time-stretcher; this build leaves that higher-quality library out rather than pretend to it.
- The tempo check compares measured duration against the setting, not listening quality, which only ears can judge.
- Very long files are bounded by browser memory, not by the tool.
- Live audition runs a lighter stretcher than the download so it can respond while you drag; treat it as a feel check, not a quality verdict.
Questions people ask
Does speeding audio up change the pitch?
Not here. The atempo filter stretches time while holding pitch, which is exactly what a speed tool is usually wanted for. Measured on a pure tone at several settings, the frequency comes back unchanged.
What if I want the tape-speed effect?
Then you want the pitch tool instead: it raises pitch and shortens duration together, like changing playback speed on a tape machine.
Why does 4x need chained filters?
One filter instance covers half to double speed. Outside that, instances are multiplied - four times is two doublings - so the extremes stay accurate instead of skipping audio.
Is anything uploaded?
No. Decoding, processing and encoding all happen in this tab; the engine itself loads once from this site.