Change audio speed

Time stretching, not pitch shifting

Playing a recording faster the naive way raises the pitch, which is why fast-forward sounds like a chipmunk. The atempo filter used here stretches time and leaves the pitch alone, the same technique your podcast app uses for 1.5× playback. The difference is that this bakes it into the file, so it plays that way in anything.

Practical speeds

1.25× and 1.5× are comfortable for most speech. 2× works for a clear, well-paced speaker. 3× is intelligible but exhausting. Below 1×, slowing to 0.75× is genuinely useful for transcription and language practice.

Why the filter gets chained

atempo only accepts a rate between 0.5 and 2.0 in a single pass, because the algorithm's quality falls apart outside that window. Anything beyond it is done by applying the filter more than once: 3× is 1.5 chained with 2.0, and 4× is 2.0 twice. Each pass is inside its comfortable range, which is why extreme speeds still sound like a person talking rather than a machine failing. It also means a little more smearing on sustained notes at the top end, since the artefacts of the first pass are stretched by the second.

Bake it in, or leave it to the player

Podcast apps and most video players offer variable speed at playback time, and if that is available to you it is the better option: nothing is re-encoded and you can change your mind. Baking the speed into the file is worth doing when the destination has no speed control, which covers car stereos, smart speakers, most embedded web players, some learning platforms and anything being handed to somebody else. It is also the only way to make the change survive a re-upload.

Length, size and bitrate

The file gets shorter in direct proportion to the speed, and smaller by roughly the same factor at a fixed bitrate. A 60 minute lecture at 1.5× becomes 40 minutes, so at 128 kbps it drops from about 55 MB to about 37 MB. Speech at 96 kbps mono is a third smaller again and loses nothing that matters, which makes this a reasonable moment to reconsider the bitrate rather than carrying the original's settings forward out of habit.

Slowing down is the case to watch. A file at 0.75× is a third longer and correspondingly larger, and if it is going onto a device with limited space that is worth knowing before you start.

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