Change audio speed

How to Change the speed

1. 1

   Add your audio file

   MP3, WAV, FLAC, M4A, AAC, OGG, Opus or WMA, up to 200 MB. Up to twenty files at once, all at the same speed.

2. 2

   Set the speed

   Anywhere from a quarter speed to four times. 1.5× is the usual choice for a lecture, 0.75× for learning a part by ear.

3. 3

   Click Change speed and download

   The pitch stays where it was unless you deliberately turn that off.

Why the voice does not turn into a chipmunk

Playing a recording faster the naive way is exactly like spinning a record faster: every frequency in it goes up along with the tempo, and a lecturer at 1.5× turns into a cartoon. That is not what anybody wants from a lecture, so this page does something else by default. Time-stretching separates the two things that a record player cannot separate, how fast the sound moves and how high it sounds, and changes only the first. The words come faster, the voice is still the same voice, and after a minute you stop noticing anything was done at all.

The fast mode, and when it is the one you want

Turning off pitch preservation gives you the record-player behaviour on purpose, and it is useful: it is how you get the chipmunk voice for a joke, how you drop a voice an octave to make it unrecognisable, and how you check what a sample sounds like transposed. It is also considerably faster, because it is a single cheap operation rather than a resynthesis. And when the output is going to be a WAV anyway, this mode needs no engine at all; the browser does the whole thing natively and nothing is downloaded.

Beyond double and below half

The filter that does the time-stretching only works between half speed and double speed in one pass. Asking for 3× therefore applies it twice, once at 2× and once at 1.5×, and asking for 0.3× applies it at 0.5× and then 0.6×. The multipliers compose exactly, so the result is the length you asked for; you get one extra pass of processing, which costs a little time and a very small amount of fidelity at the extremes. Between 0.5× and 2×, which is where nearly everybody works, there is a single pass and nothing to think about.

How long the result comes out

Exactly the original divided by the speed. A 60-minute lecture at 1.5× is 40 minutes; at 2× it is 30. Slowing down works the same way in reverse: a four-minute song at 0.75× runs five minutes and twenty seconds. If the file has tags (artist, album, title) they are carried over; the length written in the container is recalculated, so players and seek bars show the new duration rather than the old one.

Keeping the format, or not

By default the result comes back in the format it went in as: an MP3 stays an MP3, a FLAC stays a FLAC. Because the samples themselves change, there is no way to avoid re-encoding a compressed file, so the bitrate is rounded up to the nearest standard step so the second generation costs as little as possible. Choosing WAV instead avoids the question entirely at the price of a much larger file, and choosing MP3 lets you pick the bitrate yourself.

Frequently asked questions