Time Stretcher

Speed up or slow down audio without changing its pitch.

Everything runs in your browser. No audio is ever uploaded.

1.00x

Playback speed1.00x

Above 1.00x the clip plays faster and gets shorter; below 1.00x it plays slower and longer. The pitch stays the same.

This time stretcher makes an audio file play faster or slower without moving its pitch, anywhere from half speed to double speed. Slowing a recording the naive way drops it in pitch and speeding it up makes everyone sound comical; this keeps the notes and the voices where they were and only changes how long the file lasts. It all happens in your browser, and the result comes back as a WAV you can compare against the original before downloading.

How to use it

Choose an audio file or drag it onto the box. Files over 30 MB get a warning because the whole thing is decoded into memory, and files over 150 MB are refused.
Set the playback speed. The slider runs from 0.50x to 2.00x in steps of 0.05. Below 1.00x the clip gets slower and longer, above it the clip gets faster and shorter, and the pitch stays put either way.
Press Apply. A progress bar appears and Apply turns into Cancel while it runs, because the stretching happens in the page itself and a long recording takes real time to work through.
Compare the two. The Original and Result buttons swap the player between the files at the same position, which makes it easy to hear whether the speed you chose is still comfortable to listen to.
Download the result as a WAV file. A compressed input comes back larger, because WAV keeps every sample uncompressed.

What you can use it for

Transcribing an interview or a lecture. At 0.5x or 0.75x you can type at the pace of the recording instead of stopping and rewinding every sentence.

Learning a fast passage by ear. Slowing a solo down without dropping it an octave keeps the notes where they are on your instrument, which is the whole reason to use a time stretcher rather than a speed control.

Getting through a long recording faster. At 1.25x or 1.5x speech is still perfectly clear, and a one-hour lecture takes forty minutes.

Fitting a voiceover into a fixed slot. If a narration runs eight seconds long over a video cut, a small stretch closes the gap without the pitch change a plain speed adjustment would cause.

Language and pronunciation practice. Slowing native speech to 0.75x makes the sounds between words audible while the vowels still sound like themselves.

Speeds and what they are good for

Speed	A 10 minute clip becomes	Good for
0.50x	20:00	Transcription and the hardest musical passages. Also where the processing artifacts are easiest to hear.
0.75x	13:20	Practising along with a recording, and language work. Slow enough to follow, fast enough to still feel like music.
1.00x	10:00	The original. Useful as a reference point when you are deciding how far to go.
1.25x	8:00	Reviewing something you have already heard. Most people stop noticing the speed within a minute.
1.50x	6:40	Working through a backlog of talks. Clear for a measured speaker, hard work for a fast one.
2.00x	5:00	Skimming for the part you need. Comprehension drops for most listeners, so treat it as searching rather than listening.

Accuracy and limits

A phase vocoder does the stretching, and it leaves audible traces at the extremes. The audio is cut into overlapping frames of about 2048 samples and reassembled at a new spacing, which smears sharp transients such as drum hits and hard consonants, and can add a faint watery or metallic quality to sustained sound. Between roughly 0.75x and 1.5x that is hard to hear on speech; at 0.5x and 2.0x on percussive material it is obvious.

This renders a new file rather than changing playback. If all you want is to listen faster, the speed control built into your podcast app, video player or VLC costs nothing, adds no artifacts and can be changed while you listen. Use this tool when you need the file itself to be a different length, for example to hand to someone else or to drop into an edit.

The work runs on the main thread in your tab, so a long file takes a while and the tab should stay in the foreground. The size limits exist because the input, the output and the working buffers all sit in memory at once. Output is always WAV, so a compressed input comes back larger.

Frequently asked questions

Does the pitch change when I change the speed?

No. That is what separates a time stretcher from a plain speed control. Playing a recording at half speed the simple way drops it an octave; this stretches it in time and leaves every frequency where it was.

How slow or how fast can it go?

From 0.50x to 2.00x, in steps of 0.05. Half speed doubles the length of the file and double speed halves it. The range stops there because the artifacts grow quickly beyond it.

Why does slowed audio sound watery or smeared?

It is a side effect of rebuilding the sound from overlapping frames. Steady tones can pick up a slight shimmer and sharp attacks lose their edge. Staying closer to 1.00x reduces it, and speech tolerates far more stretching than drums do.

Is this the same as the speed button in my player?

The idea is the same but the result is not. A player changes speed live, for you only, with no file produced. This produces a new audio file at the new speed that you can download, share or edit. If you only want to listen faster, use the player.

What format is the download?

An uncompressed WAV. That keeps the result from going through a second lossy encoding, at the cost of a larger file. Convert it to MP3 afterwards if you need the size back.

Is my file uploaded?

No. Decoding, stretching and writing the WAV all happen in your browser. Nothing is sent to a server, and the result disappears when you close the tab.