How to use the audio speed changer
- Choose an audio file or drop it on the box. WAV, MP3, M4A, OGG and FLAC files up to 25 MB and about 5 minutes of stereo sound work. Other file types, including text and images, are refused.
- Enter the speed, from 0.25 to 4, or press one of the quick speeds. 1.5 means 50% faster. The line under the box shows how long the result will be.
- Leave Keep the pitch the same ticked to change only the timing, which suits speech. Untick it to play the sound faster or slower like a record, so the pitch rises or falls too.
- Press Change speed and download. A progress bar follows the work, the WAV file is saved, and a player lets you listen to the result. Nothing plays until you press play.
Formula
result length = original length / speed
without keeping pitch: new sample at position i = old audio at position i x speed, blended between neighbors
new pitch = old pitch x speed
At 1.5 times, 60 seconds becomes 40 seconds. When the pitch is kept, the sound is cut into short overlapping pieces that are laid down closer together or further apart, each piece nudged to the point where its wave lines up with the one before.
A tone at 1.5 times
A 440 hertz tone lasting 2 seconds played at 1.5 times lasts 1.33 seconds. Without keeping the pitch the tone rises to 660 hertz, which is 440 times 1.5. With the pitch kept, the tone stays at 440 hertz and the length is the same, so only the timing changes. A sound of 60 seconds becomes 40 seconds at 1.5 times and 120 seconds at half speed.
These numbers were checked by reading the saved WAV files with Python and measuring them with a Fourier transform: 10,666 frames at 8,000 samples per second, with peaks at 660.03 hertz when the pitch changes and 440.09 hertz when it is kept, against a true 440.
Two ways to change speed
The simplest way to speed up a sound is to play the same samples faster. That is what a record player or a tape deck does, and it raises the pitch along with the speed, so voices sound squeaky when sped up and deep when slowed down. This tool does that when you untick the pitch box, by blending between neighboring samples at the new rate.
To change only the timing, the tool uses a method called waveform similarity overlap-add. It cuts the sound into frames of about 40 milliseconds, lays them down at a different spacing, and fades them into each other. Each frame is shifted by a few milliseconds to line its wave up with the one before, which avoids clicks and keeps the pitch. It works well for speech and for single instruments, and gives a slightly watery sound at extreme speeds or on dense music.
What is saved
The result is always a 16-bit WAV file at 44,100 samples per second. Your browser decodes the original at that rate first, so a recording made at another rate is converted. The channels of a stereo file are kept together, so the left and right stay in step. A WAV file is larger than an MP3, about 10 megabytes for each minute of stereo sound. To convert it to a smaller format afterward, use another tool.
Speeds from 0.25 to 4 are allowed. The tool tells you if the result would be too large to save. It does not clean up noise or change loudness. To work out a tempo, try the BPM counter, and for practice use the metronome. To tune an instrument, see the online tuner.
Privacy and limits
The audio is decoded and changed in your browser and never uploaded. Sound is only played when you press play on the player. Files up to 25 MB are accepted, and the decoded sound may be up to about 5 minutes of stereo. If your browser cannot decode a file, you get a message, and converting the file to WAV or MP3 first usually helps.
Assumptions and limits
- The result is a 16-bit PCM WAV file at 44,100 samples per second, with one or two channels.
- Files with more than two channels use the first two.
- Keeping the pitch uses time stretching, which is an approximation and can add a slight artifact at extreme speeds.
- Speeds are limited to 0.25 through 4, and long recordings are refused to protect your browser's memory.
Frequently asked questions
Why does the pitch change when I speed up?
Playing the same sound faster raises every frequency by the same factor, as with a record played too fast. Tick the box to keep the pitch the same, and the tool changes only the timing.
How long will the result be?
The original length divided by the speed. At 2 times, 60 seconds becomes 30. At 0.5 times, it becomes 120. The line under the speed box shows the length before you save.
Why is the saved file so big?
It is an uncompressed WAV file, which holds every sample. About 10 megabytes a minute for stereo is normal. Compress it afterward with an audio converter if you need a small file.
Does it work with video files?
No. It reads audio files. An MP4 that holds only audio can work, but a video file is not supported and may be refused.