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We have a dance competition approach and the song we've chosen is 3:54. The rules say the song cannot be more than 3:30. We have to provide copies of the song on a CD, a USB stick and an iPod/iPad/iPhone which means we cannot rely on realtime slow-down software on an iPhone alone to make the tempo adjustment.

We must record it and save it as a new file and then copy it to all the medium.

I have a friend with a Mac who is willing to alter the file for us -- what does he need to do to change the tempo of the song so it fits in to the 3:30 run length requirement?

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    Would you edit in what tools you have available to do the editing? What version of iOS are you running and/or do you have a Mac for editing things?
    – bmike ♦
    Apr 14, 2014 at 17:44
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    Hi Elsie, a dance competition, hmmm. If you record it at faster speed it is not going to make it shorter but it will sound funny. You see a song that normally plays for 3:54 recorded at faster speed still lasts 3:54. So your best option is to skip a note here and there till you have the 24 seconds.
    – Ruskes
    Apr 14, 2014 at 17:47
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    @Buscar웃 recording at a faster speed does shorten the length of a song. Playback at a faster speed does this as well. The song has a fixed number of beats in it and is being performed a specific tempo which can be described in beats-per-minute. Playing those same beats at a fast beat-per-minute tempo causes the song to end sooner.
    – Ian C. ♦
    Apr 14, 2014 at 19:01
  • @IanC. As far I know the faster recording only gets better resolution (more details). If I sing a song and it last 4 minutes and it is recorder by a fast recording device it will still last 4 minutes because the source input lasted 4 minutes. Fast recording is not compression, it is the opposite, it creates more data, resolution, the files are bigger. Playback of fast recording at fast speed only gives fine resolution but no time saving. Playback of fast recording at normal speed is creates the slow motion effect.
    – Ruskes
    Apr 14, 2014 at 19:15
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    You are completely misunderstanding what the OP is asking for.
    – Ian C. ♦
    Apr 14, 2014 at 19:24

2 Answers 2

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Audacity has a change tempo effect you could try in the Effects menu. It might end up making it sound funny, so you will obviously want to listen to it once the effect is applied, but it doesn't simply play it back faster which would change the pitch (that is the change speed effect.)

If my math is right you will want to change it by +11.4% to get from 3:54 to 3:30.

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  • +1. This is fast and free. Also, the question was updated. And I think it's 10.26%
    – Tony
    Apr 14, 2014 at 18:50
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You can also use the atempo filter with ffmpeg:

ffmpeg -i input.mp3 -filter:a atempo=1.114 output.wav
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