How Do I Speed Up Hundreds Of Audio Files?

Dear Lifehacker,

I have unabridged Asimov audiobooks that are great, but read at a mind-wrenchingly slow pace. I can boost an MP3's speed 20 per cent using Audacity, but I have around 250 MP3s. How can I process these files all at once?


Sped-Up Sci-Fi Fan

Dear Sped-Up,

If you were a command line geek, or knew one who owed you a favour, speeding up all of your audiobooks at once would probably be a five-minute affair. That said, it's not that difficult to set up a "Chain" in Audacity (which runs on Windows, Mac and Linux) that you can apply to multiple files from inside the program.

You already know the effect you want to apply to all your MP3s: a 20 per cent "speed" increase, where pitch and tempo are sped up at the same time and no correction is applied, as if you were just playing a tape faster. Let's set this up as a chain by hitting the File menu in Audacity and selecting "Edit Chains" —i t's down near the bottom of the menu.

You'll get a new window with two "chains" pre-loaded as examples. Hit the "Add" button in the lower left, give your new "chain" a name like "Asimov Audiobook Speed-Up", and hit OK. You'll notice that there's a single command listed for your new chain in the right-hand window, but it's just an empty "END" command. Double-click that command, or hit "Add". You'll get a pop-up window asking you to select a command and edit its parameters.

Click on the image below for a larger view.

I double-clicked "Change Speed" in the commands windows, and it auto-filled the fields above with the basic command line operation to run a speed change on files. It's set to 0 per cent, however, which won't do us much good. Hit "Edit Parameters", and you'll get a slider and numeric input you can use to set a percentage for the speed-up or slow down. You can use the automatic vinyl conversion tools if you were copying a 33 1/3 record to some other format, but we already know our number, 20 per cent, and we'll stick with that. I'm not sure how the Preview button is supposed to work, but let's just leave it alone. Hit OK, hit OK back at the command chooser, and OK once more at the Edit Chains box, where you can see your one-line Asimov Audiobook Speed-Up chain.

Back in Audacity, close any files you happen to have open for editing. Hit the File menu and select "Apply Chain", and in the dialog that pops up, select your Asimov chain and click the "Apply to Files" button. Choose the audio files you want to run through your speed wringer. Audacity isn't the A-number-One most stable program I've ever used, so I'd recommend running around 10 files at a time through your chain — then again, maybe you can plug in 30 files at a time and just deal with the crashes when they happen, since it processes them one at a time anyway.

Audacity will run through your files and convert them, one by one, and show you its progress. Not every Audacity command can be plugged into a "chain," but speed change just so happens to be one of the lucky ones.

Good luck with your listening,


P.S. We truly do appreciate the smiling coincidence of devising an automation process for the pre-eminent author of robotic-based fiction.


    The crash problem described when applying mulitple files to a chain may be a temp file directory issue; the default temp file dir is on C:, but if that is near capacity it will fill up quickly as audacity uses it to manipulate uncompressed audio data; and with multiple files, this can consume alot of space. The temp file dir is cleared when audacity closes, so you have room to work on more files. The work-around is to Edit > Preferences > Directories, and change the temp file dir location to a different folder on a secondary drive with lot's of space. (I've never had audacaity crash when applying chains, and I chain 40-50 files at a time.) PS: Don't take the above as fact, this is only my preception of how it works, watching the temp file dir size increase during the chain process.

    If you use "ChangeTempo" instead of "ChangeSpeed" you will speed up the audio while preserving the normal pitch of the recorded voice. I regularly speed up spoken word MP3's up to 75% and can fully understand the content. No more chipmunks!

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