How to Bulk Speed Up MP3 Files for Your Audiobooks

Posted on by

I recently got a new iPhone, and I’m spoiled by its ability to play audiobooks at 2x speed in the iBooks app, and to do the same for podcasts in the podcasts app.

My iPod Nano always had a “2x” function, but it was lying to me. It says "2x," but it is actually 1.5x at most. I don't know why they've done this.

Unfortunately, although I love the new "real" 2x speed, my new iPhone is large, and I don’t like wearing those armbands when I work out or go for a run. By contrast, my old Nano is a 6th generation, so it's just a tiny clip-on thing, which is perfect for running and working out. (Note: Apple, please go back to having the clip-on thingies for your Nanos. My workout shorts don't always have pockets, and I don't want to have to buy an accessory for something so simple.) 

That's all a long way of saying I’d prefer to use the Nano, but without giving up the speed of the narrating/podcasting.

Also, it occurred to me that it’d be cool if I could listen to audiobooks at speeds even faster than 2x. Right? Right. Blazing through long-ass Victorian novels in a few days’ worth of dog walks, errands, and running is pretty aces, actually.

After some fiddling, I figured it out.

The main obstacles are as follows:

1) Doing bulk MP3s, all at once, without spending all day doing them manually, one by one, like a chump.

2) If you just speed things up, your narrator sounds like a chipmunk.

3) To account for (2) above, you end up fiddling with pitch and tempo, but this can make the narrator hard to understand.*

* Note, if you jump immediately from normal playback speed to 2X just like that, it will seem hard to understand no matter what you do. Go in increments. I promise you: your brain will adjust. I’m to the point that 1X seems agonizingly slow. I can no longer stand it. It's terrible. Literally, it sounds like someone’s slowed down the track to an absurd degree and my brain has trouble parsing the idea that this is somehow “normal.”

Anyways, I started Googling to figure out how to do this.

I first saw this article on Lifehacker that shows you how to bulk speed up MP3s using the open source Audacity app. (The Lifehacker screenshots look like they’re on Windows; I used the Mac version 2.1.2 with no problems at any stage.) It's the basis for what I recommend below.

Now, in Lifehacker's instructions, they recommend speeding up the tempo instead of the speed, in order to avoid the aforementioned chipmunk effect. I tried increasing only the tempo at first, but the result just sounds weird; it’s oh-so-slightly garbled and hard to understand once you increase it past a certain point.

To counteract this, my next experiment involved adjusting the tempo by only so much, but also adjusting the speed by a little as well, figuring I could do with “a bit” of the chipmunk effect. Turns out even a "bit" of the chipmunk effect is annoying.

Then I came across this video on Youtube:

Basically, according to it, it is the speed you adjust, but after you speed up the mp3 by [A] percentage, you adjust the pitch by that same percentage, but going the other way, so that if you increase the speed (moving the slider to the right), you decrease the pitch (move the slider left the same amount).

So, in my final version, here’s what you’re doing:

Increase the speed by [A] percent.

Decrease the pitch by [A] percent.

Increase the tempo by [B] percent.

In Audacity, you’ll want to do it in that order, using the “Move Up” or “Move Down” options to make sure. (See below.) 


The Step-by-Step Instructions

Step One.

Download the Audacity app.


Step Two.

You need your files to be in MP3 format.

If you ripped your audiobooks from CDs, you're golden. (Note: If you have family members who are readers, and who were adults in the 90s and early 2000s, you'll find they often have old CD audiobooks lying around. Trust me. Just ask. They're probably hidden in a basement, and they're probably John Grisham books.)

If you're a real cool person who likes saving money, and your audiobooks are from Librivox, this shouldn't be a problem, because they're in MP3 to begin with.

If your files are not in MP3, Audacity can convert some formats to MP3 for you. Some formats might need an app like AlltoMP3 or something. Some formats might get difficult with DRM, especially if they're downloaded from Audible or elsewhere online. I don't actually know what the legalities of that are. Use Google.

If you're doing this for podcasts, you'll be able to get MP3s somewhere on your computer. Go to iTunes, right click (or command click or whatever) on the podcast file, and click "Show in Finder" (or "Show in Windows Explorer" in Windows). 

You'll find all the mp3s for that podcast. 



Step Three.

You'll be setting up "chains" of actions in Audacity. A chain is a set of actions that allow you to tell the program to automatically perform one or several actions to the files you feed into it (a lot like Mac's Automator app). 

In Audacity, click File > Edit Chains.


From there, click "Add" in the lower left, and name the new chain whatever you want. Then from there, you'll want to insert your actions by clicking "Insert."

click-addYou'll want insert four action, so that you have five actions in total:

1. ChangeSpeed

2. ChangePitch

3. ChangeTempo

4. Export MP3

5. End. (This one will already be there.)

five-totalFor each one, you can double-click on the action to change it, then to change the percentage, click "Edit Parameters." 


edit-parameters2Back in the Edit Chains Screen, you can click "Move Up" and "Move Down" to get them in the right order.

Here's what mine looked like:


Based on the numbers, you can see I increased speed by 18%, decreased pitch by 17%, and increased tempo by 45%. Yes, that'll be crazy fast, even on my Nano. I was happy with this result. You might not be.

You can experiment with what you like by only feeding in a single MP3 file at a time in the next step, before you do all of them or the whole audiobook.

Also, note that you can either speed things up to exactly how you want to listen to them, or you can speed them just a teeny bit on your computer, so that when you then listen at 1.5x or 2x on your iPod, it's perfect. To test what the new file will sound like if sped up further by 2x or 1.5x, just open the MP3 briefly with VLC Media Player. (Click Playback > Drag the slider to 1.5 or 2 or whatever you want.) That way you don't have to transfer it to your iPod each time.


Step Four. 

Once your chain is set up with its actions, go back to the main screen of Audacity. Click "File" and click "Apply Chain." Navigate to where your MP3 files are saved on your computer, and select all of them.

At the end of the process, the "new" files will appear in a subfolder called "cleaned."


Step Five (optional)

Use Audiobook Builder to turn the MP3s into an actual audiobook file with chapters. I've been doing this for years, and quite like it.

Share Button