AudioBook Mastering

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Flynwill 20170618: This page is a work in progress, copied from a post by Koz with his permission.
This is a suite of Audacity tools and process to help you produce an audiobook reading that reaches ACX Technical Compliance. See NOTES AND COMMENTS at the end.


We designed custom AudioBook Mastering tools. You have to install them.

Get and install ACX-Check

Get and install RMS-Normalize


Use Audacity version 2.1.3 or later. Not optional.

Start this process with a raw, uncorrected reading. No processing, adjustments or fixes. Don't help.

Export a WAV (Microsoft) 16-bit sound file of your raw reading and save it in a safe place. You should never be stuck reading a chapter again because of an accident. Saving an Audacity Project is not recommended for this.


These instructions are in short-form: Location > Tool: Options > OK

Select the whole reading or chapter by clicking just above MUTE (on the left).

Effect > Equalization > Select Curve: Low roll-off for speech, Length of Filter: about 5000 > OK.

Effect > RMS Normalize: Target RMS Level -20dB > OK.

Effect > Limiter: Soft Limit, 0, 0, -3.5dB, 10, No > OK.

Analyze > ACX-Check.

The first two readings, Peak (no higher than -3dB) and RMS (between -18dB and -23dB) should be nearly perfect. If the noise is quieter than about -65dB (-60dB limit), and the show sounds reasonable, you may be done.

If you fail noise, then the process can be a great deal more interesting. See Noise Posting, below.


ACX has their own help pages, tutorials and videos.

This mastering suite addresses technical considerations. Nothing here will address theatrical errors such as harsh sibilance, gritty voice or wet mouth noises. There is no acting filter. You have to know how to read aloud.

The quality of the original recording is very important. It's easy to record mistakes that can't be fixed later. Record to a reasonable volume (occasional peaks at -6dB) in a quiet room with no echoes.

When you get done with a chapter, File > Export the work as WAV (Microsoft, 16-bit) and save the sound file with a unique name somewhere safe. That's your emergency backup so you don't have to read it again if you damage the work or lose your edit.

Only then start editing, processing, filtering and correcting. Keep testing the work with the add-on ACX Check analysis tool as you go. Select the work and Analyze > ACX Check.

ACX Technical limits:

  • Peaks no louder than -3dB.
  • RMS (performance loudness) between -18dB and -23dB.
  • Noise no louder than -60dB.

Those are the top three values in the ACX Check panel


ACX Values in English

Human Quality Control at ACX (the theatrical test after you pass ACX-Check technical test) does not like heavy processing. You should be as gentle as you can with as few corrections as possible. Don't even think of submitting readings that sound like a bad cellphone, speaking into a wineglass or reading in a bathroom.

The AudioBook metaphor is listening to someone telling you a story over cups of tea. Anything that distracts from that ideal should be avoided.

Noise Compliance

We assume you've already been through AudioBook Mastering and failed ACX Technical Compliance because of noise (louder than -60dB) or your performance has odd background sounds you don't like. Noise is common in a home studio.



This is a gentle spring rain in the trees ffffffffff sound behind your voice. Microphone systems make noises like this naturally and it's your job to make your voice loud enough so nobody notices the noise, but not so loud your voice distorts.

If the hiss isn't too bad, try a gentle correction such as Noise Reduction of the Beast (6, 6, 6) or UNDO and try 9, 6, 6. You can hear the hiss getting quieter and recede into the background as you increase the first number: Nose Reduction. Try ACX-Check. If you need reduction as high as 12, 6, 6, your voice may get wine-glassy or honky and the show may not pass ACX inspection.

The solution may be to change your announcing style or even the microphone. You should be about a shaka away from the microphone...


... or as close as a fist (louder and more intimate) but you may need a pop and blast filter.



Computer fan, air conditioning, refrigerators or other machines.

Turn off fans or machines while you're presenting if you can. And yes, we understand the contradiction of needing to watch the Audacity screen, remove the noisy computer from your room and keep the computer one USB cable away from your USB microphone all at the same time. ACX did it by having an almost completely silent laptop in the studio to do the recording.


People have done it by extending the keyboard, mouse and monitor outside their studio, but you are warned against extending a USB cable, particularly with audio (or video) production.

Do not block computer vent holes.

If machine noises are constant and not seriously loud, they may respond to Effect > Noise Reduction.

Try a gentle correction such as Noise Reduction of the Beast (6, 6, 6) or UNDO and try 9, 6, 6. You can hear the hiss getting quieter and recede into the background as you increase the first number: Nose Reduction. Try ACX-Check. If you need reduction as high as 12, 6, 6, your voice may get wine-glassy or honky and the show may not pass ACX inspection.


Some computers and USB microphones hate each other and produce mosquito whine sound.

Whine doesn't respond well to Effect > Noise Reduction.

Get and install Mosquito-Killer4.

Effect > Mosquito-Killer4: ...Mosquitoes to kill?: 8 > OK.

We expect the tool to make common USB whine vanish, but there are some versions of whine that don't perfectly respond. Listen carefully and if Mosquito_Killer4 fails or doesn't work enough, UNDO and post a help message on the Audacity Forum with a sample of the work.

Posting an Audio Sample]

I don't know of any easy, foolproof way to permanently fix a USB microphone once you have the whine sound. The most likely fix is change the computer. ACX did it by not using a USB microphone. They used a very high quality analog microphone plugged into a stable USB interface.



There are no tools to remove noises that constantly change. If jets overhead, traffic noises, dogs barking and the TV next door are included in your show, they are now your permanent performance partners. Read the work again in a quieter room. Gating doesn't help.


Nobody said you can't have more than one noise. The ACX AudioBook noise test can be rough to pass with a home recording system. If you just can't get there with the above tools, or you can't get your voice to sound right, post to the Audacity Forum with a sound sample and send a clean, raw clip. Mention the three-digit Audacity number.

Posting an Audio Sample]

Raw, please. We can't remove effects from a clip and posting your correction errors doesn't tell us anything.