Tag Archives: mastering

Mastering Audiobooks with Alex the Audio Scientist

Welcome back to Audio Science class!ADBLCRE-ACX_Character_Icon

Today’s lesson is going to be a little different from my others. Since I’m lucky enough to have such eager students, I often get questions about one of the more mystifying aspects of audiobook production: mastering. Today, I’ll answer the most common questions and give you a breakdown of the basics steps of the mastering process. But first, let’s review ACX’s Audio Submission Requirements:

Your submitted audiobook must:

Each uploaded audio file must:

Now that we’ve covered the basics, let’s get to those questions.

Q: Why do I need to master my audiobook productions?

A: Mastering is the the final step of post-production and the glue that brings the entire audiobook together. All chapters/sections are brought up to matching levels, which provides a smooth listening experience. Additionally, removing unwanted high and low frequencies can help reduce any hum or hiss that may be in a recording.

Q: Why do I need to follow all of these mastering requirements?

A: Audible offers each audiobook in a range of different audio formats to accommodate listeners on Audible, Amazon, and iTunes. This means that audio quality will range from very high fidelity to lesser fidelities that equate to smaller file sizes and quicker downloads. Basically, if your RMS is between -18dB and -23dB RMS, with peaks at -3dB, you’ll achieve the optimal sound across all formats.

Q: What is RMS?

A: RMS has many functions, but for audiobooks it’s the value assigned to the overall volume level of an audio file. Audible will apply light dynamics processing once your audiobooks are submitted, so your production’s overall levels should not be too high or too low. For example, a production with a low RMS but loud peaks could end up with technical issues within the file, such as uneven narration levels, a high noise floor, etc.

Q: What is peaking?

ACX Peaks

Examples of peaks in an audiobook recording.

A: Peaks are the loudest part or parts of an audio file. If the script calls for a change from calm to excited, or from speaking to yelling, those excited or loud parts will most likely have the highest peaks. Our Audio Submission Requirements call for peaks to be under -3dB, which helps prevent distortion. If you have any 0dB peaks after mastering, you’ll need to adjust your limiter or normalizer settings and try again on your edited audio. If you have 0dB peaks before mastering, you’ll need to find out whether those peaks occurred during recording or after. If it happened during recording, you’ll need to lower your pre-amp’s level and re-record those lines of narration.

Q: What is an EQ?

A: An EQ (short for “equalizer”) is a tool that allows you to adjust the level of any frequency in an audio file. The typical frequency range that the human ear can detect is 20Hz to 20,000 kHz. The lower frequencies in this range are the bass/low range, while the middle is the mid-range, and high frequencies are the high range. Most EQ plug-ins will have high pass filter and a low pass filter. Using the high pass will remove any unwanted bass (low) frequencies that could have occurred during recording, such as the hum of your computer. A low pass will remove high frequency noises in your audio, like an air conditioner or microphone hiss. I strongly recommend applying EQ before you master, as unwanted high or low frequencies can have an impact on the next step in your mastering process – applying a limiter. Removing a low frequency hum allows the limiter to more easily adjust to the narration at hand.

Q: What is a limiter?

A: A limiter is a dynamics processor. Applying a limiter lowers any high peaks in your audio, which allows the volume of the narration to be more even throughout. This lets you bring up the overall volume of your audio, which may be necessary to meet ACX’s RMS requirement (-18dB RMS to -23dB RMS). For example, if your max peak level is -4dB but your overall RMS level is -27dB RMS, your audio will look similar to the image below:

(Click images to expand)

ACX Screenshot 1 (Highlights) - 10.15.16

In this case, you can use a limiter to lower all peaks by -3dB. Your max peak level would now be -7dB, as illustrated below.

ACX Screenshot 2

Since ACX’s peaks requirement is -3dB, you can now raise the overall level of the audio by +4dB. That would bring your RMS to -23dB RMS, which is within our required range. Your mastered audio would then look something like this:

ACX Screenshot 3

Now that we’ve gone over mastering as a concept, I think you’re ready to take a look at my Mastering Breakdown. It’s a great checklist to mark off each time you master an audiobook.


  • Assess all audio files to ensure no peaks or clipping exist in the audio.
  • Group all similar files together during the assessment so they can be processed at the same time.
  • Apply your “Mastering Chain” by using the following processes, in order:
    • Remove all unnecessary low and high frequencies by applying EQ to clean up the sound of your recordings and provide more headroom in order to boost your files levels effectively. This is a great way to minimize hum and hiss in an otherwise good recording!
    • Bring all files up to the proper dynamic levels as specified by the ACX Audio Submission Requirements page by using normalization, compression and/or limiting, and, if necessary, a final volume adjustment.
  • Listen to your audio after mastering to ensure the operation did not over-process or under-process the recordings. If the resultant audio is at one consistent volume with no change in dynamic level, you’ve likely over- If your audio has sudden spikes and drop offs (indicating it is too dynamic), you’ve under-processed.

That wraps up today’s lesson. I hope you all have a stronger understanding of audiobook mastering than when we started. Mastering your productions can seem daunting and technical, but once you know which aspects of your voice and recording space need to be accounted for, you’ll be able to apply the same processes over and over again with minimal changes. You’ll take your audiobook productions from good to great, and your listeners will appreciate the subtle improvements in sound quality you’ve achieved.

Want audiobook production tips in your inbox? Subscribe to The ACX Blog for the latest from Alex the Audio Scientist.

How to Succeed at Audiobook Production: Part 4

Welcome back to How to Succeed at Audiobook Production. If you’ve been following this series, then you’ve read up on The ACX Mile, which helps you perfect the art of narration recording, perform a complete edit and QC your recorded audio, and learn audio mastering best practices. If you haven’t perused these posts, I recommend doing so before continuing on.

NoAndrew_250x320w that you’re caught up, let’s move on to the fourth and final part of my series: encoding and file delivery.

Rounding The Final Corner

Upon a successful master, your audiobook production is not quite finished. Keeping that in mind, watch the final video in our series, and review the key points I discuss after.

The Home Stretch

I recommend you perform another final QC pass on your audio before moving on to encoding and delivering your audio. After putting so much effort into your production, the last thing we want to do is send an audio review notice to fix missing chapters or out-of-order audio files. The post-mastering QC pass needn’t be as in-depth as what I recommend in the article on editing; it could be as simple as verifying the volume levels are meet our specifications, that the content is complete, and that the audio files are numbered and named in the proper order.

If everything looks ready, then we can begin the encoding process. I cover this in more detail in my post, Encoding Audio with Andrew the Audio Scientist, but I’ll summarize here. That post contains a link to our instructions detailing how to encode your audiobook to ACX specifications using the free and cross-platform fre:ac encoding software.

Keep ACX’s Encoding Requirements in Mind.

All files in your audiobook must:

Also be cognizant of ACX’s file-level requirements. The encoding options you choose can potentially cause the run-time or the file size to go over our specifications. Each file must contain a single chapter, be under 120 minutes in length, and be no larger than 170mb.

Crossing the Finish Line

Once you verify your audio complies with our MP3 encoding and file-level requirements, you’re ready to upload our audio to the ACX production manager! Here are some last-minute tips that could potentially save you from an audio review notice.

1. Be smart about your sample. The retail sample you provide will likely be a customer’s first glimpse into your work, so make this moment count! I recommend grabbing the audio from an early point in the book, so you don’t give away plot developments. Also, be mindful of the ACX sample requirements, which expressly prohibit erotic/mature content. If your book is on the wilder side, find a section that is appropriate for all audiences.

2. Double-check your book’s chapter order when uploading your files. Files arrive to ACX in the same order as they are uploaded to the Production Manager. Doing this last check will make the QA process simpler and faster, allowing us to get your title on sale more quickly.

3. Ensure your file names are clear and concise. To help ACX best understand your audio, I recommend naming your audio using a template such as:

  • 01-BookTitle-OPENING.mp3 <<Your book’s Opening credits.
  • 02-BookTitle-PROLOGUE.mp3
  • 03-BookTitle-CH1.mp3 <<Chapter one
  • 04-BookTitle-CH2-p1.mp3 <<Chapter two, part one
  • 05-BookTitle-CH2-p2.mp3 <<Chapter two, part two

Thank You

Since we are at the end of the How to Succeed at Audiobook Production series (but not the end of my regular contributions to the blog), I want to take this opportunity to thank our users, subscribers and readers for your loyal viewership.

But don’t worry, I’ll be back with more audiobook production tips soon. In the meantime, share your own with your fellow readers in the comments below!

How to Succeed at Audiobook Production: Part 3

Andrew the Audio Scientist here, and today I’m presenting part three of my How to Succeed at Audiobook Production series. Let’s dig into one of the most important, yet least understood aspects of audiobook production: Mastering. But before we tackle that, make sure you’ve done proper editing and QC passes on your raw audio. Check out last week’s post for more on those important steps, and read on for my advice on audiobook mastering.

Andrew_250x320Mastering the Art of Mastering

Before we get to the video below, I want to remind you of the key to producing reliably great sounding audiobooks, especially in the mastering stage: consistency.

Mastering is, in essence, the process of bringing your files closer to one another in terms of sound quality and dynamic range, so the listener will enjoy a book which sounds the same all the way through.

The most important thing to remember about mastering is that it is done not to make a poor recording sound good, but rather to make a good recording sound great. Mastering, like editing, is a key aspect of the perceived professionalism of your production. While the average customer is not thinking about the mastering of a title while listening, the boost of clarity and consistency your narration receives from a proper master cannot be ignored. So, while the process of mastering an audiobook may appear cryptic at first, it is essential to achieving an optimal sound quality for your production.

Now, let’s watch part three of How to Succeed at Audiobook Production, and after, review the mastering tips I suggest below.

Mastering Breakdown

Audiobook mastering involves a few major steps:

  1. Assess all audio files to ensure no peaks or clipping exist in the audio. A good recording and careful editing are both necessary to achieve this.
  2. Group all similar files together during the assessment so they can be processed at the same time.
  3. Bring each group of audio files into your DAW, and perform the following processes in the following order (this is referred to as your “mastering chain”):
    • Remove all unnecessary low and high frequencies to clean up the sound of your recordings and provide more headroom in order to boost your files levels effectively. This is a great way to minimize hum and hiss in an otherwise good recording!
    • Bring all files up to the proper dynamic levels as specified by the ACX Audio Submission Requirements page by using normalization, compression and/or limiting, and, if necessary, a final volume adjustment.
  4. Check the audio after mastering to ensure the operation did not over-process or under-process the recordings.

A common mastering chain for an ACX production is as follows:


Remove low (80hz and lower) and high (16kHz and higher) frequencies by using a high-pass and low-pass filter, respectively. Set the high-pass filter to remove sounds below 80hz, and set the low-pass filter to remove sounds above 16kHz. If available, set the Q to the highest-possible setting for both filters. Usually, that setting is 24dB or 48dB per octave.


Typically, you should normalize your peaks to -6dB.


We recommend using a limiter, if available, instead of a compressor. Compression can achieve similar results, but it may also decrease the dynamic range of your vocal recording if used improperly. To properly utilize limiting on your files, start by setting your limiter’s maximum output to -3dB. Then, turn up the gain on your limiter until you have achieved a loud, clear, and consistent sound. Don’t boost the level too high. Otherwise, you may distort your voice, or bring up the noise floor of your recording too much. Remember: the better your recording and editing, the easier this process will be! If you have not yet read the first and second parts of this blog post series, I strongly recommend you do so. It can greatly reduce the workload involved in mastering your audiobook.

Tips for Each Step of the Mastering Process 

Before you begin mastering, record and edit the entire audiobook to completion. Then, make a final “completed edits backup,” – which I refer to in my prior blog post on file management – of each chapter. Maintaining a backup file is imperative, in case you discover any issues with the audio while mastering.

A good recording is everything. Prior to mastering, a well-recorded audio file will have an RMS value no greater than -28dB RMS and peaks at a level no higher than -12 dB This provides the headroom needed to boost the volume of your production without needing to compress the signal heavily. If your peaks are already nearing -3dB before mastering, make sure no loud noises remain in the audio. If no erroneous sounds are found, then it’s likely you recorded too loudly. This is why learning how to properly prepare for and record your narration is essential to performing a successful master.

Plugins cannot help an inconsistent or noisy recording. Some people attempt to fix deficiencies in their recordings or their editing by using noise reduction plugins and gates. Software like this can be effective if used properly, but more often than not, the use of such plugins will cause more harm than good on an audiobook production. I strongly recommended you take the time to focus on your recording environment, as well as your recording and editing techniques, so you do not need to resort to the use of such software. It will save you valuable time as well as money – those plugins can get expensive!

In order to group your audio files together effectively, leverage the audio measurement tools available in your DAW to find the audio files that are similar to one another. If you are an Audacity user, the “stats.ny” plugin will be essential to performing this task. See this thread from the Audacity forums for installation and use instructions.

Most DAWs have a similar capability, so if you are not aware of what tools can be used to achieve these actions in your particular software, contact the manufacturer to receive assistance in their use.

Double-check your masters. Use the same function on your DAW that you used to group your pre-mastered files in order to check your new mastered files. If your audio measurements fall within the ACX Audio Submission Requirements, you should be good to go, which brings us to our final step.

Verify the following ACX Audio Submission Requirements. Are all of your files’ peaks hitting around -3dB? Is each file’s average RMS between -18 and -23dB RMS? How audible is your noise floor at normal listening levels? There’s no shame in attempting a second master if you’ve found flaws in your new mastered files – that’s why you saved your Completed Edits Backup files after you completed your edits.

In truth, the most important mastering tip I can impart upon you, is to try, try again until you get it right! Mastering is very much a process of trial and error until you learn the tips and tricks that suit your production environment. Once you find settings for your mastering chain that work well for your voice and recording space, remember to save the configurations as presets so that you can easily reference them for your next ACX title. If you’ve established a consistent workflow for the recording and editing stages of audiobook production, then your mastering workflow will be a piece of cake!

Do you have mastering tips that Andrew didn’t cover here? Share them in the comments below, and join us next week for the fourth and final part of Andrew’s series: encoding and file delivery.