Welcome back to Audio Science class!
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:
- be consistent in overall sound and formatting and be comprised of all mono or all stereo files
- include opening and closing credits and a retail audio sample that is between one and five minutes long
Each uploaded audio file must:
- contain only one chapter/section that’s shorter than 120 minutes and the section header must be read aloud
- have room tone at the head and at the tail and be free of extraneous sounds
- measure between -23dB and -18dB RMS and have -3dB peak values and a maximum -60dB noise floor
- be a 192kbps or higher MP3, Constant Bit Rate (CBR) at 44.1 kHz
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?
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)
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.
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:
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.
ALEX’S MASTERING BREAKDOWN
- 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.
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