Our commercial recording studio is setup to record all things audio, and audiobook production is our sweet spot. Here’s a 40 second sample of an audiobook we recently worked on. The children’s book author was also the narrator. 

 

Audiobook for kids

We record and produce audiobooks according to audible.com standards (shown below). 

What is RMS?

RMS has many functions, but for our purposes, it’s the value assigned to the overall volume level of an audio recording. It is a conventional way to measure the effective average value of an audio signal as well as the perceived dynamic range of that signal. In other words, how loud a recording sounds to the human ear.

Why is RMS important?

Audible will apply light dynamics processing once your audiobook is submitted to ensure your production’s sound and levels are consistent with the rest of the Audible catalog, so it is important that your levels not be too high or too low. For example, a production with a low RMS but loud peaks could end up with technical issues such as uneven narration levels, a high noise floor, or digital clipping. By keeping your files between -23dB and -18dB RMS – you help ensure listeners won’t have to constantly adjust the volume of their playback device when transitioning from chapter to chapter or book to book. An audiobook listener’s experience should be free of distractions and these requirements ensure a smooth listening experience so the listener can focus on what’s most important – the story and the narrator’s performance.

How can I optimize my recording level to conform to ACX’s RMS requirements?

Maintaining optimal RMS begins with controlling the signal level of your voice. We recommend tracking so that your RMS level is around -20dB RMS with peak levels around -7dB, but not exceeding -3dB. This level should provide a strong signal to noise ratio, while leaving headroom for gain increases as you fine-tune the recording in post-production. Giving an even performance and utilizing proper microphone technique will also factor in the consistency of your recording level, and this should be your focus at the recording stage. For help adjusting the ‘gain’ control on your microphone or preamp while setting up your recording, we recommend reviewing first file (good-production_01_raw-recording.wav) in the ACX Audio Reference Sample Pack.

How do I manage RMS level after recording?

Even optimally tracked recordings may require adjustments in order to meet our RMS requirements. Some level imperfections can be addressed in the Editing and QC stage, but any extremely high peaks or distortion discovered during these checks should be re-recorded. Mastering is the final step of post-production and the glue that brings the entire audiobook together. During Mastering, dynamic processors can be used to even out slight variations in level, bringing each recording within the target level range. The ‘Alex the Audio Scientist’ series of articles and videos covers both the editing and mastering stages of production, including tips on how to maintain proper levels.

How do I manage RMS level after recording?

Even optimally tracked recordings may require adjustments in order to meet our RMS requirements. Some level imperfections can be addressed in the Editing and QC stage, but any extremely high peaks or distortion discovered during these checks should be re-recorded. Mastering is the final step of post-production and the glue that brings the entire audiobook together. During Mastering, dynamic processors can be used to even out slight variations in level, bringing each recording within the target level range. The ‘Alex the Audio Scientist’ series of articles and videos covers both the editing and mastering stages of production, including tips on how to maintain proper levels.

How can I test the RMS levels of my files during production and post-production?

RMS values can be measured using multiple methods, so readings can differ depending on your DAW and the plugin/meter used. The following statistics plugins closely match the RMS measurements we take when analyzing recordings at Audible:

REAPER
The ACX Audio Team recommends that producers working in REAPER download the SWS extension, which includes recording statistics (peak and RMS readings) and mastering tools we have found helpful when troubleshooting with producers. You can find a link to this extension, and all of our REAPER recommendations here.

Audacity
The ACX Audio Team recommends using the “Wave Stats” plugin, which can provide reliable peak and RMS data for up to 30-second windows of a recording.

Other
You can also identify your plugin or meter of choice’s RMS (or LUFS) reading equivalent to our RMS standard by analyzing the third file (good-production_03_mastered-recording.wav) in the ACX Audio Reference Sample Pack.
This can provide a good baseline reading to compare with during metering.

Guidance – Section 1 – Adjusting Low RMS Files

When beginning to resolve a low RMS issue with a finished file, it is important to first assess the dynamic nature of the recording. We recommend reviewing your file(s) alongside the second file (good-production_02_edited-recording.wav) in the ACX Audio Reference Sample Pack. If your file is peaking evenly at or below the peak level of this sample, it may be dynamically consistent enough to meet our RMS requirements by simply raising it’s overall gain.
Click here for a beginner’s primer on mastering audio to ACX specifications.

If your file is peaking unevenly or above the peak level of this sample, it may be too dynamic to boost to the required RMS level without the uneven peaks going above -3dB, or worse, distorting at 0dB. In these cases, the high peaks need to be evened out before a gain boost can be applied. To do this, we need to use a limiter.

A limiter is a dynamics processor. Applying a limiter lowers any high peaks in our audio to the threshold dB level we choose to set. This makes the volume of the narration more even throughout, and gives us more headroom for a gain increase. The following example will show how a limiter can be applied to raise an overly dynamic file to our RMS standard. Example: The image below shows a file that peaks at -2dB (too high), with an RMS value of -27dB RMS (too low). It is peaking above -3dB, but is not distorting. With careful listening and patience, we should be able to bring it within both level standards .


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To bring this file’s RMS to our required minimum level (-23dB RMS), we need increase the track gain by 4dB. Unfortunately, this file is already peaking at -2dB and an increase of 4dB would bring these peaks 2dB over 0dB, causing distortion. So how do we avoid this distortion during our gain increase?

Here is where the limiter comes in. In this case, we need to lower the highest peaks by 5dB (1dB of reduction to bring the peaks to our target final level of-3dB, added to the 4dB of reduction we need for headroom to increase the RMS level of the file). Since our file’s highest peaks are at -2dB, we need to set our limiter threshold at -7dB to bring them down 5dB in level.


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After limiting, our maximum peak dB level is now -7dB. Since ACX’s peaks requirement is -3dB, we can now raise the overall level of the audio by +4dB without distorting. This brings our RMS to from -27dB RMS to -23dB RMS, which is within our required range.

Our finished file:


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As always, it is important to remember that frequency content, noise floor consistency, and audio quality can all affect how a recording behaves during processing, so it is necessary to use your ears and best judgement when attempting to adjust overly dynamic files. Listen back to your finished revision for any areas of distortion or unnatural sounding level fluctuation due to limiting, before replacing the file in your project.

For an example of a fully mastered audio file that meets our requirements, we recommend reviewing the third file (good-production_03_edited-mastered-recording.wav) in the ACX Audio Reference Sample Pack.

Guidance – Section 2 – Adjusting High RMS Files

Files with high RMS values are either recorded with too much gain, or over-compressed during the mastering process. Resolving issues of this nature requires identifying when in the process the recording went outside of the required RMS range.

First, return to your editing session file for the recording in question. Set your track gain to unity (no added gain) and observe your recorded microphone level. If the edited master has 0dB peaks or distortion from the recording stage, you may need to lower your microphone level and re-record the distorted lines of narration. The revised recording’s gain can then be raised or lowered to bring the file within our RMS requirements .

Files with high RMS values more typically have been over-compressed during mastering. In these cases, you will need return to your mastering session file and adjust your limiter or compressor settings to allow for more dynamics within the recording. If your compressor/limiter gain is set to unity (no added gain), try adjusting your threshold until your recording peaks at -3dB. If it is even in level, it may now pass our RMS requirement.

If you added gain during compression/limiting, try first slowly lowering the added gain, while adjusting the threshold until your recording peaks at -3dB. If it is even in level, it may now pass our RMS requirements .

Example: In the example below, the original recording was mastered using a compressor with a 10:1 ratio and threshold level at -10dB. 5dB of gain was also added during compression. The resulting file peaks at -10dB, with an RMS level of -15dB RMS. This file is not only outside of our RMS requirements, but also heavily distorted:


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Returning to this sections editing session file, we are able to see the original raw, edited recording is more dynamic and actually closer to the required RMS level than the improperly mastered “High RMS, Low Peak” file:


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Setting our compressor’s output gain to unity, then applying lighter compression settings to this file, easily brings it within the required RMS range, while remaining dynamic enough that it peaks near -3dB. As always, be sure to listen for distortion throughout this process to ensure you have the cleanest possible signal at every stage.

For an example of a fully mastered audio file that meets our requirements, we recommend reviewing the third file (good-production_03_edited-mastered-recording.wav) in the ACX Audio Reference Sample Pack.

Source: https://audible.my.site.com/acxhelp/s/article/audiobook-production-guidance-rms 

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