Understand ACX (and similar) Submission Requirements related to Loudness

Understand ACX (and similar) Submission Requirements related to Loudness

Intro

When it comes to delivering professional audiobooks, meeting specific loudness requirements is crucial. ACX, and platforms like it, measure audio loudness using decibels (dB). On a digital scale, 0 dB represents the absolute maximum level your audio can reach before it distorts and sounds terrible – a phenomenon known as digital clipping.

I like to think about it as if I am a fish and 0 dB is the lake surface. If I'm above the surface, I'm in trouble; my audio will be clipped and unusable. I need to stay safely underwater. I'll describe the three key ACX requirements within this context. While I'll reference terms used in popular audio software like iZotope RX, these concepts (and similar terms) apply across most Digital Audio Workstations (DAWs) and audio analysis tools.

1. Each file must measure between -23 dB and -18 dB LUFS (Integrated Loudness)

  • As a fish: As I'm swimming around, I'm generally going to be hanging out somewhere between 18 and 23 feet below the waterline. This is my comfortable, consistent depth.
  • Audio terms: This specifies the average loudness of your entire file, known as Integrated Loudness. This is the key metric ACX uses to ensure consistent listening levels across different audiobooks. While you might see "Total RMS level" in some older guidelines or software, the industry standard for this measurement is now typically expressed in LUFS (Loudness Units Full Scale), which is a more advanced and accurate way to measure perceived loudness than simple RMS. [You can optionally keep the "RMS stands for Root Mean Squared..." if you feel it adds value, but it's not strictly necessary if focusing on LUFS.]
  • Why this range? This specific range ensures your audiobook isn't too quiet (forcing listeners to crank their volume) or too loud (which can be jarring and fatiguing).

2. Each file must have True Peak values no higher than -3 dB

  • As a fish: Occasionally, when I get exuberant and splash around, I'm going to venture up closer to the waterline. For the sake of being safe, though, I never go closer than 3 feet below the surface. That gives me a bit of a buffer just in case.
  • Audio terms: This is the loudest single point in your file, measured as the True Peak level (sometimes called Sample Peak).
  • Why not 0 dB? By keeping your loudest points (your 'peaks') below -3 dB, you ensure your audio never clips or distorts, even if the file is converted to a different format. More importantly, this -3 dB buffer gives the engineers at ACX (or any mastering engineer) a little wiggle room – often called headroom – to make minor adjustments to the loudness of your project without introducing distortion. If your audio hits 0 dB, you've taken that critical option away from them.

3. Each file must have a noise floor no higher than -60 dB (RMS)

  • As a fish: It's important for me to see where I'm swimming and to avoid pollution. If I'm floating around in silty, cloudy water, it's difficult to see where I'm going and to avoid swimming into danger. Clear water ensures a smooth journey.
  • Audio terms: This measures the level of background noise in your recording when you are not speaking. If there's too much background noise (like a hum from your computer, traffic outside, or even just a noisy room), it gets in the way of the listener appreciating your narration. We measure the noise floor using an area of audio that contains no narration or breathing – just the sound of your recording space. This is why it's so important to record a bit of room tone at the front end of each chapter; the RMS level of that silent selection will be the loudness of your noise floor.
  • Why is this important? A low noise floor indicates a clean recording environment, which is paramount for an immersive and professional listening experience. Listeners notice distracting background noise more than you might think!