Understand ACX (and similar) Submission Requirements related to Loudness

Understand ACX (and similar) Submission Requirements related to Loudness

ACX measures loudness using decibels (dB). The scale starts at 0 dB (the point at which audio starts to distort) and moves down from there into negative numbers.

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. I need to be underwater. I'll describe the three key ACX requirements within this context. Also note that the underlined items reference the terms used within Izotope RX software.

1. Each file must measure between -23 dB and -18 dB RMS
  1. 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.
  2. Audio terms: The average loudness of the file. ACX thinks of this as the Total RMS level. 
  3. By the way, RMS stands for Root Mean Squared. There's a reason, but it's truly not the least bit important for you to know why.

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

  1. As a fish: Occasionally, when I get exuberant, 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.
  2. Audio terms: This the the loudest single point in the file - called the Sample Peak level.
  3. Why not 0 dB? By keeping the peaks below -3 dB, you give the engineers at ACX a little wiggle room (often called headroom) to make adjustments to the loudness of your project. If your audio is right at 0 dB, you've taken that option away from them.
3. Each file must have a noise floor no higher than - 60 dB
  1. 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..
  2. Audio terms: If there's too much background noise, it gets in the way of the listener appreciating your narration. In this case, we're measuring the noise floor using an area of audio without narration or breathing. Preferably, you will record a bit of room tone at the front end of the chapter. The Total RMS level of that selection will be the loudness of your noise floor.


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