ACX audio requirements: how to prepare and master an audiobook for Audible

What is ACX?

ACX (Audiobook Creation Exchange) is Amazon's platform for producing and distributing audiobooks to Audible, Amazon and Apple Books. Every file you upload passes an automated technical review covering volume, peaks, noise floor, format and structure. Any file outside the spec is rejected before the human quality check even begins, so hitting the numbers is step one.

The ACX technical specs at a glance

A compliant file is a mono (or stereo) MP3 at 192 kbps constant bit rate (minimum) and 44.1 kHz, mono recommended and stereo accepted, with an RMS level between -23 dB and -18 dB, a peak no higher than -3 dB, and a noise floor below -60 dB. It opens with 0.5 to 1 second of room tone and ends with 1 to 5 seconds.

• RMS (average level): between -23 dB and -18 dB.
• Peak: maximum -3 dB.
• Noise floor: -60 dB RMS or lower.
• Format: MP3, constant bit rate (CBR).
• Bit rate: 192 kbps CBR (minimum).
• Sample rate: 44.1 kHz.
• Channels: mono recommended, stereo accepted, but the whole book must be consistent (never mix mono and stereo files).
• Each file under 120 minutes.

RMS, peak and noise floor: what they measure

RMS is the average level of your narration over time, the closest meter to perceived volume. Peak is the single loudest sample. Noise floor is the level of the quietest part, your room tone, where no one is speaking.

A useful nuance: ACX expresses everything in dBFS, and the RMS dB the spec refers to is dBFS RMS, not LUFS. Audiobooks predate the LUFS standard used for streaming music, which is why the yardstick is different. In practice you aim for around -20 dBFS RMS, comfortably inside the -23 to -18 window.

Structuring and naming your files

ACX requires one file per section: opening credits, each chapter, and closing credits. You cannot upload the whole book as a single file, and each file must run under 120 minutes. Number your filenames (001, 002, and so on) so the playback order is unambiguous.

• Opening credits (separate file): title, author and narrator, spoken aloud.
• Closing credits (separate file): the closing line (for example "The End").
• Room tone: 0.5 to 1 second at the head, 1 to 5 seconds at the tail of every file.
• Retail sample: 1 to 5 minutes, starting with narration (not credits or music), no explicit content.
• Cover art: square, 2400 x 2400 px minimum.

Mastering to ACX, step by step

ACX's own recommended chain is simple: clean unwanted frequencies with EQ, tame peaks with a limiter to create headroom, then raise overall gain until the file sits inside the -23 to -18 dB RMS window. The practical target is around -20 dB RMS, a real peak under -3 dB, and a noise floor below -60 dB.

Set loudness last. The most common mistake is normalizing a noisy recording, which simply amplifies the problems. Clean first, then level.

• Reduce the noise floor below -60 dB with gentle passes (iZotope RX, 6 to 8 dB at a time rather than one aggressive pass).
• Reach the RMS window without crushing the natural dynamics of the voice.
• Measure the noise floor by selecting 3 to 5 seconds of silence and reading its RMS, then check overall RMS and peak.

Tools to measure and fix

A professional station like Pro Tools handles recording and editing. For the MP3, the reliable workflow is to bounce a 24-bit WAV master and encode to MP3 with CBR explicitly set to 192 kbps and 44.1 kHz, in the DAW or a dedicated encoder, then verify the final MP3, rather than assuming a default DAW MP3 bounce meets the CBR spec. iZotope RX profiles and reduces the room noise and reads the file statistics (noise floor, RMS, peak). Audacity with the free ACX Check plug-in is an entry-level option for verifying the three numbers before upload.

The errors that get an audiobook rejected

Most ACX rejections come down to a handful of measurable faults. Fixing them is mechanical once you know what the check is looking for.

• RMS outside the -23 to -18 dB window (raise or lower overall gain).
• Peak above -3 dB (apply a limiter with a -3 dB ceiling).
• Noise floor above -60 dB (denoise the room tone in RX).
• Missing or excessive room tone (trim to 0.5-1 s head, 1-5 s tail).
• Wrong bit rate or sample rate (export 192 kbps CBR, 44.1 kHz).
• Audible artifacts left in the track (mouth clicks, breaths, hiss).

Submitting your audiobook on ACX

Before any upload, you must link the print or Kindle edition of your book on Amazon. You then create an ACX project, upload each section file, add the square cover (2400 px minimum) and the retail sample. ACX does not publish a fixed review window, and turnaround times vary, so plan for the review to take some time rather than counting on a specific number of days.

ACX royalty tiers: exclusive vs non-exclusive

Royalties are a platform term, separate from the audio specs, but creators preparing a title ask about them constantly. ACX has historically paid a higher rate for exclusive distribution (sold through Audible, Amazon and Apple Books only) than for non-exclusive distribution, where you can also sell the title elsewhere. Platform terms change over time, so treat the figures below as the established baseline and confirm the rate tied to your title in the ACX dashboard.

The figures below are the neutral platform percentages, not a recommendation. The right tier depends on your own distribution plans, and the exact rate tied to a title appears in the ACX dashboard.