How to Master an Audiobook for ACX

Updated August 2026 · by Loopin

ACX (Audible and Amazon) is unusual: it has a hard technical spec your audiobook must pass or it gets rejected. Every chapter needs measured RMS between −23 dB and −18 dB, peaks no higher than −3 dB, and a noise floor no higher than −60 dB RMS. Here’s how to hit those exact numbers, free and in your browser.

The ACX spec, exactly

Unlike streaming music, ACX doesn’t just normalize your file — it checks it against a technical standard and rejects anything outside it. There are three numbers you must hit on every file:

RMS between −23 dB and −18 dB (measured, overall) — not too quiet, not too loud.
Peak values no higher than −3 dB — leave headroom, don’t hit 0.
Noise floor no higher than −60 dB RMS — near-silent between words.
Delivery format: mono 192 kbps MP3, 44.1 kHz, one file per chapter.

Why ACX is stricter than streaming

Music platforms turn loud tracks down to a target and move on. ACX instead enforces a window: your RMS has to land inside −23 to −18 dB, your peaks under −3 dB, and your noise floor under −60 dB RMS. Miss any one and the whole submission bounces back.

That’s why consistency matters so much. A single loud laugh that peaks above −3 dB, or a hissy room that pushes the noise floor over −60 dB RMS, can fail an otherwise fine recording. The master’s job is to bring the whole file inside the spec — and keep every chapter matched.

Clean the recording before you master

Two of the three ACX numbers are really about the recording, not mastering. The noise-floor requirement (below −60 dB RMS) is set by how quiet your room and mic chain are; mastering acts on the whole finished file and can’t un-record hiss or hum without also dulling the voice.

So handle the source first, then let mastering set RMS and peaks:

Record in the quietest space you can, close and consistent on the mic.
Edit out plosives, mouth noise, breaths and long silences.
Apply noise reduction to the raw recording so the silence between words falls below −60 dB RMS.
Only then move to mastering to lock in the RMS window and peak ceiling.

Step by step: master an audiobook chapter

With a cleaned, edited chapter ready, mastering brings it into spec:

Export the edited chapter as a mono WAV at 44.1 kHz so mastering has full quality.
Open Loopin’s free online mastering and drop the file in.
A/B and check the voice stays clear and consistent, with sibilance controlled and no noise creeping up between phrases.
Confirm the result sits inside the ACX window — RMS between −23 and −18 dB, peaks under −3 dB, noise floor under −60 dB RMS.
Download the mastered WAV, then convert it to mono 192 kbps MP3, 44.1 kHz — one file per chapter.
Repeat for every chapter so the whole book is matched, and submit.

Keep every chapter matched

ACX checks each file, but listeners hear the whole book. A chapter recorded on a different day, at a different distance, can drift in tone and level even while technically passing. Master with the same settings across all chapters so the book sounds like one continuous session.

The RMS window is wide enough (−23 to −18 dB) that two chapters can both pass while sounding noticeably different in loudness. Aim every chapter at roughly the same spot inside the window — around the middle — rather than letting them scatter across the whole range.

Same mic, distance and room for every recording session if you can.
Same master settings across all chapters for consistent tone and RMS.
Check the quietest and loudest chapters against the spec, not just an average one.
Add room tone or short silence at heads and tails as ACX guidance suggests, kept under the noise-floor limit.
Aim for the middle of the RMS window so chapters match each other, not just the spec.

Understanding the three numbers

It helps to know what each ACX limit is protecting against, so you can tell whether a failure is a recording problem or a mastering one:

RMS (−23 to −18 dB) is average loudness — too quiet and listeners crank the volume; too loud and it fatigues. Mastering sets this.
Peaks (under −3 dB) guard headroom so a sudden loud word never clips or distorts. The limiter in mastering enforces this.
Noise floor (under −60 dB RMS) measures how quiet the silence between words is — a recording and noise-reduction issue, not a mastering one.
Format (mono 192 kbps MP3, 44.1 kHz) is the delivery wrapper — master in WAV, convert at the end.

Common mistakes

Peaking too hot. ACX wants peaks under −3 dB — don’t maximize to 0 like a song, or it fails.
A noisy floor. If silence between words is above −60 dB RMS, clean the recording; you can’t master hiss away without dulling the voice.
Landing outside the RMS window. Too quiet (below −23) or too loud (above −18) both bounce — aim for the middle of the range.
Delivering the wrong format. Final files must be mono 192 kbps MP3 at 44.1 kHz, one per chapter.

Do it free, in the browser

You don’t need expensive software to pass ACX. Loopin’s online mastering runs a real chain — corrective EQ, multiband compression, saturation and a true-peak limiter with loudness and level control — entirely in your browser. Nothing is uploaded, it’s free, and it takes minutes per chapter.

Clean the recording, master each chapter into the ACX window, export mono 192 kbps MP3 at 44.1 kHz, and your audiobook is ready to submit.

Frequently asked questions

What are the ACX audio requirements?

Every file must measure RMS between −23 dB and −18 dB, have peak values no higher than −3 dB, and a noise floor no higher than −60 dB RMS. Final files are mono 192 kbps MP3 at 44.1 kHz, one per chapter.

Why did ACX reject my audiobook?

Usually one of the three numbers is out: RMS outside the −23 to −18 dB window, peaks above −3 dB, or a noise floor above −60 dB RMS. Fix the noise floor at the recording stage and set RMS and peaks in mastering.

What format does ACX require?

Mono MP3 at 192 kbps and 44.1 kHz, with one file per chapter. Master from a WAV, then convert the finished master to that MP3 format for delivery.

How do I get my noise floor below −60 dB RMS?

Record in a quiet space and apply noise reduction to the raw recording. Mastering acts on the whole file and can’t remove hiss without dulling the voice, so clean the source first.

Can I master an audiobook for ACX for free?

Yes. Loopin’s online mastering is free, needs no account and runs in your browser — master each cleaned chapter into the ACX window, then export mono 192 kbps MP3 at 44.1 kHz.