Audiobooks are one of the fastest-growing formats in digital media, and platforms like Audible, Amazon, and iTunes rely on ACX (Audiobook Creation Exchange) standards to keep quality consistent. If you’re narrating or producing an audiobook for release on these platforms, your audio must pass ACX technical requirements — otherwise, it will be rejected.
But ACX mastering isn’t just a checklist. It’s a way of ensuring your audiobook sounds clear, consistent, and comfortable for long listening sessions. At Moreish Studios, we’ve worked on spoken-word, narrative, and long-form audio productions where meeting these standards is essential for approval and listener experience.
Let’s break down what ACX mastering is, what it requires, and why it matters.
ACX mastering is the final stage of preparing audiobook audio so it meets the strict technical criteria required by Audible and other distributors. These specs ensure:
ACX publishes very clear requirements (see their official guidelines here:
ACX Technical Requirements).
For most audiobook projects, mastering includes:
Even excellent narration can be rejected if the mastering doesn’t meet spec.
To be approved, all audiobook files must meet:
This ensures the narrator is loud enough without being fatiguing.
This prevents distortion.
Meaning the room must be quiet enough to avoid hiss, hum, rumble, or background noise.
No chapter should sound like it was recorded in a different room or on a different day.
These requirements are designed with the end listener in mind — especially those listening through headphones for hours at a time.
For a deeper technical breakdown of noise floors, RMS, and signal-to-noise ratio, this resource provides a great overview:
Sound On Sound – Understanding Noise & Loudness.
The most practical reason: ACX will reject files that don’t meet spec. Many DIY audiobook creators end up re-recording entire chapters — or even the full book — because of poor noise floors or inconsistent levels.
Audiobooks are often consumed for hours at a time. Improperly mastered audio leads to:
Professional mastering ensures a smooth, warm, and easy-to-listen-to experience.
Your audience expects commercial-grade quality, especially if your book sits next to major publishers. A consistent tone and level across chapters helps your audiobook feel polished and trustworthy.
Fixing a rejected audiobook is far more complicated (and expensive) than simply mastering it correctly from the start.
Audiobook mastering is very different from music mastering. Spoken word demands:
Even a well-treated room can miss ACX specs due to HVAC noise, mic noise, computer fans, or inconsistent narration levels.
This is why many creators choose to record in a professional recording studio, or have their raw audio mastered by specialists who work with ACX guidelines every day.
Our audiobook mastering process follows the exact ACX requirements, using:
We also format the files to ACX spec (44.1 kHz / 16-bit WAV + MP3 deliverables), and create the required opening/closing room tone.
Our audiobook workflow blends engineering precision with a respect for the natural dynamics of spoken performance.
For authors, voice actors, and narrators who need recording support, we also offer production and session guidance to help capture clean, consistent vocals from the start.
ACX mastering isn’t just bureaucracy — it’s the difference between an audiobook that sounds professional and one that listeners struggle to finish. It ensures your story, message, or performance is delivered with clarity, comfort, and credibility.
Whether you’re narrating your first audiobook or producing titles regularly, understanding (and meeting) ACX standards is essential for getting your work onto Audible and into the hands of listeners worldwide.
If you want to ensure your audiobook passes ACX’s requirements the first time — and sounds great doing it — Moreish Studios is here to help.
