Master Your Audiobook for ACX Submission
ACX is the production and distribution platform for Audible, Amazon, and Apple Books. ACX operates the largest audiobook marketplace in the world and enforces the most detailed technical specifications of any audiobook distributor. Every chapter file submitted to ACX must pass 8 automated quality checks[1] before acceptance. ChapterPass masters audiobook chapter files to meet all ACX requirements automatically, producing MP3 files ready for submission.
What Does ACX Check? (ChapterPass Handles These)
What volume levels does ACX require?
ACX requires each chapter to have an RMS loudness between -23 and -18 dBFS. ChapterPass measures your audio and adjusts gain to land within this range, ensuring consistent loudness across all chapters.
What is the ACX peak level requirement?
True peak must stay below -3 dBFS. This accounts for inter-sample peaks that occur between digital samples. ChapterPass automatically limits true peaks to ensure compliance.
What noise floor does ACX require?
ACX requires the noise floor to be below -60 dBFS. ChapterPass manages noise in silent sections, producing a natural-sounding floor that meets the specification.
What file format does ACX require?
Mono MP3 at 44100 Hz sample rate with constant bit rate (CBR) of 192 kbps or higher. ChapterPass converts any supported input format (WAV, MP3, FLAC, AIFF, M4A) to this exact specification.
What silence does ACX require at the start and end?
ACX requires 0.5 to 1 second of room-tone silence at the head and 1 to 5 seconds at the tail of each chapter. ChapterPass adds and verifies silence padding within these ranges.
How does ChapterPass verify compliance?
Every output file is checked against all 8 ACX technical checkpoints before you can download it. If any measurement falls outside spec, the engine re-processes with tightened parameters (up to 3 correction cycles).
What Do You Need to Handle Yourself?
Clean recording
No clicks, pops, mouth noise, or echo. These are recording issues that need to be fixed in your editing software before uploading.
Consistent narration
Same tone, pacing, and volume feel across all chapters. ACX's reviewers listen for jarring differences between chapters.
Opening and closing credits
Title, author, and narrator announced at the start. Closing credits at the end. You need to record these yourself.
One chapter per file
Each file you upload should be one chapter or section. ACX requires chapters to be submitted as separate files.
Narration matches manuscript
Your spoken narration needs to match the written book. ACX's QA team checks for accuracy.
Human narration
ACX requires human narration. The only exception is ACX's Voice Replica programme (beta since September 2024), which allows narrators to create and monetize AI replicas of their own voice with explicit consent.[2]
What Are the ACX Technical Specifications?
| Requirement | ACX Spec | ChapterPass |
|---|---|---|
| RMS Loudness | -23 to -18 dBFS | ✓ |
| True Peak | Below -3 dBFS | ✓ |
| Noise Floor | Below -60 dBFS | ✓ |
| Sample Rate | 44100 Hz | ✓ |
| Channels | Mono | ✓ |
| Format | MP3 CBR 192+ kbps | ✓ |
| Head Silence | 0.5 to 1 second | ✓ |
| Tail Silence | 1 to 5 seconds | ✓ |
What Does Each ACX Requirement Mean in Detail?
What RMS level does ACX require and why does it matter?
ACX requires every chapter file to measure between -23 and -18 dBFS RMS[1]. RMS (Root Mean Square) represents the average loudness of your audio over time, not just the loudest moments. This range ensures listeners hear a consistent volume across your entire audiobook without needing to adjust their player.
In practice: If your chapter measures -30 dBFS RMS, it sounds noticeably quieter than other audiobooks on Audible. If it measures -15 dBFS RMS, it sounds uncomfortably loud. ACX enforces this window so every title on the platform sounds roughly the same volume.
How ChapterPass handles it: ChapterPass measures the RMS loudness of your entire chapter, then applies precise gain adjustment to bring it within the -23 to -18 dBFS range. If chapters vary in loudness, each one is adjusted individually so they all land consistently within spec.
Common mistakes: Raw recordings often come in too quiet, well below -23 dBFS, because narrators record at conservative levels to avoid clipping. On the other end, over-compressed audio can push RMS above -18 dBFS, making the narration sound squashed and fatiguing.
What is the ACX true peak requirement?
ACX requires all true peaks to remain below -3 dBFS[1]. True peak measurement accounts for the actual waveform between digital samples, not just the sample values themselves. Peaks that exceed this threshold cause distortion when the audio is decoded by listeners' devices and streaming apps.
In practice: A file can show sample peaks at -1 dBFS and appear fine in your DAW, but the reconstructed analog waveform between those samples may actually exceed 0 dBFS. These inter-sample peaks cause audible crackling and distortion on playback, especially through Bluetooth speakers and earbuds.
How ChapterPass handles it: ChapterPass detects and corrects both sample peaks and inter-sample peaks, then verifies the encoded MP3 output to confirm compliance.
Common mistakes: Normalising audio to 0 dBFS is the most frequent cause of true peak failures. Standard peak normalisation only looks at sample values and misses inter-sample peaks entirely. Many narrators also apply a limiter set to -1 dBFS or -0.5 dBFS, which still leaves peaks above the -3 dBFS threshold.
What noise floor does ACX require for audiobooks?
ACX requires the noise floor to measure below -60 dBFS[1]. The noise floor is the level of background noise present during silent sections of your recording, the pauses between sentences and paragraphs. Anything above -60 dBFS means there is too much audible background noise in your file.
In practice: ACX measures noise floor during the quietest portions of your chapter. If your recording room has an air conditioner, computer fan, or street noise, those sounds become clearly audible during pauses. Listeners notice background noise most when the narrator stops speaking.
How ChapterPass handles it: ChapterPass reduces noise only during silent sections, leaving your narration untouched. The result is a natural-sounding floor that measures well below -60 dBFS without the metallic artefacts that aggressive noise reduction can introduce.
Common mistakes: Recording in untreated rooms is the primary cause of noise floor failures. HVAC systems, refrigerators, and computer fans produce steady low-frequency noise that sits right around -50 to -55 dBFS. Aggressive noise reduction plugins can fix the measurement but introduce metallic artefacts that ACX reviewers will catch.
What sample rate does ACX require?
ACX requires all files to be 44100 Hz (44.1 kHz) sample rate[1]. This is the same sample rate used for audio CDs and is the standard for consumer audio distribution. Files at any other sample rate, whether higher or lower, will be rejected by ACX's automated checks.
In practice: Many audio interfaces and DAWs default to 48000 Hz because that is the standard for video production. If you record at 48 kHz and export without converting, your file will fail the sample rate check even if everything else is perfect.
How ChapterPass handles it: ChapterPass detects the sample rate of your input file and resamples to 44100 Hz if needed. The resampling uses a high-quality algorithm that preserves audio fidelity, so there is no audible difference in the output.
Common mistakes: Recording at 48 kHz and forgetting to convert before submission is extremely common. Some narrators record at 96 kHz thinking higher is better, but ACX specifically requires 44100 Hz. Submitting at the wrong rate does not just fail the check. It can also cause subtle pitch shifts if misinterpreted by the decoder.
Does ACX require mono or stereo audiobook files?
ACX requires all audiobook files to be mono (single channel)[1]. Stereo files contain two channels of audio, which doubles the file size without any benefit for spoken-word content. Audiobooks are voice recordings, and there is no meaningful spatial information that stereo would preserve.
In practice: Submitting a stereo file is an instant rejection. Even if your narration sounds identical in both channels, ACX's automated system checks the channel count and rejects anything that is not mono. This is one of the simplest requirements to meet but one of the most commonly overlooked.
How ChapterPass handles it: ChapterPass automatically detects stereo files and downmixes them to mono. The downmix sums both channels and adjusts the gain to prevent clipping, producing a clean mono file that sounds identical to your original recording.
Common mistakes: Many DAWs export stereo by default, even when you recorded with a single mono microphone. The result is a stereo file with identical left and right channels, which wastes space and fails ACX's channel check. Always check your export settings, or let ChapterPass handle the conversion.
What audio format does ACX accept for submissions?
ACX requires MP3 files encoded at a constant bit rate (CBR) of 192 kbps or higher[1]. Variable bit rate (VBR) encoding is not accepted, and neither are uncompressed formats like WAV or AIFF. The MP3 must be CBR, not just any MP3.
In practice: The CBR requirement exists because Audible's delivery platform needs predictable file sizes and consistent streaming behavior. VBR files can cause playback issues on some devices and make it harder for the platform to calculate chapter timestamps accurately.
How ChapterPass handles it: ChapterPass outputs every file as MP3 CBR at 192 kbps, exactly matching the ACX specification. It accepts WAV, MP3, FLAC, AIFF, and M4A as inputs, so you can upload from virtually any recording workflow.
Common mistakes: Submitting WAV files directly is a common error from narrators who think uncompressed means better. Using VBR MP3 encoding is another frequent issue, as many encoders default to VBR for better quality-to-size ratios. ACX requires CBR specifically, and VBR files will be rejected regardless of the average bit rate.
How much silence does ACX require at the beginning of each chapter?
ACX requires between 0.5 and 1 second of room-tone silence at the head (beginning) of each chapter file[1]. This is not digital silence. It must be the natural ambient sound of your recording space. The silence provides a brief buffer before the narration begins so listeners do not miss the first word.
In practice: Head silence gives the listener a moment to settle in when a new chapter starts. It also prevents the first syllable from being clipped by players that take a fraction of a second to begin audio output. ACX checks both the duration and the character of the silence.
How ChapterPass handles it: ChapterPass adds 1.0 second of room-tone silence at the head of each chapter, well within the 0.5 to 1 second requirement. The silence is generated to match the noise profile of your recording, not as absolute digital zero.
Common mistakes: Using absolute digital silence (all zeros) instead of room tone is the most common mistake. ACX flags digital silence because it sounds unnatural. There is an abrupt jump from complete silence to the ambient noise of your room when narration begins. Some narrators also trim too close to the first word, leaving less than 0.5 seconds.
How much silence does ACX require at the end of each chapter?
ACX requires between 1 and 5 seconds of room-tone silence at the tail (end) of each chapter file[1]. Like head silence, this must be natural room tone, not digital silence. The tail silence provides a natural fade-out that lets the listener process the chapter ending before the next one begins.
In practice: Tail silence prevents the last word of a chapter from being cut off and gives Audible's player time to transition cleanly to the next chapter. Too little silence makes the ending feel abrupt. Too much silence wastes the listener's time and can feel like a playback error.
How ChapterPass handles it: ChapterPass adds 1.5 seconds of room-tone silence at the tail of each chapter, fitting comfortably within the 1 to 5 second requirement. This duration provides a natural ending without unnecessary dead air.
Common mistakes: Cutting the audio immediately after the last word is the most common tail silence failure. Narrators often trim aggressively to keep files short, but ACX needs at least 1 second of room tone. Others leave 10 or more seconds of silence, which exceeds the 5-second maximum and also fails the check.
What Makes ACX Different From Other Audiobook Platforms?
ACX is the exclusive production portal for Audible, Amazon, and Apple Books. ACX publishes the strictest and most detailed audio specifications of any audiobook distribution platform. Where other platforms leave some specs unstated, ACX enforces precise measurements on every file through automated quality checks.
ACX offers two royalty structures: exclusive distribution at 40% royalty share (your audiobook is available only through Audible, Amazon, and Apple Books) and non-exclusive distribution at 25% royalty share (you retain the right to distribute through other platforms simultaneously). ACX is rolling out a new royalty model to select creators with different rates and calculation methods. Check ACX's current terms before committing to a distribution option. Many authors choose non-exclusive ACX distribution so they can also distribute through wide platforms.
ACX requires mono audio only. This is the strictest channel requirement of any major platform. Findaway Voices accepts both mono and stereo[3]. Authors Republic accepts both with a consistency requirement. Google Play Books accepts both mono and stereo across multiple formats.
Can You Use ACX-Mastered Files on Other Platforms?
Yes. ACX specifications are the strictest of any audiobook distribution platform, so files mastered to ACX standards meet or exceed all requirements from other distributors. ChapterPass produces mono MP3 files at 192 kbps CBR with loudness, peak, noise floor, and silence padding that satisfy ACX, Findaway Voices, Authors Republic, and Google Play Books. Narrators and authors who master their audiobook with ChapterPass can submit the same files to every major platform without reprocessing. ChapterPass is platform-agnostic: one set of mastered files works everywhere. See the Findaway Voices, Authors Republic, and Google Play Books pages for platform-specific details. See the audio mastering glossary for definitions of technical terms.
Ready to master your audiobook for ACX?
ChapterPass handles the technical specs. You handle the recording.