Getting the audio quality right is only half the battle. Your audiobook also needs to be formatted correctly: files named properly, chapters structured with appropriate silence, credits recorded as separate files, and a retail sample designated. Formatting mistakes don't fail the automated technical check. They fail the human quality review, which is harder to debug because the rejection feedback is less specific.
This guide covers every formatting requirement for audiobook chapters. For the technical specifications (loudness, peaks, noise floor, format), see the ACX audio requirements guide. For the complete pre-submission process, use the preparation guide.
What Are the File Naming Conventions?
ACX File Naming
ACX allows you to arrange files in order during upload regardless of filename. However, disciplined naming prevents mistakes during production.[1]
Recommended convention:
00_Opening_Credits.mp3
01_Introduction.mp3
02_Chapter-01.mp3
03_Chapter-02.mp3
...
32_Chapter-30.mp3
33_Closing_Credits.mp3
Key rules:
- Zero-padded numbers (01, 02, ... 10, 11). Without zero-padding, file systems sort Chapter 1, Chapter 10, Chapter 11, ... Chapter 2.
- Underscores or hyphens, not spaces. Some upload systems handle spaces inconsistently.
- Consistency. Don't mix conventions within a project.
- No special characters. Avoid apostrophes, ampersands, colons.
Other Distributors
Findaway Voices and Authors Republic have similar flexibility. Sequential naming with zero-padded chapter numbers works for every distributor.
What Goes in Opening Credits?
Opening credits are a separate file from Chapter 1 and must meet all the same technical specifications as chapter files.[1]
Required Content
At minimum, opening credits must include:
- Book title (and subtitle, if applicable)
- Author name: "Written by [Author Name]"
- Narrator name: "Narrated by [Narrator Name]"
Optional Content
- Publisher name
- Copyright notice: "Copyright [Year] [Author/Publisher]"
- Series information: "Book [Number] of the [Series Name] series"
Format
Keep opening credits brief (30 seconds to 2 minutes), mastered to the same RMS target as all other files, with correct silence padding.
What Goes in Closing Credits?
Closing credits are the last file in the audiobook, with all the same technical requirements.
Required Content
At minimum: an end statement. "End of [Book Title]" or "The end."
What to Avoid
- Extended sales pitches. A brief mention of the author's next book is fine. An extended catalogue is not.
- URLs read aloud. Listeners can't click links. Reading URLs sounds awkward and dates the content.
- Unlicensed music or sound effects. This is a copyright issue that can result in removal.
How Should You Structure Each Chapter?
Head Silence
Every chapter starts with 0.5 to 1 second of silence before speech begins.[1] This silence serves two purposes:
- Player compatibility. Some audiobook players have a brief buffering delay. Head silence prevents the first syllable from being cut off.
- Listener experience. An abrupt start is jarring, especially when chapters auto-advance.
This should be natural room tone, not digital silence (absolute zero). Digital silence creates an audible acoustic transition when speech begins. The silence padding guide covers room tone, crossfade techniques, and why digital silence causes problems.
Section Headers
Each file should contain the section header if it's in the book, such as "Chapter 1" at the start of Chapter 1's audio.[1] Be consistent. A listener may think content is missing if most headers are read but some aren't.
Section Breaks Within Chapters
Many books have section breaks (extra whitespace, asterisks in print). In audio, represent these with 1 to 3 seconds of silence, longer than a paragraph pause, shorter than a chapter break. 2 seconds is a comfortable default. Use natural room tone.
Tail Silence
Every chapter ends with 1 to 5 seconds of silence after the last word.[1] Standard recommendation: 3 seconds of tail silence for all chapters. As with head silence, use natural room tone.
What Is the Retail Sample?
ACX requires a retail audio sample, the free preview that potential buyers listen to on the Audible product page. It must be between 1 and 5 minutes long, start with narration (not opening credits or music), and must not include explicit content.[1]
How Do You Choose the Best Sample Chapter?
The retail sample is your audiobook's sales pitch. Choose a chapter that:
- Hooks early. Action, compelling dialogue, or an intriguing premise in the first 30 seconds.
- Showcases your best narration. Strong, engaging delivery that represents full audiobook quality.
- Is self-contained enough to follow. No extensive prior context required.
- Demonstrates character voices (if applicable).
Avoid very short chapters (feels insubstantial), chapters with major spoilers, and chapters heavy on internal monologue.
How Do You Handle Long Chapters and File Splitting?
ACX has a maximum file length of 120 minutes per file.[1] If a single chapter exceeds the limit, split it into multiple files with clear labelling (e.g. "Chapter 2 continued") so listeners can navigate.
How Do You Format Multi-Narrator Audiobooks?
File Consistency
All narrators' files must meet identical technical specs. Process all files through the same mastering chain to ensure consistent loudness, noise floor, and tonal character.
Recording Environment Matching
Narrators should record in acoustically similar environments. A narrator in a professional studio and another in an untreated bedroom will sound noticeably different regardless of mastering.
How Do You Handle Special Sections?
Parts
If the book has parts, either record a brief "Part One" announcement as a separate file, or announce it at the start of the first chapter in each part. Be consistent.
Appendices and Author Notes
Record each as a separate file following the same technical specs. Place after the final chapter but before closing credits.
Footnotes and Endnotes
Options: integrate into text as parenthetical, collect at chapter end, or omit if not essential. Discuss with the rights holder.
What Formatting Mistakes Cause ACX Rejection?
Technical spec failures (RMS, peak, noise floor) get caught by ACX's automated check and come back with a specific error. Formatting mistakes are different. They reach a human reviewer and come back with vague feedback like "inconsistent audio quality" or "production issues." That second rejection is slower to resolve because the problem is harder to pinpoint.
These are the formatting mistakes that most commonly trigger human review rejection.
Missing or Incorrect Credits
ACX requires opening and closing credits as separate files. If they're missing, the QA reviewer flags the submission immediately.[1] The opening credits must include the book title, author name, and narrator name. Credits embedded at the start of Chapter 1 rather than in a dedicated file are also a failure point, because ACX treats every file as a discrete section with a defined role.
Missing Section Headers
If your book has chapters and you don't announce them on audio ("Chapter One," "Part Two," etc.), ACX's human review may flag the omission. The guidance from ACX is that section headers help listeners navigate without looking at their player.[1] Be consistent: if you announce headers in most chapters but skip a few, reviewers will notice. Either announce every header or announce none.
Mixing Mono and Stereo Files
You must choose one format and use it for every file in the submission. Mixing mono and stereo files in the same audiobook causes rejection.[1] This usually happens when narrators record new chapters in a different DAW project from the original sessions, or when pickup recordings use different export settings. Check all files before upload.
Inconsistent Sound Across Files
"Inconsistent audio quality" is the most common vague rejection. It covers a wide range: one chapter with noticeably more room noise than others, a pickup session that sounds tonally different from the original recording, or a chapter with a clearly different microphone. These are not technical spec failures. They're production consistency issues that ACX's review catches subjectively.
The fix is prevention: record all sessions in the same room with the same microphone and interface settings, and process all files through an identical mastering chain.
Retail Sample Issues
The retail sample must start with narration, not music or opening credits.[1] Samples that open with credits or silence before narration can be flagged. The sample also must not contain explicit content even if the book itself is adult material.
Digital Silence Instead of Room Tone
This is subtle but noticeable on careful listening. Digital silence (all samples at zero) creates an audible switch when narration begins or ends, because the recording environment's acoustic character disappears abruptly. ACX's human reviewers listen for this, particularly at chapter starts. Use room tone, not digital silence.
How Do Platform Requirements Differ Between ACX, Findaway, and Google Play?
ACX, Findaway Voices (now Voices by INaudio), and Google Play Books all accept audiobooks, but their formatting requirements differ in ways that matter if you're distributing across platforms.
File Format
All three platforms require separate files per chapter or section. ACX requires mono MP3 at 192 kbps CBR. Findaway/INaudio accepts both mono and stereo files. Google Play Books accepts M4A with AAC encoding or MP3, and supports stereo.
If you're distributing to all three platforms, master to ACX's mono MP3 specification. Those files pass Findaway and Google Play without modification.
Chapter Length Limits
ACX imposes a 120-minute maximum per file.[1] Findaway imposes a 78-minute maximum per file. Google Play Books does not publish a hard per-file duration limit, but accepts standard chapter lengths without issue.
If you have chapters longer than 78 minutes (rare for most audiobooks, but possible for academic or reference titles), you'll need to split those chapters for Findaway even if they pass ACX.
Opening and Closing Credits
All three platforms require opening and closing credits as separate files. The content requirements are similar: book title, author, narrator. The retail sample requirement (1 to 5 minutes, starting with narration) is consistent across ACX and Findaway. Google Play Books requests a sample but has more flexibility in how it's sourced.
Retail Sample
ACX requires the retail sample to be designated during upload and must start with narration. Findaway similarly requires a 1 to 5 minute retail sample starting with narration. Google Play generates its own sample from the uploaded files rather than requiring a separately designated sample file.
The Practical Rule
Master all files to ACX specifications. This handles loudness, peak, noise floor, format, and silence across all three platforms. The only platform-specific consideration is Findaway's 78-minute chapter limit, which affects a small minority of titles.
Quality Checklist
Use alongside the preparation guide:[1]
- All files named consistently with zero-padded numbers
- Opening credits recorded as a separate file
- Closing credits recorded as a separate file
- Head silence: 0.5 to 1 second on every file (room tone)
- Tail silence: 1 to 5 seconds on every file (room tone)
- Section headers read aloud consistently
- Section breaks within chapters: consistent duration
- Retail sample chapter identified (1 to 5 minutes)
- Files in correct sequential order
- No special characters in filenames