The audiobook industry is experiencing unprecedented, exponential growth. Millions of readers are shifting from physical pages and e-readers to listening on the go—whether during daily commutes, workouts, or quiet evenings at home. For indie authors and publishers, this shift presents a massive opportunity to reach brand-new audiences and open a steady stream of passive royalties.
However, entering the world of audiobooks requires clearing one major hurdle: meeting strict industry distribution standards. Audible, Amazon, and iTunes distribute their audiobooks through ACX (Audiobook Creation Exchange), and ACX maintains rigorous technical and structural requirements. If an audio file misses a target by even a fraction of a decibel, the automated QA system will reject the upload.
To ensure your audiobook glides through the approval process on your first attempt, here are the essential rules you need to follow.
Below I have listed all of the various rules that must be followed to get a quick and easy pass from ACX. I did my first audiobook of a short story entitled "Writer's Block" and it got an immediate pass and was accepted in less than a week. So it may seem like a lot of rules, but learning how to do each one of them will save you a lot of time and disappointment.
1. MP3 Format Required
All final audio files submitted to ACX must be encoded in MP3 format. While WAV or FLAC files are preferred for master studio recordings, ACX requires MP3s for consumer distribution.
2. Mandatory Chapter & Section Separation
Audacity is the free program that I used to record the audiobooks
Audiobooks cannot be uploaded as one giant audio file. You must upload your manuscript broken down into individual files for each chapter, plus separate dedicated files for your Opening Credits and Closing Credits.
3. Handling Unchaptered Manuscripts
If your book does not have formal chapters (e.g., non-fiction titles, essays, or long-form prose), decide on a logical structure ahead of time. Break the audio into roughly equal parts, sections, or numbered segments before you begin recording so your file uploads match your navigation structure.
4. File Size & Length Limits
This is my first audiobook of a short story. Check it out!
Each individual audio file must be no longer than 120 minutes in running time and must be under 170MB in total file size. If a chapter runs longer than two hours, you must split it into "Part 1" and "Part 2" files.
This will probably not come into play at all if you are recording a series of chapters. If, however, you are recording a short story or novella, you will need to break the story into sections. You can call them Part 1, Part 2, etc., instead of chapters.
5. Bit Rate: Constant 192 kbps or Higher
I learned most of what I came to know from Mike Teaches Audacity. Click to see his YouTube Channel
Your MP3 files must have a bit rate of 192 kbps or higher, rendered with a Constant Bit Rate (CBR)—variable bit rates (VBR) will be automatically flagged and rejected.
Audacity Workflow: When exporting your project (File > Export > Export as MP3), set Bit Rate Mode to "Constant". In the quality drop-down menu, choose 192 kbps (or 256/320 kbps). Ensure your sample rate is set to 44.1 kHz.
6. Peak Level: -3 dB Maximum
No part of your audio signal can exceed -3.0 dB (or -3.08 dB). Keeping a peak cap prevents digital clipping and distortion on mobile devices and headphones.
Audacity Tip: You can use the Limiter effect or Normalize tool set to -3.0 dB to pull down transient loud spikes without altering the rest of your vocal dynamics.
7. RMS Level: Average Loudness Standard (-23 dB to -18 dB)
ACX requires an RMS (Root Mean Square) average loudness between -23 dB and -18 dB. Aiming around -19 dB to -20 dB RMS guarantees that listeners won't have to constantly adjust their volume between quiet and loud passages.
8. Noise Floor: -60 dB or Lower
If you have a small closet, you have a sound studio.
The "noise floor" measures the background noise (room tone, computer fans, electrical hums) present when you aren't speaking. ACX strictly mandates a maximum noise floor of -60 dB.
Note: A lower negative number means a quieter room (e.g., -83.84 dB is virtually dead silent and will pass with flying colors, whereas -55 dB will trigger a rejection).
If you have a small closet, you have a sound studio. I started in my closet, but now I just record anywhere in my home because I live alone in a very quiet neighborhood, under a canopy of trees. Having a proper studio turned out to be much 'easier' than I originally thought!
9. Distinct Structural File Structure
To confirm your upload order, ACX requires every section uploaded as an isolated file:
Opening Credits (1 file)
Individual Chapters / Sections (1 file per chapter)
Closing Credits (1 file)
10. Audiobook Cover Art Specifications
Audiobook artwork differs from standard eBook or print covers:
Dimensions: Must be a perfect 1:1 square ratio.
Resolution: Minimum size of 2400 x 2400 pixels (maximum 3000 x 3000 pixels).
Format:JPEG or TIFF images only, formatted in RGB color space.
11. Exact Audio-to-Text Title Matching
ACX QA engineers enforce a strict exact match rule between your spoken audio titles and your manuscript titles.
If your spoken Opening Credits say "Part One", your onscreen text chapter heading cannot say "Part 1". Keep the naming convention uniform across your script, file titles, and spoken narration.
12. Encoding Summary Checklist
Double-check your master export settings before uploading:
Format: MP3
Bit Rate: Constant (CBR) at 192 kbps or higher
Sample Rate: 44.1 kHz
Channel Mode: Mono or Stereo (Mono is recommended for standard single-narrator books).
13. Proper Spacing and Room Tone
Every audio file must include brief "breathing room" of natural background noise (room tone):
Beginning:0.5 to 1 second of room tone before the first word.
End:1 to 5 seconds of room tone at the end of each file.
Caution: Do not insert digital "dead silence" (flat line); ACX looks for clean, recorded room tone.
14. Final Checklist Reminder: Bookend Credits
Never merge your credits into Chapter 1 or the final chapter. The Opening Credits ("This is [Title], written by [Author], narrated by [Narrator]") and Closing Credits ("This has been [Title]...") must always exist as independent, standalone MP3 files.
