Understanding ACX File Requirements

Before you start labeling and organizing your audiobook files, it’s essential to know exactly what ACX expects. The Audiobook Creation Exchange (ACX) enforces strict technical specifications to ensure consistent playback quality across all retail partners. Failure to adhere to these can result in immediate rejection or lengthy revision cycles.

ACX requires audio files to meet the following standards:

  • Sample rate: 44.1 kHz (44,100 samples per second). You must not use 48 kHz or 96 kHz.
  • Bit depth: 16-bit. 24-bit recordings are allowed during production but must be dithered down to 16-bit for the final submission.
  • Format: MP3 with a constant bitrate of 192 kbps, or WAV (uncompressed PCM). The vast majority of authors and narrators use WAV for highest quality and MP3 for final upload if preferred. Important: Do not use variable bitrate (VBR) – ACX explicitly requires constant bitrate (CBR) MP3s.
  • Sample format: For WAV files, use 44100 Hz, 16-bit, mono. (Stereo files are accepted but will be downmixed to mono on playback, so it’s cleaner to submit mono.)
  • Volume: RMS level should be between -23 dB and -18 dB, with peaks no higher than -3 dB. ACX provides a free Audio Lab tool to check these metrics.
  • Noise floor: Background noise must be below -60 dB. Any residual hiss or hum will flag the file.

Always consult the official ACX Technical Requirements page for the latest specifications, as policies can change. Checking this page before starting a project will save you from having to re-export dozens of files.

Labeling Your Audiobook Files

Consistent, clear labeling is the single most effective way to avoid ACX rejection due to file naming issues. ACX’s upload system automatically sorts files alphabetically by filename; if your naming convention does not reflect the correct chapter order, the system will likely reorder them incorrectly, causing your audiobook to be out of sequence.

Naming Convention Best Practices

Adopt a system that is both human‑readable and machine‑sortable. The following rules apply:

  • Include the book title (or a short abbreviation) at the beginning.
  • Follow with the chapter or section number, zero-padded to two digits (e.g., 01, 02, …, 99). This ensures correct alphabetical sorting beyond chapter 9.
  • Use underscores (_) or hyphens (-) between elements – avoid spaces because some operating systems and cloud storage tools may handle spaces inconsistently.
  • Avoid special characters (! @ # $ % ^ & * ( )) – they can cause errors during upload or metadata extraction.
  • Do not use the word “Chapter” if it makes the filename overly long; a brief abbreviation like Ch01 or 01 is fine.

Good examples:

  • The_Hidden_Reef_Ch01.mp3
  • HiddenReef_02.wav
  • THR_Chapter_03.mp3

Bad examples to avoid:

  • Chapter 1.mp3 (space, no zero padding – will sort after Chapter 10)
  • THR_Ch_1_edit_final_v2.mp3 (too long, multiple underscores, ambiguous versioning)
  • Hidden!Reef!Ch1.mp3 (exclamation marks are unsupported)

Naming for Multi-Disc or Parted Audiobooks

If your book is long and broken into parts (e.g., Part 1, Part 2), include the part number before the chapter number, again zero‑padded. For example:

  • HiddenReef_Pt01_Ch01.mp3
  • HiddenReef_Pt01_Ch02.mp3
  • HiddenReef_Pt02_Ch01.mp3

This structure keeps everything in logical order when sorted alphabetically.

Organizing Files for Submission

Good organization goes beyond naming. Even before you upload, a clean folder structure and a master inventory list will prevent missing chapters, duplicate files, or incorrect versions.

Folder Hierarchy

Create a dedicated project folder for each audiobook. Inside it, use the following subfolders:

  • 01_Raw – Your original, unprocessed recordings (optional but good for backup).
  • 02_Edited – Files after noise reduction, retakes, and compression.
  • 03_Final – Files that have been mastered, volume adjusted, and meet ACX specs. These are the files you will upload.
  • 04_Retainers – If you have retainers or proof copies, store them here.

Label each folder with a prefix number to enforce order, and within the Final folder, keep only the exact files you intend to submit. Nothing else should reside there.

Master Spreadsheet

Create a simple spreadsheet (Google Sheets or Excel) with columns: File Name, Chapter Number, Duration, File Size, Checked (Yes/No), Notes. Populate it as you produce each track. This spreadsheet becomes your checklist at upload time. Verify that every file in the Final folder appears exactly once in the spreadsheet and that the order matches the book’s table of contents.

Metadata & ID3 Tags

ACX reads ID3 tags (in MP3 files) or equivalent metadata (in WAV files) to populate retail listings. While WAV files have limited metadata support, MP3 files allow rich tags. Many authors prefer to submit MP3 to embed metadata directly. Critical tags include:

  • Title: “Chapter 1 – The Beginning” (follow the exact chapter title as it appears in the manuscript or as agreed with the rights holder).
  • Artist: The narrator’s name (if you are the narrator) or the author’s name (if you are the author and not the narrator – check ACX’s guidance).
  • Album: The full book title as it will appear on retail stores.
  • Track Number: The chapter number (e.g., 1 / total chapters).
  • Year: The year of production or first publication.
  • Genre: Select the most appropriate genre from ACX’s list (e.g., “Fiction,” “Self-Help”).
  • Comment: Some authors add a “© [Year]” note, but this is optional.

Use a tag editor like MP3tag to batch update ID3 tags across all files. This tool lets you set track numbers incrementally, create patterns, and export/import tag data via CSV. For WAV files, you can embed basic metadata using software like Audacity (via the “Metadata Editor”) or a dedicated BWF (Broadcast Wave) tagger.

Tip: After tagging, verify a few random files on your computer. Right-click the file, go to Properties > Details (Windows) or Get Info (Mac), and ensure the tags are correct. Inconsistent or missing metadata can cause ACX’s system to reject the upload.

Cover Art Requirements

Properly labeled files also include the correct cover art file. ACX mandates that the cover art be a separate JPEG image, not embedded in the audio file. Specifications:

  • Minimum resolution: 2400 x 2400 pixels.
  • Maximum resolution: 3000 x 3000 pixels.
  • Format: JPEG only, RGB color space.
  • File size: No larger than 50 MB.
  • Text on cover: Must be fully readable, no blurriness, and the author and title must appear exactly as they will on the retail page.

Name the cover art file using the same convention as your audio files (e.g., HiddenReef_Cover.jpg). Do not embed the image in the audio file – ACX will ignore it and rely on the separate upload.

Step-by-Step Workflow for ACX Submission

Follow this checklist to ensure nothing is missed:

  1. Record and edit – Produce raw audio, then edit, noise reduce, and master each chapter.
  2. Export – Export each chapter as a 44.1 kHz, 16-bit, mono WAV (or MP3 CBR 192 kbps). Use ACX’s Audio Lab to verify volume and noise floor.
  3. Name files – Apply the naming convention described above to every file in your Final folder.
  4. Tag files – Use MP3tag or similar to embed ID3 tags. Ensure track numbers are sequential and match the chapter list.
  5. Prepare cover art – Save a 2400x2400 JPEG named with the book title.
  6. Create a spreadsheet – List every file, its duration, and check for completeness.
  7. Do a final audit – Play the first and last chapter to confirm correct file naming and ordering. Even a single mislabeled file can cause ACX’s system to reject the entire submission.
  8. Upload – Log in to ACX, create a new title, and upload each file individually (or via the bulk upload tool if available). Confirm the order shown in ACX matches your spreadsheet.
  9. Submit for review – After ACX processes the files (usually a few minutes), review the sample and then submit for final quality assurance.

Common Mistakes and How to Avoid Them

  • Misordered chapters: Caused by not zero-padding numbers. Solution: Always use two digits (01, 02, … 99).
  • Missing track metadata: Especially the “Track Number” tag. Solution: Batch tag before upload.
  • Cover art resolution too low: 2400 pixels is the minimum – do not crop a smaller image and upscale it. Solution: Use original artwork or a high-resolution photo.
  • Duplicate files: Uploading “Ch01.mp3” and “Ch01_2.mp3” from previous editing versions. Solution: Only keep the final, approved files in your upload folder.
  • More than one file per chapter: Some narrators split a long chapter into two audio files. ACX requires exactly one file per track/chapter. Solution: Combine into a single MP3/WAV if a chapter exceeds 120 minutes (rare, but possible).

Tools to Simplify File Management

Leverage these free or affordable tools to automate repetitive tasks:

  • MP3tag – Batch ID3 tag editing, tag from filenames, and export/import via CSV.
  • ReNamer – Advanced file renaming with rules, regular expressions, and preview. Great for applying a consistent pattern to hundreds of files.
  • ACX Audio Lab – A free web tool that checks volume, noise, and clipping. Always run it on a sample of your final files before upload.
  • Audacity – Audacity can export as WAV and MP3, and you can use the “File > Export Multiple...” feature to split a long recording into separate files while automatically naming them based on labels.
  • Adobe Audition – More advanced, with batch processing and scriptable file export.

Using these tools reduces human error and frees up time to focus on delivering a great performance.

Final Thoughts

Proper file labeling and organization are not just bureaucratic hurdles – they are safeguards that protect your hard work from administrative rejection. By adopting a disciplined naming convention, structuring your folders logically, embedding metadata correctly, and double‑checking everything against a simple spreadsheet, you dramatically increase your chances of a first‑time ACX approval. The effort you invest in file management pays off in faster production cycles and a polished, professional audiobook that listeners can enjoy without glitches or misordered chapters.

Remember to visit ACX’s Help Center for the latest guidelines, and consider joining online communities like the ACX Narrators Facebook group for real‑world tips. With careful preparation, your audiobook will be ready for its audience in no time.