Preparing audiobook audio files for submission to ACX (Audiobook Creation Exchange) is a critical step that can determine whether your title gets approved quickly or faces time-consuming rejections. The platform enforces strict technical and quality standards to maintain a consistent listening experience across Audible, Amazon, and iTunes. A thorough, methodical approach to file preparation saves you hours of rework and gets your audiobook in front of listeners sooner. This expanded checklist guides you through every stage — from recording to final upload — ensuring your files meet ACX requirements on the first try.

1. Record High-Quality Audio

The foundation of an ACX-ready audiobook is a clean, professional recording. Invest in a quality large-diaphragm condenser microphone and an audio interface with a noise floor below -80 dB. Use a treated recording space — even a closet filled with clothes can reduce flutter echoes. Maintain a consistent distance of 4–6 inches from the microphone to avoid proximity effect shifts. Record at a consistent speaking pace and volume, and always wear closed-back headphones to monitor for plosives, sibilance, or low-frequency rumble. A uniform vocal delivery across all sessions prevents jarring transitions during editing.

2. Edit and Master Your Audio Files

Editing removes errors, long pauses, and ambient clicks. Use a DAW like Audacity (free) or Adobe Audition (paid) to isolate and cut mistakes, lip smacks, breath catches, and mouth clicks. Trim leading and trailing silence so each file starts precisely with the first word. After editing, master the audio for loudness and tonal balance. ACX requires an average loudness of -18 dB to -23 dB RMS with a peak not exceeding -3 dB. Apply a limiter to prevent clipping. Use a gentle high-pass filter at 60–80 Hz to remove subsonic rumble, and a small amount of compression to even out dynamic range — but keep it natural to avoid an over-processed sound.

Important Mastering Tips

  • Normalize to -3 dB peak to leave headroom.
  • Check for DC offset and remove it using your software’s offset removal tool.
  • Apply a de-esser to tame harsh “S” and “T” sounds.
  • Listen on multiple playback systems (headphones, laptop speakers) to confirm clarity.

3. Maintain Technical Specifications

ACX enforces a rigid set of technical parameters. Files that deviate will be flagged for rejection. The table below summarises the core requirements:

  • Sample Rate: 44,100 Hz (not 48,000 Hz)
  • Bit Depth: 16-bit
  • File Format: MP3 (mono or joint stereo — ACX accepts both, but mono is standard for narration)
  • Bit Rate: 192 kbps constant (CBR). Do not use VBR (variable bitrate) — it can cause playback issues.

Use your audio editor’s export settings to enforce these values. Incorrect sample rate or bit depth is one of the most common reasons for rejection. Validate with a tool like ACX Check (free) or the built-in analyser in Audacity’s Plot Spectrum view.

4. Split Files Correctly

Each chapter or section of your audiobook must be saved as a separate MP3 file. Use a consistent naming convention — for example, AuthorName_BookTitle_Chapter01.mp3. Keep each file under 120 minutes in length; longer files may cause streaming interruptions. Split at natural chapter breaks, and ensure no silence is included between chapters in the final submission. If a chapter is very long, consider splitting it into parts (e.g., Chapter05_Part1, Chapter05_Part2) and noting this in the metadata.

File Naming Best Practices

  • Avoid spaces — use underscores or hyphens.
  • Use three-digit numbering (001, 002, … 012) to maintain sort order.
  • Include the title abbreviation if the book name is long.
  • Match names exactly to the chapter list you submit in ACX.

5. Follow ACX Submission Requirements

ACX provides detailed specifications beyond just audio quality. Review the ACX Audio Standards page for the latest guidelines. Key areas to verify:

  • Intro and Outro: Each chapter must start with “Chapter X” and end with “End of Chapter X.” The entire audiobook must open with the title and author, and close with “End of [Title].”
  • Metadata Consistency: The title, author, narrator, and publisher fields must match exactly across all audio file metadata (ID3 tags) and the ACX submission form.
  • ID3 Tags: Populate title, track number, album, and artist tags on every MP3 file. Use the Check application to automate this.
  • Retail Sample: Provide a 5-minute sample clip that meets the same technical specs.

Ignoring even one requirement — like a missing “End of Chapter” tag — can cause a rejection. Create a checklist and go through it file by file.

6. Prepare Cover Art

Cover art is part of your submission and must meet ACX’s exacting standards. Requirements:

  • Dimensions: Minimum 2400 x 2400 pixels; recommended 3000 x 3000 pixels for future-proofing.
  • Resolution: 300 dpi.
  • Format: JPEG or PNG, with JPEG being the most commonly accepted.
  • Aspect Ratio: 1:1 (square). Avoid any other ratio.
  • File Size: Under 50 MB.

If you are not a designer, use tools like Canva or hire a professional. Do not include the ACX or Audible logo — it’s reserved for Amazon’s use. Save the cover with the same naming convention as your audio files (e.g., AuthorName_Title_Cover.jpg).

7. Final Quality Check

Before uploading, perform a complete auditory review. Listen to every file from start to finish — ideally on both headphones and speakers. Watch for:

  • Volume jumps or dips between chapters.
  • Buried background noise (room tone, electrical hum, pages turning).
  • Mispronunciations or stumbles that were missed during editing.
  • Inconsistent pacing or breaths that sound unnatural.
  • Metadata tags matching across all files.

Use a loudness meter plugin (many free ones exist for OBS and DAWs) to verify RMS levels stay within -18 to -23 dB on every file. If you find an issue, re-export that single chapter file — don’t re-export the whole book unnecessarily.

8. Upload and Submit

Log into your ACX account and navigate to the project page. Upload each file individually (ACX supports drag-and-drop). After upload, the system runs an automated check for format, bitrate, and loudness. Wait for the green check mark next to each file. If any file shows a red warning, review the error message and fix it before submitting. Finally, fill out the remaining metadata:

  • Book title, author, narrator, publisher, language.
  • Retail release date (can be set for future).
  • Pricing and distribution territories.

Click the “Submit for Review” button. ACX’s quality assurance team will listen to a sample of your audiobook and may request changes if they detect issues. Typical review time is 10–15 business days.

Common Mistakes to Avoid

Even experienced narrators stumble on these pitfalls:

  • Noise floor differences between chapters recorded on different days — re-record or apply noise reduction consistently.
  • VBR encoding instead of CBR — always set to constant bitrate in your export settings.
  • Wrong file length — a chapter longer than 120 minutes will be rejected. Split it.
  • Missing ID3 tags — ACX will warn you, but it’s better to fill them in before upload.
  • Cover art with text too close to the edge — Amazon crops 5% from each edge; keep critical text in the safe zone.

Tools to Streamline Your Workflow

These free and paid tools can help you meet ACX standards efficiently:

  • Audacity — Free, cross-platform DAW with ACX-compatible export presets.
  • ACX Check — A free plug‑in for Audacity that validates each file against ACX specs.
  • Levelator (discontinued but still available) — Automates loudness normalization to -19 dB RMS.
  • MP3tag — Batch edit ID3 tags across hundreds of files.
  • Auphonic — Cloud-based loudness normalization and mastering service (free tier available).

Learn more about ACX production from the official ACX Help Center and from the Audible Content Standards page. Both resources are updated regularly.

Final Thoughts

Preparing your audiobook audio files for ACX submission is a detail‑intensive process, but one you can master with a methodical workflow. By recording clean audio, applying careful editing and mastering, respecting every technical specification, and double-checking all metadata and cover art, you significantly reduce the chance of rejection. Use the checklist above for every project — it transforms a nerve‑wracking submission into a routine step on your path to publishing.