Introduction to ACX Audio Submission Requirements

Publishing an audiobook on the Audiobook Creation Exchange (ACX) can open doors to a vast audience of listeners on Audible, Amazon, and iTunes. However, before your title reaches listeners, it must pass ACX’s stringent quality review. The most common reason for rejection is failing to meet the platform’s audio submission requirements. This expanded guide covers every aspect of those requirements — from technical specifications and recording best practices to post-production editing and submission procedures. Use this checklist as your roadmap to produce a professional, ACX-compliant audiobook on the first try.

ACX’s standards ensure a consistent listening experience. Whether you are a seasoned narrator or a first-time author, following these guidelines will save you time, avoid costly revisions, and help your audiobook stand out. Let’s break down each requirement step by step.

Understanding ACX’s Technical Specifications

Before recording a single word, you must understand the non-negotiable technical specs ACX demands. These are the foundation of a compliant file. Deviations — even minor ones — will cause automatic rejection during automated validation or the human quality check.

File Format and Bit Depth

ACX requires all audio files to be delivered as uncompressed WAV files with a 16-bit depth. Avoid MP3, AIFF, FLAC, or any compressed formats. The reason is simple: compressed files can introduce artifacts that degrade quality after multiple encodes. Always record and export as WAV, 16-bit. If you record in 24-bit, you may downsample — just ensure dithering is applied correctly to avoid truncation noise.

Sample Rate

The required sample rate is 44.1 kHz. Do not use 48 kHz or higher. Many audio interfaces default to 48 kHz; verify your session settings before recording. A mismatched sample rate causes pitch shifts or resampling artifacts. If you accidentally record at 48 kHz, you can convert to 44.1 kHz using spectral resampling in your DAW, but it’s best to set it correctly from the start.

Volume Levels and Loudness

ACX enforces specific loudness targets to maintain consistent playback across devices. The guidelines are:

  • Average RMS (root mean square) loudness should fall between -18 dB and -23 dB. This is often measured as the overall level of the narration, not peak values.
  • Peak levels must not exceed -3 dB. Headroom prevents clipping on playback.
  • Absolute silence between words should be kept at -60 dB or lower (noise floor).

To achieve these levels, use an audio editor with loudness metering (e.g., Audacity, Reaper, iZotope RX). Apply normalization to bring the average RMS to approximately -20 dB RMS. Then check that peaks are below -3 dB. If necessary, use a limiter to catch stray peaks — but avoid over-compression that flattens dynamics.

Noise Floor and Background Noise

ACX listens for extraneous sounds. Your noise floor — the residual background noise in silent passages — must be at least -60 dB. That means no audible hums, fans, traffic, or room echo. Use a noise gate or noise reduction tool to clean up gaps. However, aggressive noise reduction can create artifacts; aim for a clean recording environment first.

Pre-Recording Preparation

Success starts long before you press record. Set up your script, equipment, and space to eliminate problems later.

Script Preparation and Formatting

A well-prepared script prevents misreads and retakes. Use a large, easy-to-read font. Annotate pronunciation guides for unusual words. Break the text into logical recording blocks — typically 10 to 15 minutes each. Mark places where you intend to take short breaks. ACX does not prescribe script formatting, but clarity reduces errors and editing time.

Also ensure your script matches the final text of the book. Any deviations after recording may require re-recording entire sections.

Equipment and Environment

You don’t need a million‑dollar studio, but you do need reliable gear. Essential items:

  • Microphone: A large‑diaphragm condenser or dynamic microphone with a cardioid pattern. Popular options include the Audio‑Technica AT2020, Rode NT1, or Shure SM7B.
  • Audio interface: A USB interface with a clean preamp (e.g., Focusrite Scarlett, Audient iD series).
  • Closed‑back headphones: To monitor your recording without bleed into the mic. Examples: Sony MDR‑7506, Beyerdynamic DT770.
  • Pop filter and shock mount to reduce plosives and vibration.

Your recording space must be quiet. Even the best mic will pick up reflections and background hum. Treat your room with absorptive material — moving blankets, acoustic panels, or even a closet full of clothes. A portable isolation shield (reflection filter) helps but is not a substitute for a treated space.

Soundproofing Tips

  • Record in a room with carpets, curtains, and soft furniture.
  • Turn off HVAC systems, refrigerators, and computers nearby.
  • Use weather stripping on doors to reduce outside noise.
  • Place your mic at least a few inches away from any reflective surface.

Test the room by recording five seconds of silence. Play it back — if you hear any hiss or hum, take steps to reduce it. ACX’s noise floor requirement leaves no room for audible artifacts.

Recording Best Practices

Once your setup is ready, focus on delivering a clean, consistent vocal performance.

Consistent Performance

Maintain a steady distance from the microphone — typically 6 to 12 inches — and speak at a natural pace. Fluctuations in volume or tone are normal, but extreme changes (e.g., whispering then shouting) can cause uneven loudness. Use your headphones to monitor levels live. If you notice clipping (red peaks), move back or lower your gain.

Pacing, Pronunciation, and Retakes

Read naturally, as if telling a story. Avoid rushing or unnaturally slow delivery. If you stumble or mispronounce a word, pause briefly and then repeat the phrase from the beginning of that sentence. Do not stop the recording; you can mark the mistake during editing. This “punctuation method” saves time compared to stopping and starting each time.

Monitoring Levels

Aim for your recording peaks to stay around -6 dB to -3 dB on your meter. This gives plenty of headroom and ensures you’re not distorting. If your voice is too quiet, the noise floor may become too prominent after normalization. If too loud, you risk clipping. Use the meter in your DAW to adjust gain until your typical speech falls in the sweet spot.

Post‑Recording Editing and Processing

After recording all chapters, you must edit out mistakes, silence, and background noise before assembling the final files.

Removing Errors and Noise

Open your raw recordings in an audio editor. For each track, perform these tasks in order:

  1. Delete any unusable takes, mouth clicks, and breaths that are too loud. Small clicks can be removed manually or with a de‑clicker tool. Breath removal: you can leave soft natural breaths, but loud gasps should be trimmed. ACX accepts reasonable breaths.
  2. Remove excessive silence. Gaps longer than a few seconds (e.g., during retakes) must be deleted. Keep natural pauses for storytelling effect.
  3. Apply noise reduction if the background noise is above -60 dB. Use a spectral noise print (capturing a sample of silence) and reduce by 6‑12 dB. Avoid over-processing; the goal is a clean, natural sound.
  4. Normalize to -3 dB peak and adjust average RMS to around -20 dB. Some tools offer “loudness normalization” to -19 dB LUFS (integrated); ACX accepts this if it results in the correct RMS range.
  5. Use a high‑pass filter at 80 Hz to remove subsonic rumble (e.g., vibrations, low‑frequency hum). This is optional but often helpful.

After processing, always export a short test section and verify that the loudness and noise floor meet ACX specs. Use a free plugin like Youlean Loudness Meter or the built‑in metering in Reaper.

File Segmentation and Naming Conventions

ACX expects each audio file to correspond to one chapter or part. Common naming patterns:

  • AuthorName_BookTitle_Chapter01.wav
  • BookTitle_Part_01.mp3? No, WAV only

Avoid special characters (colons, slashes, punctuation). Use underscores or hyphens. Ensure the chapter number matches your metadata. For example, if your book has a Prologue, label it “Prologue” or “Chapter00”. Consistency is key.

Important: Each file must begin with a short silence (up to 1 second) and end with a silence of at least 0.5 seconds. ACX’s review includes the space between tracks.

The ACX Quality Review Process

Understanding what ACX auditors look for can help you avoid common pitfalls.

What Auditors Check

ACX performs both automated and manual quality checks:

  • Automated checks: file format, sample rate, bit depth, peak level, average RMS, and noise floor. Files that fail automated rules are immediately rejected.
  • Manual listening: a human auditor listens to a sample of each file. They evaluate clarity, consistent volume, absence of background noise, and professional pacing. They also verify that the narration matches the text (if the book is a full cast or has sound effects, special rules apply).

Common reasons for rejection include inconsistent loudness, audible mouth clicks, sibilance overload, background hum, and files that are too quiet or too loud.

Tips to Avoid Rejection

  • Listen to your entire audiobook on headphones before submission. It’s tedious, but it catches mistakes.
  • Check the first 5 minutes and last 5 minutes of every file — these are often the sections sampled.
  • Use ACX’s free Universal Audio File Checker tool (or a third‑party checker like Audiobook Mastering General) to validate your files before upload.

For more details, visit ACX’s official audio requirements page.

Final Submission Checklist

Before hitting “Submit” on ACX, run through this checklist one more time.

  • File format: All files are WAV, 16‑bit, 44.1 kHz.
  • Loudness: Average RMS between -18 dB and -23 dB; peaks no higher than -3 dB.
  • Noise floor: -60 dB or lower.
  • File naming: Consistent, no special characters, each chapter labeled.
  • Segmentation: One file per chapter. Total length matches the book.
  • Cover art: JPEG or PNG, at least 2400 x 2400 pixels, RGB. No text on the cover (unless it’s part of the official design).
  • Metadata: Title, author, narrator names correct. Book description accurate.
  • Sample upload: ACX may require a sample file (usually 5 minutes) for review. Ensure that file also meets specs.
  • Final quality audition: Listen to one minute every 15 minutes of the book on different playback systems (speakers, earbuds).

If you use a mastering service, ask them to confirm ACX compliance before sending you the files. If you are self‑producing, tools like Audacity (free) or Reaper can perform all necessary editing with the right plugins. For more advanced noise reduction, iZotope RX is an industry standard.

Conclusion

Meeting ACX audio submission requirements is not optional — it’s the gatekeeper to publishing your audiobook. By following the technical specs, preparing your environment, recording with care, editing meticulously, and validating every file, you dramatically increase your chances of first‑pass acceptance.

Remember that the review process exists to ensure a high‑quality listening experience for customers. Embrace the guidelines, invest in your craft, and your audiobook will reach ears around the world. Good luck — and happy narrating.