ACX Audiobook Requirements Explained Simply

*Last updated: June 17, 2025*

Introduction

Audiobooks have become a cornerstone of modern publishing, with millions of listeners around the world consuming books through their headphones and speakers. For authors and publishers looking to tap into this growing market, Audiobook Creation Exchange (ACX)—the platform that supplies Audible, Amazon, and Apple Books—represents the largest distribution channel available. However, ACX is also known for its strict technical requirements that can seem overwhelming to newcomers.

Many audiobook projects face rejection due to technical issues rather than content quality. In fact, according to recent industry data, approximately 40% of first-time ACX submissions require corrections before approval. Understanding these requirements before you begin production can save you significant time, frustration, and potentially costly re-recording sessions.

This guide breaks down ACX’s technical specifications into plain English, providing a clear roadmap for meeting their standards without getting lost in audio engineering jargon. Whether you’re recording at home, working with a professional narrator, or using AI voice technology, these guidelines will help ensure your audiobook passes ACX’s quality control process on the first submission.

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• [Introduction](#introduction)
• [Audio Format Requirements](#audio-format-requirements)
• [Recording Environment Standards](#recording-environment-standards)
• [Narration Performance Requirements](#narration-performance-requirements)
• [Chapter Organization and File Structure](#chapter-organization-and-file-structure)
• [Cover Art Specifications](#cover-art-specifications)
• [Quality Control Process](#quality-control-process)
• [Common Rejection Reasons](#common-rejection-reasons)
• [Testing Your Audio Before Submission](#testing-your-audio-before-submission)
• [Key Takeaways](#key-takeaways)

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Introduction

Audiobooks have become a cornerstone of modern publishing, with millions of listeners around the world consuming books through their headphones and speakers. For authors and publishers looking to tap into this growing market, Audiobook Creation Exchange (ACX)—the platform that supplies Audible, Amazon, and Apple Books—represents the largest distribution channel available. However, ACX is also known for its strict technical requirements that can seem overwhelming to newcomers.

Many audiobook projects face rejection due to technical issues rather than content quality. In fact, according to recent industry data, approximately 40% of first-time ACX submissions require corrections before approval. Understanding these requirements before you begin production can save you significant time, frustration, and potentially costly re-recording sessions.

This guide breaks down ACX’s technical specifications into plain English, providing a clear roadmap for meeting their standards without getting lost in audio engineering jargon. Whether you’re recording at home, working with a professional narrator, or using AI voice technology, these guidelines will help ensure your audiobook passes ACX’s quality control process on the first submission.

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Audio Format Requirements

ACX has specific technical requirements for audio files that must be met for submission approval. These specifications ensure consistent quality across their platform.

File Format and Encoding

The fundamental technical requirements for your audio files include:

1. File Format: MP3 files only (no WAV, AIFF, or M4A)
2. Bitrate: 192 kbps or higher, Constant Bit Rate (CBR)
3. Sample Rate: 44.1 kHz (standard CD quality)
4. Bit Depth: 16-bit
5. File Type: Mono or dual-channel mono (not stereo)

It’s important to note that while you may record and edit in higher-quality formats like WAV, your final delivery to ACX must be MP3 files meeting these specifications.

Volume and Loudness Requirements

ACX has strict requirements for audio levels to ensure consistency across their platform:

1. RMS (Average Volume): -23dB to -18dB RMS
– This measures the average volume of your recording
– Too low: Audio will sound quiet compared to other audiobooks
– Too high: May cause listener fatigue

2. Peak Values: -3dB maximum
– No part of your audio should ever exceed -3dB
– Prevents digital clipping and distortion
– Allows headroom for ACX’s own processing

3. Noise Floor: Below -60dB RMS
– The background noise when you’re not speaking
– Measured during “room tone” or silent passages
– Higher noise floors sound unprofessional and distract listeners

Silence Requirements

ACX has specific requirements for silence at the beginning and end of files:

1. Opening Silence: 0.5 to 1 second of room tone (not absolute digital silence)
2. Closing Silence: 1 to 5 seconds of room tone

This “room tone” should match the ambient sound of your recording environment rather than being digital silence (which would sound unnatural).

> Pro Tip: Always export a test file and check it with ACX’s free “Check My Book” tool before completing your entire project. This can save you from having to re-record or re-process hours of audio.

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Recording Environment Standards

The recording environment significantly impacts audio quality and ACX’s acceptance of your submission.

Background Noise Requirements

ACX requires a clean recording environment free from:

1. Constant Background Noise:
– HVAC systems (air conditioning, heating)
– Computer fans
– Refrigerator hums
– Traffic noise
– Construction sounds

2. Intermittent Distractions:
– Door slams
– Dog barks
– Phone notifications
– People talking in background
– Aircraft/traffic passing by

3. Recording Artifacts:
– Microphone handling noise
– Desk/stand vibrations
– Page turns
– Clothing rustling
– Jewelry noise

Room Treatment Considerations

While ACX doesn’t specifically mandate room treatment, meeting their noise floor requirements often necessitates some acoustic preparation:

1. Minimum Requirements:
– Record in a quiet room away from external noise sources
– Turn off all unnecessary electronics
– Use a directional microphone (cardioid pattern)
– Position away from hard, reflective surfaces

2. Recommended Improvements:
– Add acoustic absorption panels at first reflection points
– Use a reflection filter behind microphone
– Place carpets/rugs on hard floors
– Hang heavy curtains over windows
– Add bookshelves filled with books as natural diffusers

3. Professional Approaches:
– Dedicated vocal booth
– Professionally treated recording room
– Acoustic foam ceiling treatments
– Bass traps in corners
– Double-wall construction for isolation

Noise Reduction Limitations

ACX places limits on how much post-processing noise reduction can be applied:

1. Gentle Noise Reduction Only: Excessive noise reduction causes artifacts that may trigger rejection
2. No Gates: Avoid noise gates that create unnatural silence
3. Natural Room Tone: Maintain consistent room tone rather than creating artificial silence
4. Prevention Over Correction: Better to record cleanly than fix in post-production

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Narration Performance Requirements

Beyond technical specifications, ACX also evaluates the performance quality of your narration.

Voice Consistency Requirements

ACX expects professional-grade narration consistency throughout your audiobook:

1. Tone and Energy: Maintain consistent energy and delivery style throughout
2. Character Voices: Keep character voices distinct and consistent
3. Volume Levels: Avoid significant volume variations between sessions
4. Pacing: Maintain consistent reading speed (typically 150-160 words per minute)
5. Microphone Position: Keep consistent distance from microphone

Pronunciation and Articulation Standards

Clear pronunciation is essential for ACX approval:

1. Clarity: All words must be clearly articulated
2. Consistency: Pronounce names, places, and terms consistently
3. Accuracy: Research proper pronunciation of unusual terms
4. Regional Considerations: Be consistent with regional accents/dialects
5. No Mumbling: Every word must be understandable

Breath Control and Mouth Noises

ACX has strict standards regarding breathing and mouth sounds:

1. Breath Management:
– Consistent, non-distracting breaths
– No gasping or loud inhales
– Natural breathing pattern
– No edited-out breaths creating unnatural pauses

2. Mouth Noise Control:
– Minimize lip smacks, tongue clicks, and wet sounds
– Proper hydration helps (room temperature water)
– Some reduction allowed in post-production
– Excessive mouth noise is grounds for rejection

Errors and Corrections

ACX requires error-free narration:

1. No Uncorrected Mistakes:
– Mispronunciations
– Word substitutions
– Skipped text
– Repeated phrases
– Stumbles or stammers

2. Seamless Corrections:
– Edited corrections must match tone, pace, and volume
– No audible splices or jarring transitions
– Match room tone at edit points

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Chapter Organization and File Structure

ACX has specific requirements for how audiobook files should be organized and structured.

Required File Organization

Your audiobook must include these components:

1. Opening Credits File:
– Title of the book
– Author name
– Narrator name (“Narrated by…”)
– Should be a separate file
– Typically 30-60 seconds long

2. Chapter Files:
– Each chapter as a separate file
– Clear chapter headings narrated exactly as written
– No skipped text or content
– 1-3 second pause after announcing chapter title

3. Closing Credits File:
– “End of [title]”
– “Narrated by [narrator name]”
– Any copyright information
– Any acknowledgments
– Should be a separate file

Naming Conventions

Files must be named systematically:

1. Standard Naming Format:
– Opening_Credits
– Chapter_01
– Chapter_02 (and so on)
– Closing_Credits

2. No Special Characters:
– Use only alphanumeric characters, underscores, and hyphens
– Avoid spaces in filenames
– No accented characters or symbols

3. Sequential Numbering:
– Use leading zeros for chapters (01, 02… not 1, 2…)
– Maintain consistent format throughout

Runtime Considerations

ACX has requirements regarding audiobook length:

1. Minimum Length: 30 minutes total finished audio for the entire book
2. Maximum Chapter Length: No single file should exceed 120 minutes
3. Consistency: Similar chapter lengths preferred (when appropriate to content)
4. Retail Hour Calculation: Based on final total runtime (impacts pricing)

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Cover Art Specifications

While primarily an audio medium, audiobooks still require visual cover art for online stores.

Image Requirements

ACX has specific requirements for cover images:

1. Dimensions: 2400 x 2400 pixels square format
2. Resolution: 72 dpi (minimum)
3. Color Space: RGB (not CMYK)
4. File Format: JPEG or TIFF
5. File Size: 1-12MB

Design Considerations

Beyond technical requirements, effective audiobook covers should be:

1. Legible at Small Sizes: Title and author name must be readable at thumbnail size
2. High Contrast: Works better for small digital displays
3. Simple Designs: Avoid overly complex imagery that won’t render well small
4. Brand Consistent: Match your print/ebook cover when possible
5. Genre Appropriate: Should clearly signal the book’s genre

Common Cover Art Mistakes

Avoid these frequent issues that cause cover rejection:

1. Stretched/Distorted Images: Using non-square original images
2. Too Much Text: Cluttering with review quotes or excessive subtitles
3. Poor Font Choice: Using decorative fonts that are illegible small
4. Low Resolution: Images that appear pixelated
5. Rights Issues: Using stock imagery without proper licensing

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Quality Control Process

Understanding ACX’s quality control process helps you prepare appropriate materials and set realistic timelines.

ACX Review Timeline

The ACX quality control process follows this general timeline:

1. Initial Submission: Upload all files and cover art
2. Automated Technical Check: Immediate verification of basic specifications
3. Manual QC Review: 10-14 business days (typical)
4. Feedback: Pass, or revision requests with specific issues
5. Corrections Submission: Upload fixed files as needed
6. Re-Review: Typically 5-7 business days
7. Final Approval: Distribution to Audible, Amazon, and Apple Books
8. Retail Availability: 7-10 business days after approval

What ACX Reviewers Check

ACX reviewers evaluate these specific aspects:

1. Technical Compliance:
– Audio specifications (format, bitrate, levels)
– File organization and naming
– Cover art requirements

2. Performance Quality:
– Consistent narration
– Proper pronunciation
– No uncorrected errors
– Professional delivery

3. Content Accuracy:
– Matches print/ebook content
– No missing text
– Correct chapter order
– Proper credits

4. Production Quality:
– Clean sound without distracting noise
– Consistent volume and tone
– No excessive processing artifacts
– Professional overall sound

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Common Rejection Reasons

Understanding why ACX rejects audiobooks helps you avoid these issues in your production.

Technical Rejections

These issues relate to the technical specifications of your files:

1. Audio Level Problems:
– RMS levels outside -23dB to -18dB range
– Peak levels exceeding -3dB
– Inconsistent levels between chapters
– Excessive dynamic range

2. Noise Issues:
– Background noise exceeding -60dB
– Inconsistent background noise
– Intermittent noises (clicks, pops, bangs)
– Computer fan noise

3. File Format Problems:
– Incorrect bit rate or sample rate
– Stereo files instead of mono
– WAV files instead of MP3
– Improper file naming

Performance Rejections

These rejections relate to narration quality:

1. Voice Consistency Issues:
– Varying energy levels between sessions
– Inconsistent character voices
– Varying distance from microphone
– Tone/timbre changes between chapters

2. Reading Errors:
– Uncorrected mistakes (flubs, stutters)
– Skipped words or sentences
– Repeated phrases not edited out
– Incorrect pronunciations

3. Delivery Problems:
– Monotone reading
– Excessive mouth noise
– Unnatural pacing
– Poor breath control

Production Rejections

These relate to how the audio was processed:

1. Editing Issues:
– Audible splices or edits
– Missing words at edit points
– Inconsistent room tone
– Abrupt transitions

2. Processing Problems:
– Over-processed sound (excessive noise reduction)
– Unnatural equalization
– Distortion from over-compression
– Noise gate artifacts

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Testing Your Audio Before Submission

Properly testing your audio before submission significantly increases approval chances.

DIY Testing Methods

These approaches help identify issues before submission:

1. The Different Devices Test:
– Listen on headphones, computer speakers, and phone
– Check in car audio system if possible
– Try both high-end and consumer-grade playback
– Listen at various volume levels

2. The Critical Listening Process:
– Listen in a quiet environment
– Focus on background noise between sentences
– Check transitions between takes/edits
– Listen for consistency between chapters
– Pay attention to opening/closing seconds of each file

3. The Measurement Approach:
– Use RMS meters to verify average levels
– Check peak meters for maximum levels
– Analyze noise floor during silences
– Verify consistent levels between all files

ACX Check My Book Tool

ACX provides a free tool to verify technical specifications:

1. How to Access: Available in your ACX dashboard
2. What It Checks:

– File format compliance
– RMS level verification
– Peak level verification
– Noise floor measurement
3. Limitations:
– Doesn’t evaluate performance quality
– Cannot detect editing issues
– Doesn’t check for consistency between files

Third-Party Tools

Several tools can help verify ACX compliance:

1. Audio Editing Software:
– Audacity (free): RMS analyzer, noise profiling
– Adobe Audition: Loudness radar, match loudness feature
– iZotope RX: Advanced audio repair and analysis

2. Specialized ACX Check Tools:
– ACX Check (plugin for Audacity)
– Podcast Audio Checker (works for audiobooks too)
– Auphonic (online processing with ACX presets)

3. Loudness Meters:
– Youlean Loudness Meter (free and paid versions)
– TBProAudio MVRM (free)
– Waves WLM Plus (professional option)

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Key Takeaways

– Technical specifications are non-negotiable: ACX strictly enforces their audio format, levels, and noise requirements.

– File organization matters: Proper file naming, chapter ordering, and inclusion of opening/closing credits are required.

– Both technical and performance standards apply: Even technically perfect audio will be rejected if the narration has errors or inconsistencies.

– Prevention is better than correction: Recording in a proper environment with the right equipment saves enormous time compared to fixing issues in post-production.

– Testing before submission is essential: Use the ACX Check My Book tool and multiple listening methods to verify compliance before submitting.

– Cover art has specific requirements: Square format, high resolution, and readable at small sizes are mandatory.

– Allow time for the review process: Build in 2-3 weeks for potential revisions and resubmission.

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Related Resources

• [How to Achieve Professional Audiobook Sound Quality at Home](/resources/articles/quality-standards/how-to-achieve-professional-audiobook-sound-quality-at-home)

[How to Test Your Audiobook Quality Before Publishing](/resources/articles/quality-standards/how-to-test-your-audiobook-quality-before-publishing)

[Industry Standards for Audiobook Production in 2025](/resources/articles/quality-standards/industry-standards-for-audiobook-production-in-2025)

[How to Master Audiobook Files to Meet ACX Standards](/resources/articles/technical-aspects/how-to-master-audiobook-files-to-meet-acx-standards)

[How to Set Up a Home Recording Studio for Audiobooks](/resources/articles/technical-aspects/how-to-set-up-home-recording-studio-for-audiobooks)

*Tags: audiobook creation, audiobook production, quality standards, ai*