ACX Requirements for Audiobook Publishers: A Complete Guide (2025)

Introduction

Audiobook Creation Exchange (ACX) remains the industry standard platform for producing and distributing audiobooks through Audible, Amazon, and iTunes. Understanding ACX’s technical specifications and submission requirements is crucial for successful audiobook publishing. This comprehensive guide covers everything you need to know about ACX requirements in 2025, helping you avoid rejection and ensure your audiobook meets professional standards.

Technical Audio Requirements

Audio Format and Quality Specifications

| Requirement | ACX Specification |
|————-|——————-|
| File Format | WAV (Broadcast Wave Format) |
| Sample Rate | 44.1 kHz |
| Bit Depth | 16-bit |
| Channel | Mono |
| Encoding | Linear PCM (uncompressed) |
| Container | WAV or BWF (.wav extension) |

Loudness and Dynamic Range

| Measurement | Required Range |
|————-|—————-|
| RMS (Average Level) | -23dB to -18dB RMS |
| Peak Values | -3dB maximum |
| Noise Floor | Below -60dB |
| Dynamic Range | At least 60dB |

Room Tone and Silence Requirements

• Opening/Closing Silence: 0.5-1 second of room tone at beginning and end
• Chapter Breaks: 1-5 seconds of room tone between chapters
• Room Tone Quality: Must match the ambient noise profile of the recording
• Consistency: Room tone must have consistent noise profile throughout the audiobook

Production Requirements

Chapter Organization

• Each chapter must begin with a spoken chapter header (“Chapter One,” etc.)
• Opening and closing credits must be in separate files from chapter content
• File naming convention: chapterXX.wav (where XX is the chapter number)
• Maximum individual file size: 170MB (approximately 120 minutes of audio)

Credits and Supplementary Material

#### Opening Credits (Required)

• Book title
• Author name(s)
• “Narrated by [narrator name]”

#### Closing Credits (Required)

• “End of [book title]”
• “Narrated by [narrator name]”
• Production credit (optional)

Retail Audio Sample

• Length: 1-5 minutes
• Content: Engaging segment from the main text (not credits)
• Purpose: Used for marketing on Audible/Amazon
• Requirements: Must meet all technical specifications

Performance Standards

Narration Quality

• Consistent pacing (150-160 words per minute recommended)
• Natural breathing with no distracting breath sounds
• Consistent character voices throughout the book
• Clear pronunciation of all words, including specialized terminology
• No mouth clicks, plosives, or sibilance issues

Recording Environment

• Noise floor below -60dB
• No audible room reflections or echo
• No background noises (HVAC, traffic, etc.)
• Consistent acoustic environment throughout the book
• No electrical hum or digital artifacts

Editing Requirements

• No repeated phrases unless intentional
• No skipped text from the manuscript
• Natural-sounding edits with consistent pacing
• Consistent volume throughout the audiobook
• Complete removal of excessive mouth sounds

Common Rejection Reasons and Solutions

Technical Rejections

| Rejection Reason | Solution |
|——————|———-|
| Inconsistent volume levels | Use a compressor/limiter and ensure consistent RMS levels |
| Excessive noise floor | Improve acoustic treatment and use appropriate noise reduction |
| Distortion or clipping | Record at lower input levels and check peak values |
| Incorrect file format | Export as mono 44.1 kHz/16-bit WAV files |

Performance Rejections

| Rejection Reason | Solution |
|——————|———-|
| Inconsistent pacing | Use a metronome or tempo guide during recording |
| Mouth noises | Proper hydration, anti-click techniques, and de-clicking software |
| Incorrect pronunciations | Research all unfamiliar words before recording |
| Inconsistent character voices | Keep a voice sample library for reference |

Production Rejections

| Rejection Reason | Solution |
|——————|———-|
| Missing or incorrect credits | Follow the exact credit format requirements |
| Incorrect chapter organization | Ensure proper file naming and organization |
| Missing manuscript text | Compare the final audio against the manuscript |
| Inconsistent room tone | Record sufficient room tone and use it consistently |

Submission Process

Required Materials

1. Cover Art
– 2500×2500 pixels square JPG/TIFF
– RGB color space (not CMYK)
– Title and author name must be legible when reduced to thumbnail size

2. Audiobook Files
– All chapter files in correct format
– Opening and closing credits as separate files
– 1-5 minute retail audio sample

3. Metadata
– Book title and subtitle
– Author name(s)
– Narrator name(s)
– Publisher information
– Copyright information
– Book description (4000 characters maximum)
– Category/genre selections (up to 3)

Quality Assurance Checklist

Before submission, verify:

• [ ] All files meet technical specifications
• [ ] Chapter markers are properly placed
• [ ] Opening and closing credits are complete and accurate
• [ ] No background noise or technical issues throughout
• [ ] Volume levels are consistent across all chapters
• [ ] All text from the manuscript is included
• [ ] All pronunciation is consistent and correct
• [ ] Retail sample is 1-5 minutes and engaging

Advanced Tips for ACX Success

Mastering for Maximum Quality

1. Two-Stage Compression Approach
– First compression pass: 2:1 ratio with slow attack/release for overall control
– Second compression pass: 1.5:1 ratio with faster attack for peaks
– Target final RMS: -21dB to -19dB (optimized for ACX while maintaining dynamics)

2. Effective Noise Reduction
– Capture clean room tone for noise profile
– Apply minimal noise reduction (-6dB to -12dB reduction maximum)
– Use spectral editing for specific frequency issues
– Process before compression to avoid amplifying noise

3. Optimized EQ Strategy
– High-pass filter at 75-85Hz (12dB/octave slope)
– Targeted reduction at common problem frequencies (200-300Hz for muddiness)
– Gentle presence boost at 3-5kHz for clarity
– De-essing at 5-8kHz if necessary

Efficient Workflow for ACX Compliance

1. Pre-Production
– Create project template with correct technical settings
– Develop character voice reference sheet
– Prepare pronunciation guide for difficult words
– Configure recording chain for optimal levels

2. Production
– Record consistent room tone at beginning of each session
– Mark errors during recording for efficient editing
– Maintain consistent distance from microphone
– Record chapters in complete sessions when possible

3. Post-Production
– Edit for content before applying processing
– Process in stages: noise reduction → EQ → compression → limiting
– Check RMS and peak levels throughout
– Listen to final files on multiple systems before submission

2025 ACX Updates and Changes

Recent Platform Changes

• Expedited Review Process: Premium publishers now receive 5-7 day reviews (vs. 10-14 days)
• Enhanced Quality Metrics: More detailed feedback on rejection reasons
• Expanded Distribution Options: Additional retail partners beyond Audible, Amazon, and iTunes
• Algorithmic Quality Detection: AI-based initial screening for technical issues

New Requirements for 2025

• Enhanced Metadata Requirements: More detailed categorization options
• Chapter-Level Tagging: Improved chapter navigation requirements
• Audio Description Tags: Optional accessibility enhancements
• Extended Sample Options: Multiple retail samples for different platforms

Conclusion

Meeting ACX requirements might seem demanding, but the platform’s standards ensure audiobooks reach listeners with consistent professional quality. By following this guide, you’ll avoid common rejection reasons and streamline your submission process. Remember that beyond meeting technical specifications, the best audiobooks balance technical excellence with engaging performance to truly connect with listeners.

*This guide reflects ACX requirements as of July 2025. Always check the official ACX platform for the most current specifications.*