# chapterpass

> chapterpass is a browser-based tool that masters audiobook chapter files for ACX and Audible submission. Upload recorded chapters (WAV, MP3, FLAC, AIFF, or M4A), and download mastered MP3s ready for submission. All processing runs locally in your browser via WebAssembly. Your files never leave your device.

chapterpass handles the measurable technical specs that ACX checks: loudness, peak levels, noise floor, file format, sample rate, silence padding, and channel count. It does not fix recording quality issues (clicks, pops, echo, narration problems). Those are the narrator's responsibility.

## Key Pages

- [Home](https://chapterpass.com): Upload audiobook chapters and download mastered MP3s ready for ACX submission
- [FAQ](https://chapterpass.com/faq): What chapterpass does, what it costs, and what it does not handle
- [About](https://chapterpass.com/about): How chapterpass works, who built it, and why

## Guides

- [ACX Audio Requirements: All Platforms Compared](https://chapterpass.com/learn/acx-audio-requirements): The 8 technical specs ACX enforces in 2026. Full spec table. Cross-platform comparison for INaudio, Authors Republic, and Google Play.
- [ACX Rejected Your Audiobook? Fix Every Issue](https://chapterpass.com/learn/acx-rejection-fix-every-issue): Decision tree from rejection email to fix. Covers loudness, noise floor, true peak, format, and quality rejections.
- [Audacity Audiobook Mastering: Full ACX Workflow](https://chapterpass.com/learn/audacity-audiobook-mastering-acx): The exact Audacity effect chain for mastering to ACX spec. Noise reduction, compressor, normalise, limiter, and batch macro.
- [Audiobook Mastering Tools Compared: 2026 Guide](https://chapterpass.com/learn/audiobook-mastering-tools-compared): Honest comparison of Audacity, Auphonic, Hindenburg Narrator, Source, and chapterpass. Features, pricing, trade-offs.
- [Audiobook Noise Floor: What It Is and How to Fix It](https://chapterpass.com/learn/audiobook-noise-floor-guide): The noise floor paradox: why boosting loudness raises noise above ACX's -60 dBFS limit. Gating vs noise reduction.
- [What chapterpass Does (and What It Does Not)](https://chapterpass.com/learn/what-chapterpass-does): The honest scope. What chapterpass masters, what it cannot fix, and the difference between mastering and editing.

## What chapterpass Handles

- Loudness adjustment to ACX's required range (-23 to -18 dBFS RMS)
- True peak limiting below -3 dBFS using inter-sample peak detection
- Noise floor management in silent sections using speech-aware gating
- Format conversion to mono MP3 at 192 kbps CBR, 44,100 Hz
- Silence padding (1 second head, 1.5 seconds tail)
- Pre-download verification against all 8 ACX specs, with up to 3 correction cycles

## What chapterpass Does NOT Handle

- Recording quality (clicks, pops, mouth noise, echo)
- Narration consistency (tone, pacing, breath timing)
- Opening and closing credits
- Content editing (misread words, long pauses, manuscript accuracy)
- Human narration requirement (ACX prohibits AI-generated narration)

## How It Works

1. Upload audio files (WAV, MP3, FLAC, AIFF, or M4A)
2. The engine masters each file through 9 processing steps in your browser
3. Each output file is verified against all 8 ACX technical specs
4. Download mastered MP3s ready for ACX, INaudio, Authors Republic, or Google Play

## Technical Details

- Accepted formats: WAV, MP3, FLAC, AIFF, M4A
- Output format: MP3 CBR 192 kbps, 44,100 Hz, mono
- Processing: 100% client-side via WebAssembly (no audio leaves your browser)
- Browser requirements: Chrome, Firefox, Safari, or Edge on a desktop or laptop
- Pricing: First file free. Then $36/hr (under 5h), $27/hr (5-10h), $18/hr (10+h)

## Contact

- [Contact](https://chapterpass.com/contact): Questions about chapterpass or audiobook mastering