ACX Audiobook Mastering — Meet Audible Standards Online Free
A free browser-based mastering tool that checks and fixes your audiobook files to meet ACX/Audible technical requirements. Automatically normalizes RMS loudness to -20 dB, limits peaks below -3 dB, reduces noise floor below -60 dB, and exports as 192 kbps CBR MP3 — the exact specs required by Amazon ACX.
Normalize loudness, limit peaks, and eliminate noise to publish on Audible
ACX Compliance Check
🎚 Gain
🎛 Equalizer
⚡ Compressor
🔇 Noise Reduction
Batch Processing
Features
Frequently Asked Questions
What are the ACX audio requirements?
ACX requires MP3 files at 192 kbps CBR or higher, 44.1 kHz sample rate, mono channel, RMS between -23 and -18 dB, peaks no higher than -3 dB, and noise floor below -60 dB. Each chapter needs 0.5-1 second of room tone at the beginning and 1-5 seconds at the end.
Will this tool fix my audio automatically?
Yes. The Auto-Master feature normalizes RMS to -20 dB (the sweet spot within ACX range), limits peaks to -3.5 dB with a safety margin, applies noise reduction if needed, adds room tone, and converts to the required format — all automatically.
Can I process multiple chapters at once?
Yes. Upload all your audiobook chapter files and the tool will analyze and master each one individually, maintaining consistent loudness across all chapters. Download them all as a ZIP file.
Is this the same as the ACX Check plugin for Audacity?
The ACX Check plugin only checks whether your audio meets requirements — it does not fix anything. This tool both checks AND fixes your audio to meet ACX specs in one step.
Will noise reduction make my audio sound worse?
The tool uses the lightest noise reduction possible to achieve -60 dB. You can preview the result with A/B comparison before saving, and adjust the strength from Light to Heavy. If your recording environment is already quiet, no noise reduction may be needed.
Is my audio uploaded to a server?
No. All processing happens entirely in your browser using Web Audio API and FFmpeg WASM. Your files never leave your computer. There are no accounts, no file size limits imposed by servers, and no usage restrictions.
