ACX Audiobook Mastering — Meet Audible Standards Online
A 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 checks the result against Amazon ACX specs. Exporting in the ACX format — 192 kbps CBR MP3, 44.1 kHz, mono — is a Premium feature.
How to Use
Drag and drop your audiobook chapter file (MP3, WAV, FLAC, or other format) into the upload zone.
The tool instantly checks RMS loudness, peak levels, and noise floor against ACX requirements and shows pass/fail for each.
Click "Auto-Master" for one-click fixing, or fine-tune individual settings like target RMS, peak limit, and noise reduction strength.
Use the A/B toggle to compare before and after mastering. Ensure the audio sounds natural without over-processing.
Download the finished file. Exporting in the ACX format — 192 kbps CBR MP3, 44.1 kHz, mono — is a Premium feature.
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.
The check says my file fails. Will auto-mastering fix it?
It fixes the three things that are a matter of level: loudness is brought to −20 dB RMS, peaks are limited under the ceiling, and the noise floor is pushed down. What it cannot fix is a room. If the failure is the noise floor and the recording was made next to a fridge or in a bare room with an echo, the level work only makes the noise louder along with the voice — that one has to be solved before the microphone, not after.
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.
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Comments
Thank you, Tembrica. The issue that hampered me from editing and publishing my audiobook, DECEPTION: Conspiracy for Israeli Expansion and Western Hegemony, has been successfully resolved with your help. I truly appreciate your support. Keep up the great work