Audio Format Guide for Audiobooks
Amazon's ACX (Audiobook Creation Exchange) has strict technical requirements for audio submission. Non-compliant files are automatically rejected. This guide covers the exact specs, how to achieve them, and the conversion workflow to get your recordings ready for submission.
ACX Technical Requirements
ACX mandates the following for all audiobook submissions: MP3 format (no WAV, FLAC, or AAC accepted), 192 kbps CBR (Constant Bitrate — VBR files are rejected), mono (not stereo), 44.1 kHz sample rate, RMS loudness between -23 dBFS and -18 dBFS, peak level no higher than -3 dBFS, noise floor below -60 dBFS. Every chapter file must meet all of these specifications.
Recording and Editing Workflow
Step 1: Record in WAV, 44.1 kHz, 24-bit, mono. Using 24-bit during recording gives you headroom — you can bring the levels up during mastering without noise artifacts. Step 2: Edit in Audacity, Adobe Audition, or your DAW — remove breaths, room noise, and mistakes. Step 3: Apply noise reduction to achieve the -60 dBFS noise floor. Step 4: Normalize to -23 LUFS RMS and -3 dBFS peak. Audacity's ACX Check plugin can verify compliance.
Converting WAV to MP3 192 kbps CBR
After mastering your WAV files, convert to MP3 using the WAV to MP3 converter at audioutils.com. Select 192 kbps CBR in the encoding options. The free tier converts files up to 10 MB; Pro handles up to 500 MB (sufficient for chapter-length audio at WAV quality). The output MP3 will be 192 kbps CBR mono — the exact format ACX requires. Verify the output file meets specs before uploading the full project.
Non-ACX Audiobook Platforms
Findaway Voices (Spotify for Audiobooks, library platforms) accepts WAV or MP3, requires -23 LUFS average loudness, -3 dBFS peak, and -60 dBFS noise floor — the same loudness standards as ACX. Authors Republic accepts MP3 at minimum 192 kbps or WAV. Kobo Writing Life accepts MP3 at 192 kbps minimum. In practice, mastering to ACX spec and converting to MP3 192 kbps CBR works for all major platforms.
Common Rejection Reasons and Fixes
Noise floor too high: record in a quieter space, apply noise reduction plugin. Loudness too low: normalize to a higher RMS level. Loudness too high: apply a limiter to reduce peaks, then normalize down. Wrong format: convert WAV to MP3 192 kbps CBR. Stereo file submitted: re-record or convert to mono by mixing both channels. Chapter too short: ACX requires a minimum of 15 seconds of audio per file.