acx audiobook audio checker

Catch ACX rejections before you upload, on every chapter at once.

acx audiobook audio checker gives your AI agent a pre-flight check for audiobook files headed to ACX (Audible). Point it at a folder of WAV masters or MP3 exports. It measures each file against ACX's published audio submission requirements and returns a pass/fail table, the exact number behind every failure, and the usual fix. It never modifies your audio.

What it checks

• Levels (WAV): RMS between -23 and -18 dB, peak no higher than -3 dB (with a warning above -3.5 dB, because MP3 encoding can push peaks up), noise floor at -60 dB or lower, and clipped samples.
• Spacing (WAV): 0.5 to 1 second of room tone at the head and 1 to 5 seconds at the tail, plus generated digital silence where clean room tone should be.
• Format (WAV and MP3): 44.1 kHz sample rate, length up to 120 minutes per file.
• MP3 exports: reads every frame header to confirm MPEG-1 Layer III, 192 kbps or higher, constant bitrate (catches VBR exports frame by frame), channel mode, duration and the 170 MB file limit. It also flags a LAME Xing (VBR) tag on a file that should be CBR.
• Whole project: all mono or all stereo, and the RMS spread between chapters so loudness stays consistent.
• Reports: terminal table, acx-report.md and acx-report.csv. Exit code 1 when anything fails, so it also works as a gate before upload.

Problems it solves

• Files bounced by ACX QA days after upload
• Not knowing which of 40 chapters has the noise or level problem
• A master that peaks at exactly -3.0 dB and then fails after MP3 export
• Accidental VBR or 48 kHz exports from a DAW preset
• Head and tail room tone that's too short, too long, or pure digital silence

Who it's for

Self-published authors narrating their own audiobooks, indie narrators and producers on ACX, small audiobook studios, and KDP authors expanding into audio.

What's included

• SKILL.md: how to gather files, run the check, explain each result, a fix table for Audacity, Audition and Reaper users, and how the measurements are made
• scripts/acx_check.py (main CLI), audio_io.py (pure-Python WAV and MP3 frame readers), make_samples.py (passing and failing test files)
• references/acx-requirements.md: every requirement with its source on acx.com
• references/measurement.md: exactly how RMS, peak, noise floor and room tone are measured, how MP3 headers are read, and validation results

Standard-library Python only. No audio libraries, no API keys, no network calls, no installs.