ACX compliance checker for audiobook files

An audiobook can sound finished and still fail a technical review. The usual problems are not dramatic: the narration is a little too quiet, a peak jumps above the limit, the room tone is louder than expected, or the export uses the wrong MP3 settings. This ACX compliance checker is built for that last-mile check before you upload a chapter, retail sample, or full audiobook file.

The tool runs in your browser and checks the audiobook file itself. It looks at RMS loudness, peak level, estimated noise floor, sample rate, MP3 bitrate, and constant bit rate status. Those are the items that often send authors back into editing after they thought mastering was done.

Use it after editing, after mastering, and again after exporting your final MP3. If you change compression, normalization, limiter settings, chapter joins, or the export preset, check again. A clean source file can fail after a bad export.

The ACX requirements are technical, but the fixes are usually practical. RMS loudness tells you whether the chapter is sitting in the expected average loudness range. Peak level catches loud spikes that may distort after encoding. Noise floor estimates the quietest parts of the recording, where room hum, fan noise, mouth noise, or aggressive compression can become obvious.

Format checks matter too. ACX expects audiobook uploads to be prepared as MP3 files with the right sample rate, a high enough bitrate, and constant bit rate encoding. A file may pass loudness and still be rejected because it was exported as variable bit rate, at the wrong sample rate, or from a workflow that changed the final MP3 settings.

• RMS loudness for overall listening level.
• Peak level for clipping and unsafe transient spikes.
• Noise floor for hiss, room tone, and background buildup.
• MP3 bitrate and CBR status for upload-readiness.
• Sample rate so the final file matches audiobook delivery expectations.

A failed result does not always mean the narration is bad. It means the file needs another pass before submission. If RMS is too low, raise level gently or remaster the chapter. If peaks are too high, use a limiter with enough headroom. If the noise floor fails, listen to the quiet passages first; the fix may be noise reduction, cleaner edits, or less compression.

Do not chase the numbers so hard that the audiobook becomes tiring to hear. Audiobook mastering is a balance between compliance and comfort. A chapter that is technically loud enough but harsh, clipped, or over-processed will still feel amateur to listeners.

For human-recorded audiobooks, run the checker after your final mastering chain and export. For AI audiobook narration, use it as a quality gate after chapter assembly. Long-form audio can shift when you concatenate files, add pauses, normalize chapters, or convert formats, so the last exported file is the one that matters.

NarrationBox is built around long-form audio production: manuscripts, chapters, voice selection, pacing, pauses, pronunciation, and export. The checker is useful even if you produce elsewhere, but it pairs naturally with a workflow where you can regenerate only the lines that changed and then export another clean audiobook file.