ACX Audiobook Checker
Check an audiobook chapter against the ACX numbers: loudness, peak, noise floor, sample rate and MP3 bit rate, then fix and save a compliant MP3.
Drop a chapter file
or click to browseor tap to choose
MP3 or WAV
Large files (over 5 GB) may take longer or run out of browser memory
About ACX Audiobook Checker
Checks an audiobook file against the published ACX audio requirements: loudness (RMS), peak level, noise floor, sample rate, MP3 bit rate and length. Each line shows your value, the limit and pass or fail. If the levels are off, one button saves a corrected 44.1 kHz, 192 kbps constant bit rate MP3 and checks that file again. Runs in your browser; the audio is not uploaded.
Drop one chapter file (MP3 or WAV works best). Read the table, then press Fix and download if anything failed. Check the new file's table before you upload it.
ACX also listens to the files and has rules about the opening, ending, room tone and quality that a meter cannot judge. Treat a pass here as the numbers being in range, not as an approval.
- What does this check?
- Loudness (RMS), peak level, noise floor, sample rate, MP3 bit rate and constant bit rate, and length, each compared with the ACX audio requirements. A table shows your value, the limit and the result.
- Does a pass mean ACX will accept my file?
- No. It means the measured numbers are inside the limits. ACX also checks the sound itself, such as the opening and ending, room tone, noise and clipping, so listen to the file as well.
- Why does the noise floor still fail after the fix?
- Changing the volume raises or lowers the noise together with the voice. A noisy room has to be fixed at the recording, or with a noise reduction pass, and then the file can be checked again.
- Is my recording uploaded?
- No. The file is decoded and measured in your browser, and the corrected MP3 is built in your browser too.
How to use ACX Audiobook Checker
Drop one chapter file, MP3 or WAV. The tool decodes it in your browser and shows a table: loudness (RMS), peak level, noise floor, sample rate, MP3 bit rate and length, each next to the ACX limit with Pass or Fix needed.
If something failed, press Fix and download. It sets the loudness to about -20.5 dB RMS, holds peaks under -3 dB, converts to 44.1 kHz and saves a 192 kbps constant bit rate MP3. The new file is measured again and its table appears below, so you can see the result before you upload it.
When to use it
Use it before uploading a chapter to ACX, when a submission was rejected for levels or format, or to check a file from a studio or editor against the numbers without opening a mastering program.
When not to use it
A pass here means the numbers are in range, not that ACX will approve the file. ACX also reviews the sound itself: clean openings and endings, consistent room tone, no mouth noise or clipping. And if the noise floor fails, changing the volume will not help; the recording or a noise reduction pass has to.
Tips for best results
Fix recordings that are too quiet or too loud with the button, but record in a quieter room if the noise floor fails: raising the volume raises the noise with it. Check each chapter separately, since ACX wants one file per chapter.
Limitations
Loudness is plain RMS over the whole file, which is how the ACX numbers are described, not LUFS. The noise floor is the quietest half second in the file, so a file with no quiet gap can read higher than its room tone really is. Sample rate and bit rate are read from the MP3 or WAV header; other formats are decoded but the sample rate is shown as not read. The ACX numbers are written from their published requirements, so check their current page before a submission.
Privacy
The file is decoded and measured inside your browser tab and never sent to a server. The fix runs in an engine in the tab too; the only download is that engine, pulled from a public CDN the first time an audio tool needs it.
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