Free ACX audio check
Pick an audiobook chapter (MP3 or WAV) and see whether it meets the audio specs ACX publishes for Audible: loudness, peak level, noise floor, room tone at the start and end, length and MP3 format.
Your file stays on your computer. This page reads it inside your browser; nothing is uploaded, and it sets no cookies.
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Fix a failing chapter
Our Audiobook ACX Check & Master tool on Apify fixes what it can: it sets the loudness and peaks, trims or tops up room tone with your own room's sound, and exports 192 kbps MP3s. Then it measures every finished file the same way as this page, and tells you when a recording can't be fixed automatically, such as a noisy room.
What it checks
| Check | ACX spec | How this page measures it |
|---|---|---|
| Loudness (RMS) | −23 to −18 dB | RMS of the whole file; stereo: the average power of both channels |
| Peak | at most −3 dB | highest sample, and the peak between samples (4× oversampled), in the louder channel |
| Noise floor | at most −60 dB | RMS of the quietest half-second, not counting pure digital silence |
| Room tone | recommended 1–5 s at the start and end; over 5 s not allowed | time before the first and after the last speech |
| Length | at most 120 min | decoded length |
| File format | MP3, 192 kbps or higher CBR, 44.1 kHz | read from the file's MP3 headers |
These are the same measurements our Apify tool reports for a file before mastering. ACX doesn't say how long a stretch to use for the noise floor; like Audacity's ACX Check, we use the quietest half-second. Stereo files are measured per channel, as Audacity does: loudness and noise floor are the average power of the two channels, and the peak is the highest in either channel, so a stereo file with the same sound in both channels reads the same as its mono original. Room tone under 1 s is below ACX's recommendation, not a stated rule; only more than 5 s breaks their limit.
What it doesn't check
ACX also has a person listen to every book, for things no meter measures: mouth noise, plosives, edits, background sounds, and consistent performance. A pass here covers the published technical specs only and doesn't guarantee approval. ACX's own tools may measure slightly differently; small differences near a limit are possible.
Very long files use a lot of memory while they are read; if your browser gives up on a long chapter, check it in a desktop browser.