Free tool
ACX audio checker
Got an ACX rejection, or about to upload? Check one chapter against the ACX audio requirements: loudness, peak level, noise floor, sample rate, MP3 bitrate and clipping. You get a pass or fail for each, with your number next to the one ACX asks for.
Your file is measured in your browser and your audio never leaves your computer. This checker only reports; it does not change your file. No training on your voice, we simply don't do it at all.
Drop one chapter file here
MP3 or WAV, up to 300 MB. No account needed.
What happens to your file
- The file is read and measured by this page, in your browser. The audio is never uploaded.
- We count that a check ran, the file type (MP3 or WAV), which checks passed and the campaign tag of the link you came from, if it had one. No file name, no email, no audio.
- If you ask for the full report by email, we receive your email address and the measured numbers, and keep them with your choice about marketing email. Marketing email is off unless you tick the box.
- No training on your voice, we simply don't do it at all.
ACX audio requirements: questions narrators ask
What are the ACX audio requirements?
Each file must measure between -23 and -18 dB RMS, peak no higher than -3 dB, and keep its noise floor at -60 dB or lower. Files are uploaded as MP3 at 192 kbps or higher with a constant bit rate, at 44,100 Hz, and every file in a book must be the same, all mono or all stereo. A single file can run up to 120 minutes.
Why did ACX reject my audio?
The most common reasons are loudness outside the -23 to -18 dB RMS range peaks above -3 dB, a noise floor above -60 dB, and an MP3 exported at the wrong bitrate or with a variable bit rate. This checker measures each of those on your file.
What does RMS mean, and why is my chapter too quiet?
RMS is the average loudness of the whole file. A raw recording usually sits well under -23 dB because the pauses pull the average down. Mastering raises the whole chapter by one amount and then limits the peaks, so it lands inside the range without sounding squashed.
What is the noise floor?
The noise floor is the level of the background sound in your pauses: room noise, computer fans, preamp hiss. ACX asks for -60 dB or lower. This checker measures it inside the pauses between your phrases, not in the quietest single moment, because the pauses are what a listener actually hears.
Does this checker upload my audio?
No. Your file is read and measured in your browser, and the audio never leaves your computer. If you ask for the full report by email, we receive your email address and the measured numbers, nothing else.
Does it fix my audio?
No. The checker only measures and reports. AudLit, the full app, masters a chapter to the ACX specification and shows the same measurements before you export.
Want the whole job in order, from manuscript to ACX upload? Read the audiobook guide.
More on recording, cleanup and mastering: the AudLit blog.