ACX Checker: RMS, Peak, Noise Floor Online

Where it runs

Works in a current Chrome, Edge, Firefox or Safari, on Windows, Mac, Linux and Chromebook computers, Android phones, iPhones and iPads. Nothing to install, no account.

The first file loads the audio engine (ffmpeg, about 31 MB) once, and your browser keeps it. Very long books are easiest on a computer, which has more memory than a phone.

4 simple steps.

Drop your chapters

Add every chapter file, or the whole book folder. Files are checked one after another.

Watch the results

Each file gets a pass, review or fail for every ACX requirement as soon as it is measured.

Find the problem

Click a file to see its meters, a level view with the failing spots in red, and how to fix each issue.

Share the report

Download a CSV, or print or save a PDF report for your author, producer or yourself.

ACX audio requirements at a glance

Requirement	Target	Why it matters
RMS loudness	-23 to -18 dB	Consistent listening volume across books
Peak level	-3 dB or lower	Headroom so audio never distorts
Noise floor	-60 dB or lower	Quiet background between words
Format	MP3, 192 kbps+, CBR	The format ACX ingests
Sample rate	44.1 kHz	Standard for audiobook delivery
Channels	All mono or all stereo	Consistent playback between chapters
File length	120 minutes or less	One chapter per file, of manageable size
Room tone	1–5 s at the start and at the end	Natural transitions between chapters

Understanding the three levels

RMS loudness

RMS is the average loudness of the whole file. Narration that sits around −20 dB RMS sounds full without being tiring. Too quiet and listeners turn the volume up, then get blasted by the next book; too loud and the audio sounds squashed.

Peak level

The peak is the single loudest sample. Keeping peaks at or below −3 dB leaves room for the encoding and playback chain. Plosives (“p” and “b” sounds), laughs and shouted lines are the usual culprits; the level view marks every place in red.

Noise floor

The noise floor is the level of the quietest half-second: your room tone. Computers, fans, traffic and electrical hum raise it. A floor at −60 dB or lower means the pauses between sentences sound truly quiet.

A good order of operations

Record in the quietest space you can and check the noise floor first: it is the hardest thing to fix later.
Edit, then raise the level so the RMS sits near −20 dB.
Add a limiter with a −3.5 dB ceiling to tame peaks.
Export MP3 at 192 kbps CBR, 44.1 kHz, then run this checker on every chapter.

Questions, answered.

What are the ACX audio requirements?

Each file must have RMS loudness between -23 dB and -18 dB, peaks no higher than -3 dB and a noise floor of -60 dB or lower. Files must be MP3 at 192 kbps or higher, constant bitrate, 44.1 kHz, all mono or all stereo, no longer than 120 minutes, with 1 to 5 seconds of room tone at the start and at the end (no silence longer than 5 seconds).

How is the noise floor measured?

As the RMS level of the quietest half-second in the file, which is how ACX-style checkers define it. We slide a 0.5-second window through the whole file in 0.1-second steps, so the quietest moment is found wherever it falls.

Are my files uploaded?

No. Every file is decoded and measured inside your browser. Nothing leaves your computer, so you can check a whole book without any upload.

If every check passes, will ACX accept my audiobook?

Passing the measurable checks is necessary but not the whole story. ACX also reviews files by ear for things like mouth noise, consistent levels between chapters, background sounds and pronunciation.

Why does the room tone check say “review” instead of “fail”?

Room tone is detected automatically by finding where speech starts and stops. That works well for most files, but a breath or a quiet first word can shift it slightly, so we ask you to review rather than marking it as a hard failure.

What if my RMS is too low or my peaks are too high?

Raise the overall level so the RMS is around −20 dB, then use a limiter with its ceiling at −3.5 dB so peaks stay under −3 dB. If the noise floor rises above −60 dB as a result, you need a quieter recording space or gentle noise reduction.

Can I check WAV or other formats?

Yes. Loudness, peaks, noise and room tone are measured for any audio file, so you can check your masters before exporting. The format checks will remind you that ACX needs MP3 at 192 kbps CBR, 44.1 kHz.