Free ACX check: test your audiobook audio against Audible's requirements

Drop a narration file (WAV, MP3, M4A, FLAC, or OGG) and get your peak, RMS, and noise floor measured against ACX's submission limits: free, in your browser, with nothing uploaded. Your audio never leaves your computer.

The three numbers ACX checks, and how to fix each one

Peak: must be −3 dBFS or lower

Your loudest single moment. To fix a hot peak: lower your interface's input gain, not your distance from the mic, and re-record if the take actually clipped, because flattened waveforms can't be repaired. For stray peaks on an otherwise good take, a limiter around −3.2 dB works. Track with peaks landing around −12 to −6 dB.

Average (RMS) level: must be between −23 and −18 dBFS

The average loudness of the file, and the number that most often fails first-time submitters, usually on the quiet side. Fix low RMS with loudness normalization targeting about −20 dB RMS (Audacity: Effect → Loudness Normalization, RMS mode). Note that raising gain also raises your noise floor by the same amount. If your noise floor isn't at least 45 dB below your RMS target, fix the room before you normalize.

Noise floor: must be −60 dBFS or lower

The sound of your room when you aren't talking, measured as the RMS of the quietest half-second in the file, using the same method as Audacity's ACX Check plugin. Fix it at the source: kill fans, HVAC, and computer noise, treat reflections with soft material, and track at a healthy level. Gentle noise reduction of 6 to 12 dB can close the final gap; heavy noise reduction leaves artifacts that fail ACX's human quality review.

The rest of the spec people forget

• 192 kbps or higher MP3, constant bit rate (CBR), 44.1 kHz for uploads
• Room tone at both ends: 0.5 to 1 second at the head, 1 to 5 seconds at the tail
• One file per chapter, section read aloud at the top, no file over 120 minutes
• All files consistently mono or stereo (mono is the norm), matching tone across chapters
• A retail sample of one to five minutes plus opening and closing credits files

Frequently asked questions

What are the ACX audio requirements?

A peak no higher than −3 dBFS, RMS between −23 and −18 dBFS, and a noise floor at or below −60 dBFS, plus delivery as 192 kbps or higher CBR MP3. The level measurements are where most automated rejections happen.

How do I check my audio against ACX requirements for free?

Drop a WAV, MP3, M4A, FLAC, or OGG file into the checker at the top of this page. It measures peak, RMS, and noise floor in your browser. Nothing is uploaded.

How is the noise floor measured?

As the RMS of the quietest half-second window anywhere in the file. That window is your room tone, the honest measure of how quiet your recording space is.

Why did ACX reject my audiobook audio?

Most commonly: peaks above −3 dB, RMS outside −23 to −18 dB, or a noise floor above −60 dB. A free level check before submission catches all three.

Does ACX require silence at the start and end of each file?

Yes: 0.5 to 1 second of room tone at the head and 1 to 5 seconds at the tail, and it should be real room tone rather than digital silence.

Is my audio uploaded when I use this checker?

No. Your browser decodes and measures the file locally; it never touches a server, which makes it safe for client work under NDA.

Pass the check while you record, not after

VoxPrompter runs this exact check on every take you record, with a voice-following teleprompter, broadcast-style metering, studio-grade WAV export, and punch-and-roll. Learn more about VoxPrompter or see pricing. The 14-day trial is free, and this checker stays free either way.