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Owais Khan Software Reviews

ACX Audio Spec Checker

Drop in a chapter file and check it against every ACX audio requirement — RMS, peak, noise floor, bit rate, sample rate and room tone — before you upload it, with the exact adjustment for anything that fails.

Everything runs locally: your audio never leaves your browser.

Check an audio file against ACX requirements

Example: a synthetic 14-second test recording, measured when this page was built.

CheckMeasuredResultWhat to do
RMS -25.8 dB Fail Too quiet by 2.8 dB. Raising the gain by 2.8 dB keeps peaks at -10.8 dB, under the limit.
Peak -13.6 dB Pass At or below -3 dB.
Noise floor -66.2 dB Pass At or below -60 dB.
Bit rate 192 kbps CBR Pass 192 kbps or higher, constant.
Sample rate 44,100 Hz Pass 44.1 kHz.
Channels Mono Pass Either is accepted, but every file in the book must match.
Length 0 min 14 s Pass Under 120 minutes.
Room tone, start 0.70 s Pass 0.5–1 s.
Room tone, end 2.25 s Pass 1–5 s.

Reading the example

The example recording is well made but too quiet: -25.8 dB RMS against a minimum of -23 dB. Its loudest peak is -13.6 dB, so there is room to simply raise the gain — the fix line works that out for you. The noise floor, -66.2 dB, is comfortably below -60 dB, and the room tone at each end is within ACX’s ranges. If the peaks had been near -3 dB, the advice would have been to limit them first.

An ACX RMS checker that matches Audacity

Narrators usually check levels with Audacity’s ACX Check, and ACX’s own Audio Lab only works after you upload. This ACX RMS checker uses the same method as ACX Check: peak is the largest single sample, RMS is taken over every sample of every channel, and the noise floor is the quietest 0.4-second window. So the numbers should agree closely with what Audacity reports for the same file; small differences can come from how each program decodes an MP3.

ACX noise floor checker and room tone

As an ACX noise floor checker it also reports where the quietest window is, and measures the room tone at the head and tail — the stretch that stays within 6 dB of the noise floor before the first word and after the last. ACX asks for 0.5–1 second at the start and 1–5 seconds at the end; trimming too tight is a common reason files get sent back.

Checking the format: 192 kbps CBR at 44.1 kHz

Levels come from decoding the audio; the format comes from the file itself. The checker walks every MP3 frame header to find the real bit rate and whether it ever changes — a single variable-bit-rate frame fails ACX’s constant-bit-rate rule — and reads the sample rate and channel mode from the same headers. A WAV file can be checked for levels, but ACX only accepts MP3, so export the final version as a 192 kbps CBR MP3.

Frequently asked questions

What are the exact technical audio requirements for ACX?
Each file must measure between -23 dB and -18 dB RMS, with peaks no higher than -3 dB and a noise floor no higher than -60 dB RMS. It must be a 192 kbps or higher MP3, constant bit rate, at 44.1 kHz; hold one chapter or section under 120 minutes with its header read aloud; and have 0.5–1 second of room tone at the start and 1–5 seconds at the end. The whole book must be all mono or all stereo, with opening and closing credits and a 1–5 minute retail sample.
What is the acceptable RMS range for ACX audio files?
From -23 dB to -18 dB RMS, measured over the whole file. RMS is the average loudness, so pauses count: a chapter with long gaps measures quieter than the same narration read straight through. If you are below -23 dB, the checker tells you how much gain you need and whether your current peaks can take it without crossing -3 dB.
Why does my audio pass peak level checks but fail ACX RMS standards?
Peaks and RMS measure different things. A recording can have a few loud plosives or laughs near -3 dB while the narration itself sits around -26 dB RMS; raising the gain enough to fix the RMS would push those peaks over the limit. The fix is to tame the peaks first — a limiter at about -3.5 dB, or gentle compression — and then raise the overall gain until the RMS lands between -23 and -18 dB.
How is the noise floor measured?
Like Audacity’s ACX Check: the file is scanned in 0.4-second windows, one every 0.1 seconds, and the noise floor is the RMS of the quietest window — normally a pause between sentences. It must be -60 dB or lower. Below -90 dB it is digital silence, which usually means a noise gate cut the room tone out entirely; ACX asks for natural room tone, and silence that drops to nothing sounds unnatural.
Why does ACX reject my file when the levels pass?
Usually the format. ACX needs a constant bit rate (CBR) MP3 at 192 kbps or more and exactly 44.1 kHz; a variable bit rate export, 128 kbps, or 48 kHz audio all fail. This checker reads the MP3’s frame headers directly, so it reports what the file really is, not what the export dialog said.
Does this ACX checker upload my audio files to a server?
No. The file is decoded and measured by your browser; it never leaves your browser, and there is no upload, no API call and no analytics here. That is enforced rather than promised: this site’s test suite scans the shipped HTML for every browser API capable of sending data off the page and fails the build if it finds one.