To check a batch of audiobook files against ACX and get a report, use a batch checker. WavSpec (Windows 11) checks every chapter’s RMS, peaks, noise floor, and room tone, corrects levels, peaks, and room tone, re-measures, and writes PDF, HTML, CSV, and JSON reports offline. For pass or fail on a few files, a simpler checker may be enough.
A finished audiobook goes to ACX as dozens of chapter files, and every one of them has to clear the same audio rules. Fix a flubbed line and a chapter’s RMS moves. Trim a breath and the tail gets too short. Checking the whole book again by hand, chapter by chapter, is the part that eats an evening.
Check the whole book against ACX in one pass and keep a report for every chapter.
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What is the short answer?
Load every chapter into a batch checker, pick an ACX profile, and read the per-file results. The tools differ in three ways that matter:
- Where the audio goes. Some run on your computer, some in the browser without uploading, and some send files to a service.
- Whether they fix anything. Some only report. Others correct RMS, peaks, room tone, and the MP3 format.
- What you keep. A report you can file with the delivery is useful when a chapter comes back from QA.
WavSpec runs on Windows 11, corrects what can be corrected, re-measures every delivered file from disk, and writes the report in four formats. The sections below compare it fairly with the other ACX checkers people name.
What does ACX check?
ACX publishes its audio submission requirements on its own site. The core numbers, as ACX states them:
| Requirement | ACX value |
|---|---|
| RMS level | Between -23 dB and -18 dB RMS |
| Peak level | No higher than -3 dB |
| Noise floor | No higher than -60 dB RMS |
| Room tone | No more than 5 seconds of room tone at the beginning and end of each file |
| File format | 192 kbps or higher constant bit rate MP3, 44.1 kHz |
| Files | One chapter per file, each under 120 minutes |
| Channels | All mono or all stereo, consistently |
| Also required | Opening and closing credits, and a retail sample of 1 to 5 minutes |
Sources: ACX’s guides to mastering audiobooks, submitting your audiobook, and its December 2021 spacing update, which replaced the older head and tail timings. ACX’s human QA reviews the audio too, so meeting the numbers is necessary, not a guarantee. For the terms, see our guide to LUFS, LKFS, dBTP and LRA.
What are the ways to batch-check audio against ACX?
Facts about other tools come from their own sites, checked on 17 October 2026.
| Tool | Price model | Where the audio is processed | Platform | Batch | Corrects files | Report |
|---|---|---|---|---|---|---|
| WavSpec | One-time purchase in the Microsoft Store, 15-day free trial | On your PC, no network connections | Windows 11 version 22H2 or later, x64 | Yes: files, folders, watch folder, command line | Yes: RMS, true-peak limiting, resampling, dither, room-tone pad and trim, CBR MP3, file names | PDF, HTML, CSV, JSON, with SHA-256 per file |
| Narrator Check Batch Checker | No price shown for the checker | In the browser, nothing uploaded | Browser | Yes | Yes, fixable chapters into a ZIP, re-measured | JSON and CSV |
| Specula | $99.99 one-time, 7-day trial | On your Mac | macOS 14 or later | Yes, per chapter | Yes: level chapters, level dialogue, limit peaks | Per-chapter results with consistency against the median |
| Sonatune Checker | Account with 250 free credits, then paid credits | Measurements in the browser; some checks use partners in the EU | Browser | Yes | Auto-fix for WAV | Not compared here |
| ACX Audio Lab and Audio Analysis | Part of ACX | Uploaded to ACX | Browser, on ACX | Checks each uploaded file | No | Feedback on ACX |
| A DAW or audio editor | Varies | On your computer | Varies | Usually one file at a time | By hand | Whatever you write down |
Sources: ACX spacing update, Narrator Check Batch Checker, Specula, Sonatune Checker.
ACX’s own Audio Lab
ACX has its own checks. Audio Lab gives instant feedback on any audio you upload to it, for metrics such as peak value and RMS, and Audio Analysis checks production files as you upload them to a project, including the spacing at the beginning and end. If you only want to know whether a file passes before it goes to QA, start there. What it does not do is correct files on your PC or give you a batch report to keep.
Narrator Check Batch Checker
Narrator Check runs in the browser and says nothing is uploaded. Its batch checker fixes the chapters that fail into a ZIP, re-measures them, writes JSON and CSV reports, and flags chapters that stray from the batch median. It declines chapters it cannot fix, and it also checks for mixed mono and stereo files and for duplicates. Narrator Check also offers noise removal, which WavSpec does not. If you work on a Mac or a Chromebook, or you want noise reduction in the same place, it is a strong choice.
Specula (macOS)
Specula from Headroom Studio is a Mac app, $99.99 one-time with a 7-day trial, for macOS 14 or later. It measures ACX RMS, true peak, and noise floor per chapter, compares each chapter with the median, and offers Level chapters, Level dialogue, and Limit peaks. For a narrator on a Mac, it is the closest equivalent to WavSpec.
Sonatune Checker
Sonatune Checker runs in the browser with an account and starts with 250 free credits, which Sonatune puts at about 12.5 hours of checks. It handles batches and custom profiles, and auto-fixes WAV files. Sonatune says the measurements run in the browser while other checks use trusted partners in the EU, so read its privacy terms if your narration is unreleased.
Metering chapter by chapter in a DAW
Any audio editor with an RMS meter and a peak meter can check ACX by hand. It costs nothing if you already own the editor. The catch is time and record keeping: every edit means measuring again, and nothing writes the results down for you.
WavSpec: check, correct, re-measure, report
WavSpec checks a batch of audio files against a named delivery spec, corrects what the spec needs, re-measures every delivered file from disk, and writes the delivery report as PDF, HTML, CSV, and JSON. Its ACX audiobook (Audible) preset judges RMS between -23 and -18 dB, peaks at or below -3 dB, and a noise floor at or below -60 dB RMS, and it judges one to five seconds of room tone at each end of every chapter.
When you correct the batch, WavSpec brings RMS to -20.5 dBFS, limits peaks, resamples to 44.1 kHz, pads a short head or tail with the chapter’s own room tone, trims a long one, replaces digital silence, renames files to plain file names, and writes 192 kbps CBR MP3. Every delivered file is read back and measured again, with up to three passes to land on target. Originals are opened read-only and never overwritten.
Run your last audiobook through the ACX preset and compare the report with what ACX said.
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How do I check an audiobook against ACX in WavSpec?
- Add the chapters. Drag the chapter files or the whole folder into the window, or choose Add folder (Ctrl+Shift+O). WavSpec reads WAV, BWF, RF64, AIFF, FLAC, MP3, AAC, M4A, and WMA.
- Choose the ACX preset. Under Client specification, pick ACX audiobook (Audible), and set the Delivery folder.
- Check only (F6). WavSpec measures every chapter and marks it Pass or Fail with the reason, writing nothing. Select a row to see each criterion, its measured and required values, and the loudness timeline.
- Check and correct batch (F5). WavSpec corrects what the preset needs, writes the files to the delivery folder, re-measures each one from disk, and saves the report beside them.
- Open the report. Read the batch verdict and any chapter still marked Fail before you upload.
For recurring work, the watch folder (Ctrl+W) checks and corrects new files as they arrive, and the command line takes --spec, --out, and --check-only with exit codes.
What is in the ACX report?
The report lists every file with its measurements before and after correction, a pass, fail, or advisory result for each criterion, the correction applied, and a short-term loudness timeline. It adds batch-level warnings, a consistency line across the batch, the spec and its reference document, your client and project details, and a SHA-256 digest for every delivered file and its source. It is saved as PDF, HTML, CSV, and JSON.
Keep the PDF with the delivery. If a chapter comes back from QA, you can see what was measured when it went out. The CSV is handy for a narrator’s own tracking sheet, and the JSON suits a script. Our audio delivery QC checklist covers what else to check before you upload.
What does WavSpec not fix?
Be clear about the limits before you buy:
- Noise floor. WavSpec measures the noise floor (the quietest 500 ms window) and judges it against ACX. It does not reduce noise. A chapter with a noisy room stays marked Fail until you treat it in an editor or a tool with noise reduction, such as Narrator Check.
- Performance problems. Mouth clicks, plosives, edits, and pacing are for your ears and ACX’s QA, not a meter.
- Mac and Windows 10. WavSpec needs Windows 11 version 22H2 or later on an x64 PC.
- ACX approval. Passing WavSpec’s ACX check does not guarantee ACX or Audible will accept the book.
If true peaks are the problem on other deliveries, see how to fix true peak overs in a WAV.
Frequently asked questions
What software can check if my audiobook files meet ACX requirements?
Batch checkers such as WavSpec on Windows 11, Specula on macOS, and the browser tools Narrator Check and Sonatune Checker measure each chapter against ACX’s RMS, peak, and noise floor rules. WavSpec also corrects levels, peaks, room tone, and the MP3 format, and writes a report.
Can I check a whole audiobook against ACX at once on Windows?
Yes. Add the folder of chapters to WavSpec, pick the ACX audiobook (Audible) preset, and press Check only (F6) for results without writing anything, or Check and correct batch (F5) to fix and re-measure.
What are the ACX RMS, peak and noise floor requirements?
ACX asks for RMS between -23 and -18 dB, peaks no higher than -3 dB, and a noise floor no higher than -60 dB RMS, delivered as 192 kbps or higher CBR MP3 at 44.1 kHz.
Does WavSpec fix room tone for ACX?
Yes. It pads a short head or tail with the chapter’s own room tone, trims one that is too long, and replaces digital silence.
Will passing WavSpec’s ACX check guarantee ACX approval?
No. ACX’s own QA has the final word, and it listens for problems that numbers do not catch.
Does my narration get uploaded anywhere?
No. WavSpec runs on your PC and opens no network connections.
Does WavSpec work on Mac?
No. It runs on Windows 11 version 22H2 or later on an x64 PC. Specula is a macOS option, and Narrator Check runs in a browser.
Try it on your last delivery
WavSpec is a one-time purchase in the Microsoft Store, and the Store runs a 15-day free trial. Load a book you have already delivered, pick the ACX preset, and compare the report with what you sent. Read more on our ACX requirements check page, or go to the WavSpec product page.
Check every chapter, fix what can be fixed, and keep the report.
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ACX and Audible are trademarks of Audible, Inc. or its affiliates. Microsoft and Windows are trademarks of the Microsoft group of companies. Other product names are trademarks of their owners. WavSpec is not affiliated with or endorsed by ACX, Audible, or any other company named here.
Last updated 17 October 2026. Published by Automata Labs, a software company in Canada.