How to Prepare Your Audiobook for ACX Submission

Getting your audiobook accepted by ACX isn't just about hitting the right loudness numbers. It's about the full package, from recording environment to the final pre-upload verification. ACX runs two independent checks: automated technical verification against eight measurable specs, then a human quality review that listens for recording issues.[1] Your audiobook must pass both. This guide walks through the entire process: what to do before recording, what to watch during sessions, how to edit, how to master, and the systematic pre-submission checklist that separates first-time passes from rejection cycles.

What Should You Do Before Recording?

Your recording environment determines the ceiling of your final audio quality. No amount of post-production can fix a fundamentally noisy or echoey recording, and ACX requires a noise floor below -60 dBFS.[1] If you've already been rejected for noise floor, see the noise floor rejection guide for the fastest path to fixing it.

Find a quiet space. Turn off HVAC, close windows, power down electronics you don't need. Record a minute of silence and listen back on headphones. You'll hear things you didn't notice sitting in the room.

Treat the room. Hard walls create reflections that make your voice sound boxy. Bookshelves full of books, heavy curtains, rugs, and acoustic panels all absorb sound. You don't need a professional studio, but you need a room that doesn't add its own character.

Set up consistently. Mark your mic position so you can replicate it across sessions. Same distance (6–8 inches with a pop filter), same angle, same height. ACX reviewers listen for consistency across chapters, and a mic that's 6 inches away in chapter 1 and 12 inches away in chapter 8 produces noticeably different sound.

Do a test recording. Before committing to a full session, record 5 minutes at your normal narration pace. Play it back on headphones. Listen for echo, background noise, plosives, and sibilance. Measure the noise floor of a silent section. Below -65 dBFS gives you comfortable margin for gain adjustments during mastering.

What Should You Watch for While Recording?

Monitor your levels. Watch your recording meter while you narrate. Peaks should land around -12 to -6 dBFS. Too quiet means you'll need to boost gain later, which also boosts noise. Too loud risks clipping.

Stay consistent. Keep the same distance from the mic throughout your session. If you lean forward for dramatic moments or pull back for loud passages, the volume and tone changes will be audible.

Take notes. When you make a mistake, note the timestamp. When you hear a noise (phone vibration, door closing, stomach rumble), note it. This saves enormous time during editing.

Record room tone. At the start of each session, record 30 seconds of silence. This gives you clean room tone for patching and silence padding during editing.

Hydrate and warm up. Dry mouth produces clicks and lip smacks that are tedious to remove. Keep water nearby and take sips between chapters. A brief vocal warm-up reduces mouth noise.

How Should You Edit Your Recording?

Editing is where you fix problems before they become rejection reasons. ACX's human reviewers listen for clicks, pops, mouth noise, and inconsistencies, none of which mastering can fix.[1]

Remove mistakes. Cut retakes, flubs, and false starts. Listen to edit points carefully. Bad crossfades sound like clicks or unnatural gaps.

Clean up artefacts. Listen for clicks, pops, mouth noise, and any sounds that aren't your narration. Use de-clicking tools, spectral editing, or manual cut-and-paste repair.

Check for echo. If you hear reverb or a boxy quality, there's not much you can do in post without degrading the audio. Heavy de-reverb creates artefacts that sound worse than the original problem. This is why room treatment matters.

Add your credits. ACX requires opening credits (title, author, narrator) as a separate file at the beginning and closing credits at the end. Record these as standalone files following the same technical specs as chapter files.

Organise your files. One chapter per file. Name them sequentially with zero-padded numbers (e.g., 00_Opening_Credits, 01_Chapter-01). Make sure every file is present. For file naming conventions and the retail sample chapter, see the audiobook chapter formatting guide.

How Do You Master Audio for ACX?

Mastering adjusts your audio to meet ACX's eight measurable technical specifications: RMS loudness between -23 and -18 dBFS, true peak below -3 dBFS, noise floor below -60 dBFS, 44,100 Hz sample rate, mono channel, MP3 192 kbps CBR format, 0.5–1 second head silence, and 1–5 seconds tail silence.[1]

The standard signal chain is: format conversion, high-pass filtering, noise management, loudness adjustment, peak limiting, silence padding, MP3 encoding, and verification. The complete mastering guide walks through each step with specific settings, the Audacity ACX settings guide covers the exact parameters for free-tool mastering, and the audiobook format requirements guide compares format specs across all major platforms.

ChapterPass automates the entire mastering step. Upload your edited chapters and the engine handles loudness, peaks, noise floor, format, silence padding, and verification. Every file is checked against all eight specs before download.

Either way, mastering only works with what you give it. A clean recording plus good mastering equals acceptance. A bad recording plus any mastering equals rejection.

What Should You Check Before Submitting?

This pre-submission checklist is the final quality gate. Don't submit the moment files are mastered. A systematic pass catches problems that would otherwise cost a rejection cycle.

Content Completeness

• All chapters are recorded and edited, with no placeholders remaining
• Opening credits file includes book title, author name, and narrator name
• Closing credits file is complete with at minimum an end statement
• Retail sample chapter identified (1–5 minutes, represents your best narration)
• Chapter count matches the manuscript
• Files are named sequentially and backed up

Technical Verification (Every File)

For each file in your audiobook, open the final MP3 and verify:[1]

1. RMS loudness: -23 to -18 dBFS (target -20 dBFS). Select all → measure full-file RMS.
2. True peak: Below -3 dBFS. Use a true-peak meter (Youlean Loudness Meter, free).
3. Noise floor: Below -60 dBFS. Select a silent section → measure RMS.
4. Format: 44,100 Hz, mono, MP3, 192 kbps, CBR. Verify with MediaInfo or ffprobe.
5. Silence padding: 0.5–1 second head, 1–5 seconds tail. Room tone, not digital silence.

Cross-File Consistency

After verifying each file individually:

1. Compare RMS values. All chapters should be within 2 dB of your target.
2. Listen to transitions. Play the last 5 seconds of Chapter 1 then the first 5 seconds of Chapter 2. Volume, tone, and ambience should be seamless.
3. Verify file sequence. Opening credits → chapters in order → closing credits.
4. Check the retail sample. Play through the entire designated sample chapter.

Listening Check

Automated measurements catch technical failures but not quality issues. A file can pass every metric and still sound poor due to artefacts, distortion, or missed editing errors.

• Play back the actual MP3s you'll submit, not source files
• Listen on headphones
• Check first and last 30 seconds of each chapter for clean silence transitions
• Spot-check the middle of several chapters

What Happens After You Submit to ACX?

Automated technical check: Runs immediately, with fast turnaround, typically within hours. Measures all eight specs on every file. If any chapter fails any spec, the whole book gets kicked back.[1]

Human quality review: Listens for recording quality issues: clicks, pops, echo, narration consistency, missing credits, manuscript accuracy. Takes anywhere from a few business days to a couple of weeks depending on ACX's review queue. For a deeper look at both layers, see what ACX actually checks.

If rejected: The notice specifies which chapters failed and why. Fix the flagged issues, re-master if needed, and re-verify all specs on all files before resubmitting. Fixing one problem can shift other measurements.

If only one chapter fails: Fix and resubmit only the failed chapter(s). But if the problem was in your mastering chain, check all chapters processed through that chain, since they may have the same issue even if only one was flagged.

What Are the Most Common Mistakes That Cause First Rejection?

Most first-submission rejections fall into a handful of repeatable patterns. Knowing them in advance is the cheapest form of quality control.

Verifying the WAV, not the MP3. The file you submit is the MP3. MP3 encoding raises peak levels by 0.5 to 1 dB and can shift RMS slightly. A chapter that passes every check in your DAW can fail ACX because the measurements changed during export. Always verify the final MP3, not the source file.

Using a sample-peak limiter instead of a true-peak limiter. Audacity's built-in limiter measures sample peak. ACX checks true peak, which accounts for inter-sample peaks via 4x oversampling per the ITU-R BS.1770 standard. The difference is typically 0.5 to 1.5 dB. A chapter limited to -3 dBFS sample peak can have a true peak of -1.5 dBFS, which fails ACX. Use a true-peak-aware limiter, or set a sample-peak limiter to -3.5 dBFS as a compensating margin. The true peak guide explains this in detail.

Measuring noise floor before mastering, not after. Background noise gets louder when you boost gain to hit the RMS target. A raw recording at -65 dBFS noise floor that needs 10 dB of gain to reach -20 dBFS RMS ends up at -55 dBFS noise floor after mastering. That fails the -60 dBFS limit. Always measure noise floor on the processed, mastered file.

Digital silence instead of room tone. Generated silence passes the automated duration check but fails human review. ACX reviewers listen for the abrupt transition from absolute zero to room ambience. Use room tone for all silence padding. See the silence padding guide for the full explanation.

Missing or incomplete credits. Opening credits must include the book title, author name, and narrator name. Closing credits must include at minimum an end statement. Files without proper credits trigger an immediate human review rejection. Record these first, not last, so they're ready before you start the chapter mastering workflow.

Inconsistent room treatment across sessions. Chapters recorded in the same room but on different days can sound noticeably different if the room was rearranged, furniture was added or removed, or a window was open. ACX's human reviewers listen for these tonal inconsistencies across chapters. Record in the same configuration each session and capture fresh room tone every time.

Submitting without a full listening pass. Automated measurements catch technical failures. They don't catch outtakes left in the file, stumbled sentences that weren't re-recorded, or audio artefacts from over-aggressive noise reduction. Listen through your final MP3s on headphones before submitting. The ACX QA team will.

How Do You Avoid Rejection Cycles?

The pattern behind first-submission acceptance is systematic verification. Check every spec on every file. Don't trust spot checks. Don't assume one chapter passing means they all pass. A single failed chapter means the entire audiobook gets kicked back.

If you're producing books regularly, the math favours automation. Hours spent on manual per-chapter verification are hours not spent on narrating, editing, or marketing. The complete ACX requirements guide covers all eight specs, the ACX platform page has a quick reference, and the mastering tools comparison helps you choose the right workflow for your volume.