The ACX audio requirements are eight technical specs every audiobook chapter must meet: RMS loudness between -23 and -18 dBFS, peak level below -3 dBFS, noise floor below -60 dBFS RMS, a 44,100 Hz sample rate, MP3 at 192 kbps or higher CBR, one channel format across the whole title (all mono or all stereo), and 1 to 5 seconds of room tone at both the head and the tail.[1] ACX's audio review team holds every submission to those specs, and each file must also be no longer than 120 minutes.[1]
ACX (Audiobook Creation Exchange) is Amazon's route to Audible, Apple Books, and Amazon. Its ACX technical requirements are among the strictest of any major audiobook platform. Three numbers cause most of the rejections: RMS loudness between -23 and -18 dBFS, peak level below -3 dBFS, and noise floor below -60 dBFS RMS.[1] Get those right (along with format, sample rate, channel, and silence rules) and your files pass everywhere, including Voices by INaudio, Authors Republic, and Google Play Books.
Want to know where your own chapter stands on every one of these specs? Run the Free ACX Check. The file stays in your browser, and the report shows each value (RMS, true peak, noise floor, head and tail silence, sample rate, and channels) against the ACX target.
The ACX figures on this page were verified against the ACX help centre on 21 September 2026.
How do ACX requirements compare to other platforms?
ACX has the tightest constraints. The other three accept wider formats or looser silence windows, but all of them accept files mastered to ACX standards. This is why the ACX values are the ones worth memorising. For the full ACX vs Voices by INaudio vs Google Play comparison, including where each platform actually diverges, see the platform comparison guide.
| Platform | Format | Rate | Ch. | Bitrate | RMS dBFS | Peak | Noise | Head | Tail | Stereo | Max len. |
|---|---|---|---|---|---|---|---|---|---|---|---|
| ACX[1] | MP3 | 44.1 kHz | Mono/Stereo | 192k CBR | -23 to -18 | <-3 | <-60 | 1-5 s | 1-5 s | Yes | 120 min |
| INaudio[4] | MP3, FLAC | 44.1 kHz | Mono/Stereo | 192k CBR | -23 to -18 | <-3 | <-60 | n/p | 1-5 s | Yes | Yes |
| Authors Republic[5] | MP3 | 44.1 kHz | Mono/Stereo | 192k CBR | -23 to -18 | <-3 | <-60 | 1-5 s | 1-5 s | Yes | 170 MB |
| Google Play[6] | MP3, M4A, FLAC, WAV | 44.1 kHz | Mono/Stereo | 128k+ | n/p | n/p | n/p | n/p | n/p | Yes | None |
Peak and noise figures are dBFS; head and tail are silence windows in seconds. "n/p" means the platform does not publish a figure openly (INaudio's full technical requirements sit behind its sign-in).
Master to ACX and your files meet or exceed every other platform's requirements. If you distribute through multiple platforms (and most narrators do), mastering once to ACX saves you from maintaining separate versions for each distributor.
What are the 8 specs ACX measures?
ACX reviews every chapter file against these 8 audio specs.[1] The other platforms use a subset of the same list, so master to these once and your files are ready everywhere.
- RMS loudness: -23 to -18 dBFS
- Peak level: below -3 dBFS
- Noise floor: below -60 dBFS RMS
- Sample rate: 44,100 Hz
- Channels: mono or stereo, one format across every file
- Format and bitrate: MP3, 192 kbps or higher CBR
- Head silence: 1 to 5 seconds of room tone
- Tail silence: 1 to 5 seconds of room tone
These numbers exist because ACX encodes titles in a variety of formats before they go on sale in the Audible store.[1] The headroom in peaks, the CBR encoding, and the silence padding all protect your audio from degradation during that process.
One further rule sits outside the eight measured values: each file must be no longer than 120 minutes.[1] Most chapters are nowhere near that, but a single-file omnibus or a long unbroken part will need splitting before you upload.
What does each spec mean?
RMS Loudness (-23 to -18 dBFS)
The average volume of your narration across the file. Too quiet and listeners strain to hear. Too loud and it distorts or fatigues. Most mastering tools target -20 dBFS. That sits comfortably inside the range: 2 dB below the ceiling, 3 dB above the floor, with room for natural variation between chapters. Passing this window per file is not the same as sounding consistent across a whole book. See keeping chapters consistent for why chapters can each pass and still sound uneven.
If your meter shows a different scale (integrated loudness, VU, or something else), switch it to RMS before comparing against ACX's range.
Peak Level (Below -3 dBFS)
The loudest single moment in the file. ACX asks for peak levels less than -3 dB and does not state whether it reads sample peak or true peak.[1] That matters, because the two readings differ: true peak also counts the peaks that fall between digital samples, and ITU-R BS.1770 shows a sample-peak meter can under-read those by several decibels on transients with strong high-frequency content.[2] With the method unpublished, the safe target is true peak below -3 dBFS, which satisfies both readings. The headroom itself is there because ACX encodes titles in a variety of formats before they go on sale in the Audible store, and a signal without margin can distort on the way through.[1]
Noise Floor (Below -60 dBFS)
The volume of background sound during silence. HVAC hum, computer fans, refrigerators in the next room, and room ambience all contribute. It is read in the quiet gaps between words and phrases, not under the narration. If your studio is not acoustically treated, this is usually the first spec to flag. The audiobook noise floor guide covers exactly how to fix it.
Sample Rate (44,100 Hz)
44,100 Hz, the CD standard. Files recorded at 48,000 Hz (common for video work) need to be converted before submission. If your interface supports it, record at 44,100 Hz from the start and skip the conversion entirely.
Channels (Mono or Stereo)
ACX accepts either mono or stereo. What matters is consistency: every file in a project must share the same channel format, all mono or all stereo, or the submission errors out.[1] Audiobooks are spoken word, not spatial audio, so mono is the sensible choice. It halves the file size for no listening benefit and keeps every chapter uniform, which is why chapterpass masters to mono. If you recorded in stereo, collapsing to mono in your DAW is a one-click fix.
Format and Bitrate (MP3, 192 kbps CBR)
ACX asks for 192 kbps or higher, at a constant bit rate rather than variable.[1] CBR ensures the file plays reliably across all devices and apps. Variable bit rate MP3s can cause playback glitches on older hardware and streaming apps, which is why ACX enforces CBR.
Head Silence (1 to 5 seconds)
Room tone (not digital silence) at the start of each chapter. ACX recommends between 1 and 5 seconds, and the spacing must not exceed 5 seconds.[1] Gives the listener an audio cue that the chapter has begun and lets automated splitting tools locate chapter boundaries cleanly.
Record a few seconds of silence in your booth during the same session and use that as your room tone source. Digital silence (absolute zero) sounds unnatural and can cause audible clicks at the transition to speech.
Tail Silence (1 to 5 seconds)
Room tone at the end of each chapter. Provides a clear pause before the next chapter starts. Same source as your head silence: recorded room tone from your booth, not generated silence.
Why is the ACX noise floor set to -60 dBFS?
ACX does not publish a rationale for the number, but the arithmetic explains the choice. A file sitting at the middle of the RMS window, around -20 dBFS, with its noise floor at -60 dBFS, has a 40 dB gap between narration and background. That gap is what keeps a hiss or hum from reading as part of the recording when the room goes quiet between sentences. Raise the floor towards the speech level and the silences stop sounding like silence.
The practical problem is the gain budget. Raising loudness to reach -20 dBFS raises the noise floor by the same amount. A raw recording at -65 dBFS plus 8 dB of gain lands at -57 dBFS, which fails. That tension, and the approach that solves it, is the subject of the audiobook noise floor guide.
What are the specific platform details?
ACX
ACX has the strictest technical requirements of any major platform. Every file is held to the 8 specs above during audio review, and every measurement needs to pass. Note the shape of it: the ACX standards are a set of windows, not single targets, so there is room to land comfortably inside each one rather than on its edge. For a full walkthrough of every rejection type, see the guide on how to fix ACX rejection.
Voices by INaudio (Formerly Findaway Voices)
Findaway Voices rebranded as Voices by INaudio in August 2025.[4] INaudio accepts FLAC alongside MP3, handles stereo with Joint Stereo encoding, and sets its own maximum chapter length. Its loudness, peak, and noise floor targets match ACX's, so check its current silence padding rules in the INaudio knowledge base before you upload.
Authors Republic
Authors Republic accepts both mono and stereo files with a 170 MB per chapter limit.[5] Its silence padding matches ACX: 1 to 5 seconds of room tone at both the head and the tail. The main difference is that Authors Republic requires separate opening and closing credits tracks with the book title, author, and narrator name. You will need to create the credits tracks separately.
Google Play Books
Google Play Books accepts MP3, M4A (AAC), FLAC, and WAV at 128 kbps or higher.[6] Google does not publish specific loudness, peak, or noise floor targets. If your file meets ACX's specifications, it exceeds everything Google Play requires.
What are the most common questions about ACX specs?
Are the ACX audio requirements (RMS, peak, noise floor) the same for audiobook samples?
ACX does not publish separate specs for the sample. It asks for a retail sample of 5 minutes or less, ideally the opening five minutes, cut from the finished audiobook.[1] Master the whole book to spec and the sample takes care of itself: an excerpt of a compliant production comes from the same chain that already meets the loudness, peak, and noise floor targets.
Is there a maximum file length for ACX?
Yes. Each file must be no longer than 120 minutes.[1] Chapters rarely come close, so this one usually only bites when a whole part or a short book has been exported as a single file. Split it at a chapter or section boundary and give each piece its own room tone at the head and the tail.
What is the difference between sample peak and true peak for ACX?
Sample peak is the loudest single digital sample in the file, and it is what Audacity's ACX Check and most DAW meters report. True peak estimates the actual analogue level between samples by oversampling the signal 4x, the method ITU-R BS.1770 specifies.[2] The gap is not fixed: ITU-R BS.1770 demonstrates a 3 dB under-read on an unfortunately phased tone at a quarter of the sampling rate, and notes that under-reading on real transients with significant high-frequency content "can commonly be several dBs".[2] ACX asks for peaks less than -3 dB and does not publish which reading it takes,[1] so the safe move is to land true peak under -3 dBFS. A file that clears true peak clears sample peak automatically.
Can I submit a WAV or FLAC file to ACX?
No. ACX requires MP3 at 192 kbps or higher CBR. WAV and FLAC are accepted by INaudio, Authors Republic, or Google Play, but ACX itself accepts only MP3 uploads.[1]
Does ACX require mono, or is stereo allowed?
Either is allowed. ACX accepts mono or stereo, as long as every file in the project uses the same format: all mono or all stereo. Mixed formats cause errors.[1] Mono is still the sensible choice for spoken word: a single narrator in stereo is two identical channels, doubling the file size for no listening benefit, so chapterpass masters to mono.
What happens if my chapter is longer than the silence window allows?
Silence rules are per file, not per chapter length. ACX recommends 1 to 5 seconds of room tone at both the head and the tail, and the spacing must not exceed 5 seconds, regardless of runtime.[1] If your tail silence is 8 seconds, trim it back. If it is 0 seconds, add room tone recorded from your booth.
If you are weighing your options for how to hit these targets, the audiobook mastering tools compared guide puts chapterpass next to Auphonic, Audacity, and the rest. For quick answers on pricing and privacy, see the FAQ.
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