The three platforms share one core loudness standard and differ mainly on file format. ACX, Voices by INaudio (formerly Findaway Voices) and Google Play Books all sit happily with RMS loudness between -23 and -18 dBFS and true peak below -3 dBFS. ACX and Voices by INaudio both also require the noise floor below -60 dBFS.[1][2] ACX is the strictest on format: MP3 at 192 kbps CBR, 44.1 kHz, silence padding at the head and tail, and no file longer than 120 minutes, and it accepts mono or stereo as long as every file in the submission matches.[1] Voices by INaudio wants the same loudness targets and is format-flexible too, accepting mono or stereo and MP3 or FLAC.[2] Google Play Books publishes only format and bitrate rules, not loudness targets: it accepts MP3, M4A (AAC), WAV and FLAC, mono at 128 kbps or higher, stereo at 256 kbps or higher, at 44.1 kHz or above, with a book length of 5 minutes to 100 hours.[3]
The practical upshot is simple. Master once to ACX spec and the same file clears all three, because ACX is the tightest standard on format. Build for ACX and you are already inside everyone else's requirements. This guide states the numbers side by side, walks each platform's audio submission requirements in turn, and shows where the real differences hide. For the full ACX spec and what each metric means, the flagship guide covers every number in depth. This page compares them.
How do ACX, Voices by INaudio, and Google Play audio requirements compare?
ACX is the strictest of the three on file format, so a file mastered to ACX spec clears the other two without any reprocessing. The table below lines up the specs that decide whether a file passes: RMS, true peak, noise floor, sample rate, channels, format and bitrate, room tone, and maximum file length.
| Requirement | ACX[1] | Voices by INaudio[2] | Google Play[3] |
|---|---|---|---|
| RMS loudness | -23 to -18 dBFS | -23 to -18 dBFS | not published |
| True peak | below -3 dBFS | below -3 dBFS | not published |
| Noise floor | below -60 dBFS | below -60 dBFS | not published |
| Sample rate | 44.1 kHz | 44.1 kHz | 44.1 kHz or higher |
| Channels | mono or stereo (must match across files) | mono or stereo | mono or stereo |
| Format and bitrate | MP3 only, 192 kbps CBR | MP3 192 kbps CBR, or FLAC | MP3, M4A, WAV, FLAC; 128 kbps+ mono, 256 kbps+ stereo |
| Room tone (head and tail) | 1 to 5 s, recommended | broadly tracks ACX | not published |
| Max file length | 120 minutes | 120 minutes | book: 5 min to 100 hrs |
The takeaway sits in the loudness rows. ACX and Voices by INaudio ask for the same RMS, peak and noise floor numbers, and Google Play does not gate on any of them. So the loudness question is settled the moment you hit the ACX window. All three accept mono or stereo, so channels are not where the platforms diverge. What is left is format: ACX accepts MP3 only, and there it is the narrowest of the three. If your file is a genuine ACX master, every column above is already satisfied. For the file-by-file mechanics behind the silence padding and the 120-minute cap, see the step-by-step ACX submission checklist.
What are the ACX audio requirements?
ACX is the strictest of the three, and the only one that runs a fully automated analysis before a human listens. A file has to clear these specs to pass.[1]
- RMS loudness: between -23 and -18 dBFS.
- True peak: below -3 dBFS.
- Noise floor: below -60 dBFS.
- Sample rate: 44.1 kHz.
- Channels: mono or stereo, and every file in the submission must use the same one.
- Format and bitrate: MP3 at 192 kbps CBR or higher.
- Room tone: ACX recommends 1 to 5 seconds of room tone at the head and tail of each file, and asks that it not exceed 5 seconds.
- Maximum file length: 120 minutes per file.
That is the whole list. This page states the numbers rather than teaching each one from scratch, because the flagship guide already explains the full ACX spec and what each metric means in depth. The short version worth carrying with you: most solo narrations are mono anyway, since a single narrator in stereo is just two identical channels doubling the file for no listening benefit, and ACX asks for CBR encoding and peak headroom because Audible re-encodes your file into several formats for different devices. Everything about ACX's format rules is built around surviving that re-encoding cleanly.
What are the Voices by INaudio (formerly Findaway Voices) audio requirements?
The loudness and noise floor targets are identical to ACX. Voices by INaudio wants RMS between -23 and -18 dBFS, true peak below -3 dBFS, and noise floor below -60 dBFS, the same three numbers that decide most ACX rejections.[2] If you have mastered to ACX spec, your RMS, peak and noise floor are already exactly where Voices by INaudio needs them.
Findaway Voices rebranded to Voices by INaudio in August 2025, and the audio requirements did not change with the name.[2] Where it differs from ACX is format. Voices by INaudio accepts MP3 at 192 kbps CBR or FLAC, and, like ACX, it accepts both mono and stereo. Stereo submissions use joint stereo encoding, which stores the shared information between the two channels once rather than twice, keeping the file efficient. Room tone and file-length rules broadly track ACX's own, and the same 120-minute cap applies per file.[2] So a mono MP3 built for ACX passes here untouched, and if you ever want a FLAC version, this is one of the platforms that will take it. The noise floor is still the gate to watch: if you raise loudness and push the noise floor above -60 dBFS, the file fails here for the same reason it would on ACX.
What are the Google Play Books audiobook audio requirements?
Google Play Books is the outlier. It publishes format, bitrate, sample rate and length rules, but no loudness, peak or noise floor targets at all.[3] Here is what it does state:
- Formats: MP3, M4A (AAC), WAV (PCM, 16-bit) and FLAC (16-bit).
- Bitrate: 128 kbps or higher for mono, 256 kbps or higher for stereo, CBR preferred.
- Sample rate: 44.1 kHz or higher.
- Length: the book must run from 5 minutes to 100 hours.[3]
That is genuinely the whole audio specification. Google does not ask for an RMS window, a peak ceiling or a noise floor at all; its published policies cover format, bitrate and content quality, not loudness targets.[3][4] The honest guidance: the absence of a published target is not permission to be inconsistent. If you master to ACX's RMS -23 to -18 dBFS and true peak below -3 dBFS, your book sits at a sensible, even level across phones, tablets and speakers, and a clean noise floor still matters to the listener even when no automated analysis is measuring it. Master to the ACX numbers and Google Play is comfortably covered.
Where do the three platforms actually differ?
Loudness is the shared part, so it is not where the differences live. ACX and Voices by INaudio ask for the identical RMS, peak and noise floor, and Google Play is silent on all three. Once you are inside the ACX loudness window, that whole question is answered for every platform at once.
The real differences are two:
- Format. ACX is MP3 only. Voices by INaudio adds FLAC. Google Play is the widest, accepting MP3, M4A, WAV and FLAC.
- Whether loudness is enforced at all. ACX and Voices by INaudio both gate on loudness and noise floor. Google Play does not.
Channels are not a real point of difference: all three accept mono or stereo, though ACX requires every file in a submission to match. Silence padding and the 120-minute file cap match on ACX and Voices by INaudio, so those are not points of difference either. Here is the trap worth knowing about: a FLAC file passes cleanly on Voices by INaudio and Google Play, then gets rejected by ACX on format alone, because ACX takes MP3 only. The direction of failure always runs the same way. A file built for the flexible platforms can fail the strict one, never the other way around. That single fact is what makes the ACX submission checklist the right thing to build against first.
Can I use the same audio file for all three platforms?
Yes, if you master to ACX spec, because it is the tightest of the three on format. One ACX-compliant file, an MP3 at 192 kbps CBR and 44.1 kHz, with RMS inside the window, peak and noise floor in spec, and room tone padded at the head and tail, is accepted by Voices by INaudio and by Google Play Books without any reprocessing.[1][2][3] You build one file and it travels. That is the entire payoff of mastering to the strictest standard.
The one caveat is not about audio at all. It is about distribution rights. ACX offers exclusive and non-exclusive distribution, and choosing exclusive distribution means you cannot sell or distribute the audiobook anywhere outside Audible, Amazon and Apple Books while that term runs.[5] To reuse your master on Voices by INaudio and Google Play, you need to choose non-exclusive distribution on ACX, or distribute your own files directly. That is a contract choice, and it also touches who is allowed to narrate and how ACX handles eligibility. The audio itself is portable the moment it is ACX-compliant. Only the paperwork decides whether you are allowed to move it.
Which platform should you master for first?
Master for ACX, always. It is the strictest on format, which makes it the universal baseline. A file built to ACX spec already meets Voices by INaudio and Google Play Books in full, so you never have to master the same book twice.[1][2][3] The reverse never holds: a FLAC file built for the flexible platforms will fail ACX on format alone, so starting anywhere else just means redoing the work.
Think of it as one target that happens to satisfy everyone. Hit the ACX RMS window, keep the peak below -3 dBFS and the noise floor below -60 dBFS, export an MP3 at 192 kbps CBR with your room tone padded, and you have a single file ready for all three storefronts. If you are weighing how to hit those numbers, the guide on audiobook mastering tools compared lines up the options side by side.
Common Questions
Do Findaway Voices and ACX have the same audio requirements?
Does Google Play Books have a loudness or noise floor requirement?
No. Google Play Books publishes only format, bitrate, sample rate and length rules, not RMS, peak or noise floor targets.[3] Because nothing gates loudness on submission, mastering to ACX's loudness spec is the safe way to stay consistent across devices.
Can I upload my ACX file to Findaway Voices and Google Play without changes?
Yes, if it is a genuine ACX master: MP3 at 192 kbps CBR, 44.1 kHz, with in-spec loudness, peak, noise floor and room tone.[1] ACX is the strictest standard on format, so a file that passes ACX also clears Voices by INaudio and Google Play Books.[2][3] Reprocessing is only needed if you started from a looser platform's file.
What file formats does each platform accept?
ACX accepts MP3 only, mono or stereo, at 192 kbps CBR, with every file in a submission matching format.[1] Voices by INaudio accepts MP3 at 192 kbps CBR or FLAC, mono or stereo.[2] Google Play Books is the widest: MP3, M4A (AAC), WAV (PCM 16-bit) and FLAC, mono at 128 kbps or higher and stereo at 256 kbps or higher.[3]
Did Findaway Voices change its requirements after becoming Voices by INaudio?
No. Findaway Voices rebranded to Voices by INaudio in August 2025, and the audio requirements stayed the same: RMS -23 to -18 dBFS, true peak below -3 dBFS, noise floor below -60 dBFS, MP3 or FLAC, 44.1 kHz.[2]
Does ACX require mono, or does it allow stereo too?
ACX accepts mono or stereo, on the condition that every file in your submission uses the same channel format.[1] Most solo narrations are mastered mono anyway, because a single narrator in stereo is just two identical channels, doubling the file size for no listening benefit. Voices by INaudio and Google Play also accept either, so mono works everywhere and stays the most portable choice, but it is not an ACX-only rule.
One master to ACX spec clears all three platforms. Master your first file free in your browser, no account, no upload, at chapterpass.