Most audiobook distributors use the same core audio specs as ACX. If your files pass ACX's eight-point quality check, they pass everywhere: Findaway Voices, Author's Republic, PublishDrive, and the rest all require the same loudness, peak, and noise floor targets.[1] The differences come down to format flexibility (stereo, FLAC) and file size limits, not stricter numbers. This post compares the exact technical requirements across every major platform so you can master once and distribute everywhere.
| RMS | Peak | Floor | Rate | Channels | Format | |
|---|---|---|---|---|---|---|
| ACX | ||||||
| Findaway | ||||||
| Author’s Rep | ||||||
| PublishDrive | ||||||
| Kobo | ||||||
| Google Play |
What Audio Requirements Does ACX/Audible Have?
ACX enforces the most detailed and strictly automated quality checks in the audiobook industry.[1] Every chapter file must pass all 8 specifications before acceptance. The ACX audio requirements guide explains each spec in depth. Here are the exact numbers:
- RMS Loudness: -23 to -18 dBFS. RMS (root mean square) measures the average signal power of your audio file, expressed in decibels relative to full scale
- True Peak: below -3 dBFS. True peak is the maximum amplitude of the reconstructed analog waveform between digital samples, measured via 4x oversampling per ITU-R BS.1770
- Noise Floor: below -60 dBFS. The ambient background level during silent sections, measured as RMS of the quietest passages
- Sample Rate: 44,100 Hz
- Channels: Mono
- Format: MP3, 192 kbps CBR (constant bit rate)
- Head Silence: 0.5 to 1 second of room tone
- Tail Silence: 1 to 5 seconds of room tone
ACX runs automated checks on every file. Fail any single spec and the file gets rejected. There is no manual override for the technical requirements. See the ACX platform page for a full breakdown of what makes ACX different from other distributors.
Worth noting: ACX's specs aren't arbitrary. The -23 to -18 dBFS RMS range overlaps with broadcast loudness standards (EBU R128), though the measurement systems differ. ACX uses RMS (average signal power in dBFS) while EBU R128 uses LUFS (loudness units relative to full scale, with frequency weighting per ITU-R BS.1770).[1] The lower bound of -23 is the same number, but the units and methodology aren't identical. For spoken word, RMS and LUFS typically track within 1–2 dB, which is why the overlap exists. The -3 dBFS peak ceiling accounts for inter-sample peaks that MP3 encoding can introduce. The -60 dBFS noise floor threshold reflects what's audible on earbuds and headphones in quiet environments. These numbers are engineered for the end listener's experience, which is why other platforms have adopted the same targets.
What Audio Requirements Does Findaway Voices Have?
Findaway Voices (rebranded as Voices by INaudio in August 2025) uses requirements that match ACX almost exactly.[2] RMS must measure between -23 and -18 dB, peaks must stay below -3 dB, and the noise floor must not exceed -60 dB. Files must be MP3 at 192 kbps or higher CBR, 44.1 kHz sample rate. The Findaway Voices platform page covers all Findaway-specific details.
The key differences from ACX:
- Channels: Findaway accepts both mono and stereo (stereo should be exported as Joint Stereo). ACX requires mono only.
- Format: Findaway also accepts FLAC, a lossless compressed format that preserves the full audio quality of the original recording, in addition to MP3. ACX only accepts MP3.
- Silence: 0.5 to 1 second at the head, 1 to 5 seconds at the tail, identical to ACX.
In practice, if your files meet ACX specs, they'll pass Findaway's checks. The extra flexibility Findaway offers (stereo, FLAC) is optional, not required.
One thing to keep in mind with Findaway: because they distribute to dozens of retailers, each with their own playback systems, mastering to the stricter ACX loudness and peak specifications ensures your audio sounds consistent everywhere Findaway sends it. A file that's technically acceptable at Findaway's minimum could sound noticeably different on one retailer's app versus another's. ACX-level mastering eliminates that variability.
What Audio Requirements Do Other Distributors Have?
Most audiobook distributors have adopted the same core specs as ACX because they distribute to overlapping retail partners. Here's what each platform specifies.
Author's Republic publishes explicit requirements that mirror ACX: RMS between -23 and -18 dB, peaks no higher than -3 dB, noise floor below -60 dB, MP3 at 192 kbps CBR, 44.1 kHz.[3] Individual files must be under 170 MB and no longer than 119 minutes. Each file must have 1 to 5 seconds of silence at the beginning and end. All chapters must use either all mono or all stereo channel configuration.
Author's Republic offers tiered royalties (60%, 70%, or 85%) depending on production method.[3] The standard self-produced tier is 70/30, meaning the author keeps 70% and Author's Republic takes 30%. Authors using Author's Republic's production partners or marketplace narrators receive different splits. This is worth checking before committing to the platform, since the tier structure affects your per-sale revenue significantly.
PublishDrive specifies the same loudness range (-23 to -18 dB RMS), peak ceiling (-3 dB), and format requirements (MP3, 192 kbps CBR, 44.1 kHz). They require 0.5 to 1 second of silence at the head and 1 to 5 seconds at the tail. Individual chapters cannot exceed 78 minutes. PublishDrive recommends stereo over mono, which is the opposite of ACX's mono-only requirement.
Kobo Writing Life does not publicly document strict loudness, peak, or noise floor specifications. Their published requirements focus on file format (MP3), file size (individual files under 200 MB, total under 2 GB), and metadata. Kobo compresses audio to 64 kbps upon ingestion, so higher-fidelity source files are processed down regardless.
Google Play Books has the most flexible format requirements. They accept MP3, AAC/M4A, FLAC, and WAV. For MP3, the minimum is 128 kbps (mono) or 256 kbps (stereo), with CBR preferred. For lossless formats, they require 16-bit samples at 44.1 kHz or higher. Google does not publish specific RMS, peak, or noise floor targets. They recommend starting and ending files with silence but don't specify exact durations.
Do You Need Different Masters for Each Distributor?
No. ACX has the strictest and most specific automated requirements of any audiobook distributor.[1] If your files pass ACX's 8-point quality check, they will meet or exceed the requirements of every other major platform.
The distributors that publish explicit specs (Findaway, Author's Republic, PublishDrive) all use the same loudness, peak, and noise floor targets as ACX. The ones that don't publish detailed specs (Kobo, Google Play) accept a wider range of formats and don't enforce stricter numbers.
This means mastering for ACX is the safest default strategy. You produce one set of compliant files and distribute them everywhere. ChapterPass masters your chapters to ACX specs, the most demanding standard, so the output works across all platforms without reprocessing. See the audiobook requirements page for the full list of specs ChapterPass targets.
What About Platform-Specific Quirks?
While you don't need different masters, it helps to know about a few practical differences that can catch authors off guard during submission.
File naming conventions. ACX is strict about file naming: each chapter file should be clearly labelled in sequence (e.g., Chapter01.mp3, Chapter02.mp3). Some platforms are more flexible, but keeping a clean naming convention avoids confusion during upload. Opening credits should be the first file and closing credits the last.
File size and duration limits. Author's Republic caps individual files at 170 MB and 119 minutes. PublishDrive caps at 78 minutes per chapter. ACX doesn't publish a hard file size limit, but their system handles files up to 170 minutes. In practice, most audiobook chapters are 15 to 45 minutes, well under any platform's limit. If you have chapters exceeding an hour, consider splitting them.
Metadata requirements. Every platform requires metadata (title, author, narrator, chapter titles), but the submission process differs. ACX has a structured web form. Findaway uses a dashboard with bulk upload. Author's Republic and PublishDrive have their own metadata templates. None of this affects your audio files, but incomplete metadata will stall your submission regardless of audio quality.
Re-submission policies. If ACX rejects a file, you can fix and re-upload individual chapters without re-submitting the entire audiobook. Findaway works similarly.[2] Author's Republic may require re-uploading the complete project depending on the issue. Knowing this upfront saves time: if you master to ACX specs from the start, re-submissions for technical failures become extremely rare across all platforms.
Format conversion between platforms. If you originally recorded and edited in WAV or FLAC, keep your lossless source files archived. Converting from one lossy format to another (for example, decoding an MP3 and re-encoding it) introduces generational quality loss, degradation that accumulates each time audio is decoded and re-encoded through a lossy codec. When Google Play accepts FLAC and Findaway accepts both FLAC and MP3, export separate deliverables from your original lossless session files rather than transcoding existing MP3s. This preserves the full dynamic range and avoids stacking compression artifacts. Tools like ChapterPass work from your source audio so each export targets the correct format without quality degradation.
Which Distributor Should You Choose for Your Audiobook?
This depends on your distribution strategy, not your audio specs. ACX distributes exclusively to Audible, Amazon, and Apple Books. You can go exclusive (higher royalty share) or non-exclusive (lower royalty, but you can use other distributors simultaneously). Findaway Voices/INaudio distributes to dozens of retailers including Apple Books, Kobo, Google Play, Scribd, and library platforms.[2] Author's Republic offers similar wide distribution with a focus on indie authors.[3] PublishDrive provides global distribution including some regional stores.
Many authors use ACX for the Audible/Amazon channel and a wide distributor like Findaway or Author's Republic for everywhere else. The audio files are the same either way; the difference is business terms, royalty splits, and retail reach.
One practical tip: even if you plan to distribute only through Findaway or Author's Republic initially, master your files to ACX specs anyway. You might decide to list on ACX later, and having ACX-compliant files from the start means you won't need to reprocess anything. The stricter standard costs you nothing in quality and keeps every option open.
This post covers audio specs only. Each platform has different royalty structures, exclusivity terms, and distribution networks that are worth researching separately before you commit. For a broader comparison that includes royalties, reach, and strategy, see the audiobook distribution platform comparison.
Master once, distribute everywhere. ChapterPass masters your audiobook to ACX specs, the strictest standard. Your files will meet requirements across all major platforms.