ACX Audio Requirements
Explained — Clearly.
ACX technical rejections are one of the most frustrating experiences in self-publishing — they delay releases by weeks and often require re-recording. Every single one is preventable. This guide explains all seven ACX specifications in plain language, why the platform enforces them, and where home recordings most commonly go wrong.
Covers: all 7 ACX specs · common rejection causes · how each failure happens · how PHQ prevents every one
Quick Reference: ACX Technical Requirements at a Glance
| Specification | Required Value | Common Failure Mode | PHQ Status |
|---|---|---|---|
| RMS Loudness | -23 dB to -18 dB | Home narrators often record too quietly and over-normalize in post, creating audible "pumping" artifacts. | Always met |
| Peak Level | -3 dB maximum | Home narrators who record with excessive gain often clip during moments of emphasis — a "d" or "b" consonant, an excited phrase — without realizing it. | Always met |
| Noise Floor | Below -60 dB RMS | Home recording almost always fails noise floor requirements. | Always met |
| Sample Rate | 44.1 kHz | Home narrators using video or podcast settings often record at 48 kHz (the video standard) and submit without converting. | Always met |
| Bit Rate (MP3) | 192 kbps constant bit rate | Many audio editing programs default to VBR encoding. | Always met |
| Room Tone | 0.5–1 sec at head and tail | Home narrators often begin recordings immediately without capturing room tone, or use dead silence (completely noise-free) rather than the room's actual ambient sound. | Always met |
| File Format | Mono or joint stereo MP3 | Home narrators using multi-channel interfaces sometimes record and export in true stereo without realizing the difference. | Always met |
Each Requirement — In Full Detail
For each specification: what it is, why ACX requires it, how PHQ ensures compliance, and where home recordings typically fail.
RMS Loudness
-23 dB to -18 dBWhat It Is
RMS (Root Mean Square) is a measure of average perceived loudness — how loud the audiobook sounds to the listener over time. Think of it as the "volume level" of your finished audio.
Why ACX Requires It
Too low (-24 dB or quieter) and the audiobook sounds thin and quiet — listeners will constantly need to adjust their volume. Too high (-17 dB or louder) and it sounds over-compressed and harsh. Outside this range: automatic rejection.
How PHQ Ensures Compliance
We monitor RMS during recording (to avoid grossly over- or under-level source audio) and apply compression and normalization during mastering to land every chapter consistently within the -23 to -18 dB range.
Where Home Recording Fails
Home narrators often record too quietly and over-normalize in post, creating audible "pumping" artifacts.
Peak Level
-3 dB maximumWhat It Is
Peak is the loudest single instantaneous moment in your audio — the highest spike. Even one sample that clips above -3 dB triggers a rejection, even if the rest of the file sounds perfect.
Why ACX Requires It
Peaks above this threshold introduce audible distortion. On headphones and in-car speakers — the most common Audible listening environments — this distortion is clearly noticeable to listeners.
How PHQ Ensures Compliance
Transparent peak limiting is applied during mastering to ensure no chapter exceeds -3 dB. The limiter is set conservatively so it catches spikes without audibly affecting the narration.
Where Home Recording Fails
Home narrators who record with excessive gain often clip during moments of emphasis — a "d" or "b" consonant, an excited phrase — without realizing it.
Noise Floor
Below -60 dB RMSWhat It Is
The noise floor is the level of background sound during silence — the room's ambient noise between words, sentences, and chapters. This is measured during the quiet gaps in your audio.
Why ACX Requires It
Any hum, hiss, HVAC rumble, electrical noise, or room tone above -60 dB is audible during quiet moments and chapter breaks. Audible's quality review team checks this manually in addition to automated detection.
How PHQ Ensures Compliance
Acoustic treatment in our isolation booths eliminates the problem at the source. Noise reduction is applied as a secondary measure where needed. The result is a noise floor reliably below -65 dB on every chapter.
Where Home Recording Fails
Home recording almost always fails noise floor requirements. HVAC systems, refrigerator compressors, neighborhood traffic, and electrical hum from untreated rooms all show up in the noise floor measurement.
Sample Rate
44.1 kHzWhat It Is
The number of audio samples captured per second. 44.1 kHz is the CD standard and what ACX specifies for submission.
Why ACX Requires It
Incorrect sample rates (48 kHz, 96 kHz) cause technical errors during upload processing, even when the audio itself sounds perfectly fine. The platform's technical review rejects mismatched sample rates automatically.
How PHQ Ensures Compliance
All session recordings and final exports are configured to 44.1 kHz. This is set in the session template and never varies.
Where Home Recording Fails
Home narrators using video or podcast settings often record at 48 kHz (the video standard) and submit without converting.
Bit Rate (MP3)
192 kbps constant bit rateWhat It Is
The amount of data per second used to encode the audio file. ACX requires constant bit rate (CBR) MP3 — not variable bit rate (VBR), which is common in music and podcast workflows.
Why ACX Requires It
Variable bit rate files can cause upload and playback errors on Audible's platform. Some also produce audibly lower quality during quiet sections where VBR reduces bit rate.
How PHQ Ensures Compliance
Export settings are locked to 192 kbps CBR MP3 in our delivery workflow. Files arrive in the exact encoding ACX requires, every time.
Where Home Recording Fails
Many audio editing programs default to VBR encoding. Authors who export "to MP3" without checking the settings often export variable bit rate files without realizing it.
Room Tone
0.5–1 sec at head and tailWhat It Is
Each chapter file must begin and end with a brief period of silence — specifically the room's ambient sound (not dead silence) — to give listeners and platforms a clean entry and exit point.
Why ACX Requires It
Missing or incorrectly formatted room tone causes abrupt chapter starts and ends that trigger human review rejection. It also sounds jarring to listeners when the file cuts directly into narration.
How PHQ Ensures Compliance
Room tone is captured at the start of every session and added to the head and tail of each chapter file during post-production as part of our standard workflow.
Where Home Recording Fails
Home narrators often begin recordings immediately without capturing room tone, or use dead silence (completely noise-free) rather than the room's actual ambient sound.
File Format
Mono or joint stereo MP3What It Is
ACX does not accept true stereo files (where left and right channels carry different audio). Audiobooks must be mono or joint stereo — both channels identical.
Why ACX Requires It
True stereo files can cause playback issues on Audible's platform, particularly on mono speakers. Audiobook narration is inherently mono content.
How PHQ Ensures Compliance
All final exports are mono or joint stereo. This is a non-optional step in our delivery workflow.
Where Home Recording Fails
Home narrators using multi-channel interfaces sometimes record and export in true stereo without realizing the difference.
The 6 Most Common ACX Rejection Causes
Every one of these is preventable with professional production. Every one causes delays of 2–4+ weeks when they happen.
Noise floor too high
HVAC, street noise, electrical hum in home recording environment. Cannot be fully removed in post without damaging the voice.
RMS out of range
Recording too quietly and over-normalizing in post, or recording too hot without headroom management. Both cause quality issues even if the number looks right.
Incorrect sample rate
Recording or exporting at 48 kHz (the video/podcast standard) rather than 44.1 kHz.
Variable bit rate MP3
Exporting with a VBR setting rather than 192 kbps CBR. Common default in many editing programs.
Missing room tone
Starting the recording immediately without a 0.5–1 second ambient silence buffer at the head and tail of each file.
Peak clipping
Recording with gain set too high, causing distortion on consonant attacks and emphasis moments.
Related ACX & Production Resources
ACX Requirements — FAQs
More Audiobook Resources
Never Get Rejected by ACX Again
When you record with PHQ, technical compliance is our responsibility. Every file we deliver meets every ACX specification — we verify before we send.