ACX Audio Requirements: The Complete Guide for 2026

Quick Reference: ACX Audio Specs

Before we dive into the details, here's what ACX requires for every audiobook file you submit:

If your files don't meet every one of these specs, ACX will reject your submission. Let's break down what each one means and why it matters.

Format: Why MP3 at 192 kbps?

ACX requires MP3 files encoded at 192 kbps CBR (constant bit rate). Not 128, not 320 — specifically 192.

Why constant bit rate? Variable bit rate (VBR) files can cause playback issues on some devices. CBR ensures consistent streaming quality across every Audible-compatible player.

Common mistake: Recording in MP3 format. Always record in WAV or FLAC (lossless), then convert to MP3 as the final step. Converting MP3 to MP3 degrades quality.

Sample Rate: 44.1 kHz

Your files must be 44.1 kHz — the same sample rate used for CDs. If you recorded at 48 kHz (common for video work) or 96 kHz, you'll need to downsample.

Tip: If you recorded at a higher sample rate, use a proper sample rate converter rather than a simple resample. Poor sample rate conversion can introduce artifacts.

Channels: Mono Only

ACX requires mono files. Stereo audiobooks waste bandwidth and offer no benefit for spoken word content.

If you recorded in stereo (most USB microphones default to stereo), you'll need to downmix to mono. Make sure to check levels after converting — mono summing can occasionally cause clipping if both channels had different content.

Loudness Levels: RMS and Peak

This is where most ACX rejections happen. Your audio must meet two loudness requirements simultaneously:

RMS Level: -18 dB to -23 dB

RMS (Root Mean Square) measures the average loudness of your file. ACX wants your audiobook to sit between -18 dB and -23 dB RMS.

• Too quiet (below -23 dB): Listeners have to crank their volume
• Too loud (above -18 dB): Audio sounds compressed and fatiguing

Target -20 dB RMS for the best results — right in the middle of the acceptable range.

Peak Level: -3 dB Maximum

No single peak in your audio can exceed -3 dB. This leaves headroom for MP3 encoding, which can sometimes push peaks slightly higher than the source material.

Common mistake: Normalizing peaks to -3 dB without checking RMS. Your peaks and RMS need to be correct simultaneously. Use a limiter, not just normalization.

Noise Floor: -60 dB

The noise floor is the level of background noise when nobody is speaking. ACX requires this to be -60 dB or lower.

Common noise floor problems:

• Room noise: Air conditioning, refrigerator hum, computer fans
• Self-noise: Cheap microphones with high self-noise ratings
• Electrical interference: Ground loops, USB noise, nearby electronics

How to check: Look at a silent section of your recording. The level should be at or below -60 dB. If it's not, you may need to re-record in a quieter environment rather than trying to fix it with noise reduction (which can introduce artifacts).

File Structure Requirements

Beyond audio specs, ACX has structural requirements:

• Opening credits: A separate file with title, author, and narrator
• Closing credits: A separate file with production credits
• Chapter files: One file per chapter, with 0.5-1 second of room tone at the head and 1-5 seconds at the tail
• Max length: No single file longer than 120 minutes
• Naming: Files should be clearly labeled with chapter numbers

The Retail Audio Sample

Your first 5 minutes serve as the retail sample on Audible. Make sure your first chapter sounds exceptional — it's what potential buyers will hear before purchasing.

Common Rejection Reasons

Based on narrator forums and ACX documentation, here are the most frequent reasons for rejection:

1. RMS outside range — by far the most common issue
2. Noise floor too high — especially with home recording setups
3. Peaks above -3 dB — often from plosives (p/b sounds) or mouth clicks
4. Room tone inconsistency — different noise levels between recording sessions
5. Incorrect format — WAV instead of MP3, wrong bitrate, stereo instead of mono
6. Missing head/tail room tone — files that start or end abruptly

How to Convert Your Audio for ACX

If you've already recorded your audiobook and need to convert it to ACX specs, you have a few options:

Manual approach: Use a DAW like Audacity or Adobe Audition to adjust levels, convert to mono, and export as MP3 192 kbps CBR. This requires knowing how to use a limiter, compressor, and the export settings.

One-click approach: ShiftAudioFormat has an ACX preset that handles all of this automatically — format, bitrate, sample rate, channels, and loudness normalization. Drop your files in, select the ACX preset, and download files that are ready to submit. Your files never leave your browser, so your unreleased audiobook stays private.

Pre-Submission Checklist

Before uploading to ACX, verify every file meets these criteria:

• MP3 format, 192 kbps CBR
• 44.1 kHz sample rate
• Mono channel
• RMS between -18 dB and -23 dB
• Peak at or below -3 dB
• Noise floor at or below -60 dB
• 0.5-1 second room tone at the start
• 1-5 seconds room tone at the end
• No file exceeds 120 minutes
• Opening and closing credit files included

Get these right and your submission should sail through ACX's quality check on the first attempt.