ACX Audiobook Mastering — Meet Audible Standards Online Free

A free browser-based mastering tool that checks and fixes your audiobook files to meet ACX/Audible technical requirements. Automatically normalizes RMS loudness to -20 dB, limits peaks below -3 dB, reduces noise floor below -60 dB, and exports as 192 kbps CBR MP3 — the exact specs required by Amazon ACX.

How to Use

Upload Your Audio

Drag and drop your audiobook chapter file (MP3, WAV, FLAC, or other format) into the upload zone.

Review ACX Analysis

The tool instantly checks RMS loudness, peak levels, and noise floor against ACX requirements and shows pass/fail for each.

Auto-Master or Adjust

Click "Auto-Master" for one-click fixing, or fine-tune individual settings like target RMS, peak limit, and noise reduction strength.

Preview and Compare

Use the A/B toggle to compare before and after mastering. Ensure the audio sounds natural without over-processing.

Download ACX-Ready File

Export as 192 kbps CBR MP3 at 44.1 kHz mono — the exact format required by ACX/Audible.

Normalize loudness, limit peaks, and eliminate noise to publish on Audible

Drop your audiobook file here or click to browse

MP3, WAV, OGG, FLAC, AAC, M4A, M4B, WMA, AIFF, OPUS, WEBM, CAF, 3GP, MP4, MOV, MKA ·

Loading file...

🎙

ACX Compliance Check

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RMS Level

—

-23 to -18 dB

—

Peak Level

—

Below -3 dB

—

Noise Floor

—

Below -60 dB

—

Sample Rate

—

44.1 kHz or higher

—

Channels

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Mono

—

Format

—

192 kbps CBR MP3

0:00 / 0:00

ACX targets: RMS −20 dB · Peak ≤ −3 dB · Noise ≤ −60 dB · Mono · 192 kbps CBR MP3

Mastering Settings ▼

Target RMS

-20 dB

ACX requires -23 to -18 dB. Default -20 dB is the sweet spot.

Peak Limit

-3.5 dB

ACX maximum is -3 dB. We use -3.5 dB for safety margin.

Noise Reduction

Heavy reduction may introduce artifacts. Use the lightest setting that achieves -60 dB.

Room Tone

Head: s Tail: s

ACX requires 0.5-1s at head, 1-5s at tail. Uses matched room tone, not digital silence.

🎚 Gain

Gain 0 dB

Limiter -1.0 dB

🎛 Equalizer

Low (200 Hz) 0 dB

Mid (1 kHz) 0 dB

High (5 kHz) 0 dB

⚡ Compressor

Threshold -24 dB

Ratio 4:1

🔇 Noise Reduction

Heavy reduction may introduce artifacts. Use the lightest setting that achieves -60 dB.

💡 Adjust parameters and see changes in real-time on the waveform. Press A/B to compare before and after during playback.

Mastering...

✓

Mastering Complete!

Format Bitrate Channels

Batch Processing

Features

ACX Compliance Check One-Click Auto-Mastering Smart Room Tone Batch Chapter Processing

Frequently Asked Questions

What are the ACX audio requirements?

ACX requires MP3 files at 192 kbps CBR or higher, 44.1 kHz sample rate, mono channel, RMS between -23 and -18 dB, peaks no higher than -3 dB, and noise floor below -60 dB. Each chapter needs 0.5-1 second of room tone at the beginning and 1-5 seconds at the end.

Will this tool fix my audio automatically?

Yes. The Auto-Master feature normalizes RMS to -20 dB (the sweet spot within ACX range), limits peaks to -3.5 dB with a safety margin, applies noise reduction if needed, adds room tone, and converts to the required format — all automatically.

Can I process multiple chapters at once?

Yes. Upload all your audiobook chapter files and the tool will analyze and master each one individually, maintaining consistent loudness across all chapters. Download them all as a ZIP file.

Is this the same as the ACX Check plugin for Audacity?

The ACX Check plugin only checks whether your audio meets requirements — it does not fix anything. This tool both checks AND fixes your audio to meet ACX specs in one step.

Will noise reduction make my audio sound worse?

The tool uses the lightest noise reduction possible to achieve -60 dB. You can preview the result with A/B comparison before saving, and adjust the strength from Light to Heavy. If your recording environment is already quiet, no noise reduction may be needed.

💡 Want us to improve this tool just for you?

We can — and it's free! Just send us a quick message with your idea. If you'd like to discuss it in detail, leave your email and we'll get back to you. You can stay anonymous.

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Published 24 Mar 2026 Updated 06 May 2026