Understanding ACX Audio Specifications in Depth

Before you can verify ACX compliance, you must know exactly what Audible, Amazon, and iTunes require for audiobooks. ACX (Audiobook Creation Exchange) enforces strict technical standards to ensure consistent playback across all devices. Failure to meet any of these specs leads to rejection. Here’s what you need to check.

Format and Technical Requirements

ACX demands audio files in MP3 format encoded at a constant bit rate of 192 kbps. The sample rate must be 44,100 Hz (44.1 kHz) with stereo channels. Although audiobooks are usually mono narration, ACX requires a stereo file – the two channels carry identical content. The bit depth is implicitly 16-bit because MP3 encoding starts from a 16-bit PCM source.

Do not submit files in WAV, FLAC, or AAC. Always output from your DAW as 192 kbps MP3. You can check these details with free tools like MediaInfo or Audacity.

Peak Level vs. RMS Level

ACX specifies that peak levels must fall between -3 dBFS and 0 dBFS. Peaks above 0 dB cause digital clipping; peaks below −3 dB are considered too quiet. However, peak level alone doesn’t guarantee good loudness. ACX also expects a RMS level (Root Mean Square) between −18 dB and −23 dB. RMS reflects average loudness – your narration should sit comfortably within that range during normal speech.

Many free tools, including Audacity, can measure RMS. Ensure your file’s RMS is consistent across all chapters; large RMS variations between files will also cause rejection.

Noise Floor and Background Noise Requirements

ACX requires a noise floor below −60 dB with minimal variation. Background noise must be consistent and not intrusive. A silent passage (room tone) should show no sudden hums, clicks, or hisses. Use free analyzers to check the noise profile – a spectrum view is often the fastest way to spot problem frequencies.

If you record in an untreated room, expect room rumble or fan noise. These can be reduced with free tools like Audacity’s noise reduction or a high-pass filter. But always verify after processing.

Free Tools for Verification: A Comprehensive Overview

You don’t need expensive software to check ACX compliance. Below are the best free tools, each with a specific strength.

MediaInfo – Checking File Format and Metadata

MediaInfo is a lightweight utility that displays every technical attribute of your MP3 file in a human-readable form. Drag your file into the window, and in “View” select “Tree” or “Text” to see:

  • Format: MPEG Audio
  • Bit rate mode: Constant (CBR)
  • Bit rate: 192 kbps
  • Sampling rate: 44.1 kHz
  • Channel(s): 2 (stereo)
  • Bit depth: 16 bits (when viewing raw stream)

MediaInfo does not analyze peak levels or noise. Use it as a first-pass check before moving to Audacity.

Audacity – The Swiss Army Knife of Audio

Audacity is the go-to free audio editor for ACX compliance checks. It can measure peak levels, RMS, sample rate, and even apply corrections. With its built-in spectrum analyzer and noise reduction, you can fix minor issues without buying plugins.

Audacity also supports FFmpeg to export MP3 at exactly 192 kbps. Install FFmpeg separately (free) and enable it in Audacity’s preferences to export as MP3.

FFmpeg – Command-Line Power for Advanced Users

If you prefer command-line automation, FFmpeg can batch-check and fix files. Use:

ffmpeg -i input.mp3 -f null - 2>&1 | findstr /i "Stream Audio"

to display codec, sample rate, channels, and bit rate. For peak level analysis, use the volumedetect filter:

ffmpeg -i input.mp3 -af volumedetect -f null - 2>&1 | findstr /i "max_volume peak"

FFmpeg is excellent for batch processing – for example, normalizing all chapters to a target RMS with a single command.

Online Analyzers – Quick Checks On the Go

When you need an instant peak level readout without installing software, AudioCheck Peaking Scale or Bear Audio Tool work in your browser. Upload a short clip (under 10 MB) and review the peak meter. These tools cannot check sample rate or noise floor thoroughly, but they are useful for quick sanity checks after editing.

Spek – Visual Spectrum Analyzer

Spek is a free, portable spectrum analyzer. Open an MP3 file and instantly see a spectrogram. Use it to visually confirm frequency content up to 22 kHz (expected for a 44.1 kHz file) and detect any high-frequency hiss or low-frequency rumble. It is not a replacement for Audacity, but it helps spot issues fast.

Step-by-Step Guide to Verifying ACX Compliance with Audacity

Follow these steps to thoroughly check one audio file. After you master the procedure, you can repeat for every chapter.

Step 1: Import and Initial Check

Open Audacity and drag your MP3 file into the workspace. At the top-left of the waveform window, Audacity displays the project rate. Verify it is 44100 Hz (44.1 kHz). If it shows a different number, the file is not ACX compliant.

Step 2: Verify Sample Rate and Bit Depth

Click on the Audio Track dropdown menu (the small arrow at the left edge of the track). Under “Set Sample Format,” ensure it reads “16-bit PCM” (that’s the default). If it says “32-bit float,” that’s fine – Audacity uses float internally – but the original file bit depth must be 16-bit.

To check the original bit depth more accurately, use File > Export > Export as MP3 and open the exported file in MediaInfo. The original must be 16-bit per sample.

Step 3: Check Peak Levels and Ensure No Clipping

Select the entire track (Ctrl+A) and go to Analyze > Measure RMS. Check the “Peak” value (at the top of the dialog). It must be between −3 dB and 0 dB. If the peak is above 0 dB, you have digital clipping – the waveform will show flat tops. Audacity warns you with a red bar at the top of the track when clipping occurs.

If peaks are too low (below −3 dB), you can use Effect > Amplify to boost the level. Enter the desired peak value (e.g., −1 dB) and click OK. Then verify again.

Step 4: Analyze RMS Levels

In the same Measure RMS dialog, look at the RMS (average) value. For ACX, it should be between −18 dB and −23 dB. If your RMS is too low, use Effect > Loudness Normalization (set to “Perceived Loudness” with a target of −20 LUFS – roughly equivalent to −20 dB RMS for speech). Always measure after normalizing to ensure you stay within the range.

Step 5: Evaluate Background Noise

Select a short segment of pure silence or room tone where no speech occurs. Go to Analyze > Plot Spectrum. The noise floor should be below −60 dB across all frequencies. If you see a spike (e.g., 60 Hz hum), you need to remove it with Effect > Filter Curve EQ or Noise Reduction.

For noise reduction: capture a noise profile from a silent section, then apply Noise Reduction to the entire file. Reduce noise by 12-18 dB at most to avoid artifacts. Re-check the noise floor after processing.

Step 6: Apply Corrections if Needed

Always save a copy before editing. After making any changes, re-export as MP3 at constant bit rate 192 kbps: File > Export > Export as MP3, choose “Constant” for bit rate mode, and set quality to “192 kbps”. Then rerun all checks on the exported file.

Common ACX Rejection Reasons and How to Fix Them

Understanding why files fail saves you time. Here are the top reasons and free solutions.

Clipping or Overly Loud Peaks

Peaks above 0 dB cause distortion. Fix: Apply Effect > Amplify with a negative dB value until peaks fall to −1 dB. Use “Allow clipping” unchecked. For overly quiet files, amplify to −1 dB but check RMS – you may need a compressor to bring up average level without pushing peaks.

Too Much Background Noise

Noise floor above −50 dB or audible hiss/rumbles will get your file rejected. Fix: Use Audacity’s Noise Reduction (effect > noise reduction). Capture a clean noise profile from a silent part. Reduce by 12-18 dB. For persistent hum, apply a High-Pass Filter at 80 Hz (speech rarely uses lower frequencies) and a Low-Pass Filter at 10 kHz to cut high-frequency hiss.

Incorrect Sample Rate or Bit Depth

If Audacity shows a project rate other than 44100 Hz, the file was recorded or exported wrong. Fix: You cannot up-sample correctly without quality loss. Re-record or re-export from your DAW at 44.1 kHz, 16-bit. For bit depth, avoid 24-bit files – ACX requires 16-bit. Convert with File > Export > Export as WAV (choose 16-bit PCM) then re-encode as MP3.

Poorly Mixed Stereo or Phase Issues

If your file sounds hollow or phase-cancelled when summed to mono, ACX may reject. Fix: In Audacity, go to Tracks > Mix > Mix Stereo Down to Mono and listen. If volume drops drastically, you have phase problems. Re-record with identical left/right channels. For a quick fix, duplicate one channel to both (Split Stereo Track, delete left, copy right to both).

Final Checklist Before Submission

Use this list to confirm every chapter is ready:

  • Format: MP3, constant bit rate 192 kbps
  • Sample rate: 44,100 Hz (44.1 kHz)
  • Channels: Stereo (identical content in both)
  • Peak level: Between −3 dB and 0 dB (no clipping)
  • RMS level: Between −18 dB and −23 dB
  • Noise floor: Below −60 dB with consistent background
  • No DC offset: An average offset near zero (check with Analyze > Plot Spectrum, look at the 0 Hz line)
  • File duration: Each chapter between 20 minutes and 60 minutes (ACX prefers 20-30 minute chapters)

Run these checks on every file before uploading to ACX. A single non-compliant chapter can delay your entire audiobook.

Conclusion

Verifying ACX compliance doesn’t require expensive mastering suites. With free tools like Audacity, MediaInfo, FFmpeg, and Spek, you can thoroughly test every technical aspect of your files. Follow the steps outlined here, fix common issues, and submit with confidence. Consistent compliance not only speeds up approval but also ensures your listeners enjoy a professional, frustration-free experience across all Audible devices.

Remember: always check the official ACX audio specifications for the latest updates. Good luck with your audiobook production.