How to Use ACX Check in Audacity for Audiobooks

The ACX Check plugin is a free, downloadable Nyquist tool for Audacity that allows voice actors and audiobook producers to instantly test their audio files against the technical delivery standards mandated by ACX (Audible's Audio Creation Exchange). This article explains what the plugin measures, how it interprets your audio data, and the exact steps required to analyze your tracks to guarantee compliance before submitting them for publication.

What is the ACX Check Plugin?

ACX Check is an analysis script created for Audacity that evaluates whether a track meets the three foundational technical requirements for audiobook distribution on Audible, Amazon, and iTunes: Peak Level, RMS (Root Mean Square) Level, and Noise Floor. Instead of requiring users to manually calculate these measurements using multiple meters and math formulas, the plugin scans the selected audio file and presents a clear summary panel indicating whether the track passes or fails.

Key Metrics Verified by ACX Check

To pass ACX quality control, an audio file must meet specific statistical benchmarks. ACX Check calculates and grades your recording against the following:

• Peak Level: The plugin measures the highest point of volume in the audio track. ACX requires peak levels to not exceed -3.0 dB. This headroom prevents clipping and digital distortion during file compression. If a peak crosses above -3.0 dB, the plugin marks this test as a failure.
• RMS Level: RMS represents the average loudness, energy, or perceived volume of the performance over time. ACX requires an RMS level between -23.0 dB and -18.0 dB. The plugin calculates the average energy across the selected waveform to verify that the track is neither too quiet nor excessively compressed.
• Noise Floor: The noise floor measures the residual background sound present during moments of room tone or speech pauses (such as HVAC hum, computer fans, preamp hiss, or preamp thermal noise). ACX demands that the noise floor be -60.0 dB or lower. ACX Check detects the quietest passage in your selection to determine if background noise remains within this threshold.

How to Run ACX Check in Audacity

1. Install the Plugin: Download the acx-check.ny file. Place it into Audacity's Plug-Ins directory. Open Audacity, navigate to Tools > Plugin Manager, locate ACX Check, and set it to Enable.
2. Select Your Audio: Highlight the entire track or a representative chapter section that includes both speech and typical room-tone pauses.
3. Run the Analysis: Go to the top menu bar, select Analyze, and click on ACX Check.
4. Read the Results Dialog: A pop-up box will appear displaying the exact numbers for your Peak, RMS, and Noise Floor, alongside green "Pass" tags or red "Fail" indicators.

Troubleshooting Failed Results

• Peak Level Fails (Too High): Use Audacity’s Limiter effect to set a hard limit at -3.0 dB, or slightly lower (such as -3.5 dB), to catch stray peaks without audibly altering the performance.
• RMS Fails (Too Low or Too High): If RMS is below -23 dB, the audio is too quiet; apply slight compression and amplify. If RMS is higher than -18 dB, the track is over-processed and loud; lower the overall gain.
• Noise Floor Fails (Higher than -60 dB): A noise floor higher than -60 dB (for example, -55 dB) indicates excessive background sound. Apply gentle noise reduction, check for microphone preamp gain issues, or eliminate physical noise sources in the recording environment. Note that applying excessive digital noise reduction to pass the test may degrade voice quality, which can trigger a human quality control rejection during manual review.