Audacity ACX Settings Guide: Effect Chain, Export Settings, and Macros

Audacity can produce ACX-compliant audiobook files. What you need are the exact settings: every effect, every parameter, in the right order. This guide is the condensed reference: no theory, just the numbers. If you want the full tutorial with explanations for why each setting matters, see the complete Audacity audiobook mastering tutorial.[1]

For context on what ACX requires and why, the ACX audio requirements guide covers every specification in depth.

What Is the ACX Mastering Chain in Audacity?

These steps must be applied in order. The sequence matters because each step depends on the output of the previous one. Changing the order will produce different results and likely cause failures.[2]

Prerequisites

• Audacity 3.x or later
• Source file: WAV or FLAC (never start from an MP3)
• Recording is fully edited, with all mistakes, clicks, and pops already removed
• File imported into Audacity

Step 1: How Do You Convert to ACX's Required Format?

If stereo → mono:

• Select all: Ctrl+A (Cmd+A on Mac)
• Tracks → Mix → Mix Stereo Down to Mono

If sample rate isn't 44,100 Hz:

• Set Project Rate (bottom-left corner) to 44100
• Tracks → Resample → 44100 Hz

Verify: Bottom-left of Audacity window should show 44100 Hz, Mono.

Step 2: How Do You Remove Low-Frequency Rumble?

A high-pass filter removes frequencies below the speech range that contribute to noise floor without adding audible content.

• Select all: Ctrl+A
• Effect → High-Pass Filter
• Frequency: 80 Hz
• Roll-off: 12 dB/octave

If your version of Audacity doesn't have a standalone High-Pass Filter, use Effect → Filter Curve EQ and manually set a 12 dB/octave rolloff below 80 Hz.

Step 3: When Should You Apply Noise Reduction?

Only apply noise reduction if your raw noise floor is above -65 dBFS. Measure first: select a silent section → Analyze → ACX Check → read the noise floor value.

Get Noise Profile

• Select 0.5+ seconds of pure silence (room tone only, no speech or breaths)
• Effect → Noise Reduction → Get Noise Profile

Apply Noise Reduction

• Select all: Ctrl+A
• Effect → Noise Reduction
• Noise reduction (dB): 6
• Sensitivity: 6
• Frequency smoothing (bands): 3

Maximum single-pass reduction: 8 dB. If you need more, apply a second pass with a fresh noise profile. Multiple light passes sound better than one heavy pass. The noise floor guide explains why.[3]

Step 4: What Compression Settings Work for Audiobooks?

Compression reduces dynamic range to make the subsequent loudness adjustment more consistent.

• Select all: Ctrl+A
• Effect → Compressor
• Threshold: -20 dB
• Noise Floor: -60 dB
• Ratio: 2:1
• Attack Time: 0.20 seconds
• Release Time: 1.0 seconds
• Make-up gain: Checked
• Compress based on Peaks: Unchecked

Do not increase the ratio beyond 2:1. Higher ratios (4:1, 8:1) are music mastering settings that destroy the natural dynamics of speech. Audiobook narration needs gentle compression to sound natural over extended listening. The mastering vs mixing guide explains the difference between music and audiobook processing.

Step 5: How Do You Normalise Loudness to ACX's Target?

This is where many guides introduce confusion. Audacity 3.x supports both LUFS and RMS modes in its Loudness Normalization effect. ACX measures RMS, not LUFS, so use RMS mode.[2]

• Select all: Ctrl+A
• Effect → Loudness Normalization
• Normalize to: -20 dBFS
• Type: RMS
• Treat mono as dual-mono: Unchecked

Why -20 dBFS: ACX requires -23 to -18 dBFS. Targeting -20 puts you dead centre with 2 dB margin on each side. This margin accounts for tool measurement differences and MP3 encoding shifts.

Important note on LUFS vs RMS: Audacity's Loudness Normalization defaults may show LUFS mode. For ACX compliance, explicitly select RMS. While LUFS and RMS typically track within 1–2 dB of each other for spoken word, using the same measurement system ACX uses eliminates any discrepancy risk.[4]

Step 6: How Should You Set the Limiter?

The limiter catches any peaks that exceed ACX's -3 dBFS ceiling after loudness adjustment.

• Select all: Ctrl+A
• Effect → Limiter
• Type: Hard Limit
• dB limit: -3.5
• Hold (ms): 10
• Apply Make-up Gain: No

Critical: Use -3.5, not -3.0. Audacity's limiter operates on sample peaks only. It does not perform true-peak limiting. ACX checks true peaks, which can exceed sample peaks by 0.5–1 dB due to inter-sample reconstruction. The 0.5 dB margin accounts for this difference.[5] See the true peak explanation for why this distinction matters.

Step 7: How Do You Add Silence Padding?

Head silence (beginning of file):

• Place cursor at position 0:00
• Generate → Silence → 0.75 seconds

Tail silence (end of file):

• Place cursor at the very end
• Generate → Silence → 3.0 seconds

ACX requirement: 0.5 to 1 second head, 1 to 5 seconds tail. The values above give safe margin within both ranges.[1]

Note: Generated silence is digital silence (absolute zero). Ideally, paste room tone from a clean section of your recording instead. Room tone sounds more natural at transitions. The silence padding guide covers this in detail.

Step 8: How Do You Verify Before Export?

• Select all: Ctrl+A
• Analyze → ACX Check

Expected results:

• RMS: approximately -20 dBFS (must be -23 to -18)
• Peak: approximately -3.5 dBFS (must be below -3.0)
• Noise floor: below -60 dBFS

What ACX Check actually measures: The ACX Check plugin reports sample peak, not true peak. It also measures full-file RMS and the noise floor of the quietest 0.5-second window. This means a file can pass ACX Check but still fail ACX's true-peak requirement if inter-sample peaks exceed -3 dBFS, which is why the -3.5 dBFS limiter ceiling is essential.[5]

Step 9: What Are the Correct MP3 Export Settings?

• File → Export → Export as MP3
• Bit Rate Mode: Constant
• Quality: 192 kbps
• Channel Mode: Mono

After export: Import the MP3 back into Audacity and run ACX Check again. MP3 encoding can raise peaks by 0.5–1 dB. If the MP3 fails the peak check, set your limiter to -4.0 dBFS and re-export.[5]

How Do You Create an Audacity Macro for ACX Mastering?

Audacity Macros automate multi-step processes so you can apply the entire chain with one click. This is essential for multi-chapter audiobooks where repeating 9 manual steps per chapter is impractical.

Creating the Macro

1. Tools → Macros → New
2. Name: "ACX Master"
3. Add these commands in order:

Important: Set NormalizeTo=RMS in step 3. Audacity's Loudness Normalization effect supports both LUFS and RMS modes in version 3.x. For ACX compliance, always use RMS.[2]

The macro does not include: format conversion (do this before running the macro), noise reduction (requires manual noise profile), silence padding (depends on existing silence), or MP3 export (requires manual settings verification).

Running the Macro

1. Open your WAV file (already converted to 44,100 Hz mono)
2. Select all: Ctrl+A
3. Tools → Apply Macro → ACX Master
4. Manually add silence padding (Step 7 above)
5. Run ACX Check to verify
6. Export as MP3

How Does Batch Processing Work?

For multi-chapter audiobooks, you can apply the macro to multiple files at once:

1. Tools → Macros → select "ACX Master"
2. Click "Files..." button
3. Select all chapter WAV files
4. The macro processes each file and saves the output

Limitation: Batch mode applies identical settings to every file. This works well if your chapters were recorded in the same environment with consistent levels. If chapters vary significantly, individual processing may be needed.

ACX Export Settings Quick Reference

What If My Settings Aren't Working?

RMS Still Too Low After Normalization

Your dynamics are too wide. Quiet passages are pulling the average down. Increase compression before normalization:

• Try Threshold: -18 dB (lower threshold engages compression sooner)
• Keep Ratio at 2:1 (don't increase ratio)
• Re-run Loudness Normalization (set to RMS mode) after adjusting compression

RMS Too High After Normalization

This is rare with -20 dBFS targeting. If it happens:

• Check that Loudness Normalization is set to RMS, not LUFS or peak
• Verify the target is -20, not -18 or -16

Peak Exceeds -3 dBFS After Limiting

Your limiter ceiling isn't low enough for the true peak difference. Lower it:

• Try -4.0 dBFS instead of -3.5
• Re-run ACX Check after adjusting

Noise Floor Above -60 dBFS After Processing

Loudness adjustment amplified your background noise beyond the limit. Go back to Step 3:

• Apply another pass of noise reduction (6 dB maximum)
• Get a fresh noise profile from the current state of the file
• Re-process through Steps 4–8

MP3 Export Fails Peak Check

MP3 encoding shifted your peaks. Lower the limiter ceiling:

• Set limiter to -4.0 dBFS
• Re-export and re-check the MP3

What Are Audacity's Limitations for Audiobook Work?

Audacity handles the fundamentals well, but has specific limitations for audiobook production:

What Audacity does well:

• Full effect chain for ACX mastering
• ACX Check verification (RMS, sample peak, noise floor)
• Macro automation for repeatability
• Free and open source

What Audacity lacks:

• True peak limiting (sample peak only; use -3.5 dBFS ceiling to compensate)
• True peak measurement (ACX Check reports sample peak, not true peak)
• Efficient multi-chapter consistency checking
• Sophisticated noise reduction (compared to iZotope RX)

For a comprehensive comparison of mastering tools including Audacity, see the best audiobook mastering tools comparison. If you want to understand the full mastering process beyond just Audacity's implementation, the complete audiobook mastering guide covers the conceptual signal chain.

The free audiobook quality checker guide explains how to supplement Audacity's checks with additional free tools, particularly for true peak measurement, which Audacity's built-in ACX Check doesn't cover.