Reference · Audacity workflow
Audacity ACX mastering: the exact chain, and where it breaks
The standard free answer to an ACX rejection is a three-effect chain in Audacity. It works — when the values are right, when the order is right, and when your Audacity version is one the macro was written for. This page gives the chain with its exact settings, the reasons behind each number, and the failure modes people only discover from a second rejection.
Checked against Audacity’s guide and the open bug on 2026-08-31.
The chain
Three effects, in this order, with these values
This is the sequence Audacity's own audiobook mastering guide documents. The order is not decoration: each step depends on the one before it.
| Step | Effect | Setting | Why this value |
|---|---|---|---|
| 1 | Filter Curve EQ | factory preset “Low roll‑off for speech” | Removes rumble below the voice. Without it, inaudible low‑frequency energy counts toward RMS, and step 2 sets your level against sound nobody hears. |
| 2 | Loudness Normalization | normalize RMS to −20 dB | −20 sits in the middle of ACX's −23 to −18 window, so ordinary chapter‑to‑chapter variation stays inside the band instead of straddling its edge. |
| 3 | Limiter | Threshold −3.5 dB, make-up target −3.5 dB, knee 0, lookahead 0.1 s, release 10.1 s Audacity 3.6 renamed these controls. The older guides — and the macro in the bug below — still say “Soft Limit, gain 0/0, limit −3.5 dB, hold 10 ms”, which is the same setting under the previous names. |
ACX's ceiling is −3 dB. Limiting half a decibel below it leaves margin for what the MP3 encoder does to peaks afterwards. |
Then run the ACX Check plugin (Analyze menu) and read the three numbers. Note what the chain does not touch: the noise floor. Steps 1–3 move level and peaks; if your floor fails, see below.
Where it breaks
Five ways the standard chain still produces a rejection
The macro is broken in Audacity 3.6 and later
The downloadable Audiobook‑Mastering‑Macro produces blown‑out audio that fails ACX Check in current Audacity: the Limiter effect changed its parameters, and the macro still feeds it the old ones. The bug is tracked as audacity #8651, reported April 2025 and still open as of August 2026. The community keeps a corrected “36Audiobook‑Mastering‑Macro” on the Audacity forum; or apply the three effects by hand with the values above, which is thirty seconds per chapter.
The chain never fixes a noise floor
Nothing in EQ → normalize → limit lowers the room between your words. Audacity's guide itself warns that pushing Noise Reduction hard enough to rescue a bad room makes the voice “wine‑glassy or honky” and fails human review even when the meter passes. If the floor is the failure, the honest question is whether the room can be fixed — what actually helps.
Fixing one chapter is not fixing a book
The chain is per file, and so is the re‑check. Miss one chapter and you find out when ACX bounces the title. A book also has to match itself: a chapter mastered on a different day can pass alone and still sit 3 dB from its neighbours. The batch checker measures every chapter and flags the outliers against the book's own median.
The export can undo the master
ACX wants a 192 kbps constant‑bitrate MP3 at 44.1 kHz, and the encoder moves peaks a little. A file limited exactly at −3.0 dB can come back over the ceiling after encoding — which is why the chain limits at −3.5, and why the only trustworthy check is one made on the encoded file, not the project.
ACX Check measures three of the five
The plugin reports peak, RMS and noise floor. It says nothing about room tone at the ends, the sample rate, the bitrate mode, or a chapter running past 120 minutes — four of the reasons files bounce with “passing” numbers. The full list is on the requirements page.
Or skip the chain
The same correction, measured from the encoded bytes
The chapter checker on the home page measures all five ACX numbers, applies the correction in the right order, exports the 192 kbps CBR MP3 — and then re‑opens its own output and measures the encoded file the way ACX will, refusing to offer a download that would bounce. Nothing is uploaded and there is nothing to install. If the verdict is that your room, not your levels, is the problem, it says so instead of selling you a louder failure.