ACX Audio Requirements: Export Your Audiobook as MP3 on a Mac

ACX wants MP3 files at a constant 192 kbps or higher and 44.1 kHz, loudness between -23 dB and -18 dB RMS, peaks no higher than -3 dB, and a noise floor no higher than -60 dB RMS. The table is ACX’s own page, read 11 October 2026.

Only the first two rows are encoding choices. Everything from loudness down is a property of the recording or the edit, and it has to be right before the MP3 is made. ACX also asks for the section header (“Chapter 1”) in each file, and opening and closing credits in their own files.

It means 192,000 bits a second, every second, at 44.1 kHz. That is 24,000 bytes a second, or 86.4 MB an hour. “Or higher” covers 256 and 320 kbps, which ACX says are allowed.

“CBR” means constant bit rate. Constant bit rate spends the same number of bits on every second. Variable bit rate spends fewer on silence and more on busy passages, and ACX asks for the first. Voices by INaudio, the service people search for as Findaway Voices, says the same in its CBR versus VBR error article: it “requires all audio files be encoded as Constant Bit Rate and not as Variable Bit Rate”, and the fix is to re-export as CBR. That article gives no bitrate and no sample rate, so 192 kbps at 44.1 kHz is the ACX target, not a figure quoted for Voices.

Higher is not better here. Each step up in bitrate makes the book larger without changing what the ACX levels checks measure. Background on what bitrate buys is in audio bitrate, explained.

ACX’s requirements page states a length limit, 120 minutes a file, and lists no byte cap. At 192 kbps a full-length file is about 173 MB, so size follows from bitrate and length.

The WAV row is why the MP3 step matters for storage: an eight-hour book of mono WAV chapters is around 2.5 GB, and the same book at 192 kbps is about 690 MB. Other platforms and upload tools may have their own caps, so read the upload screen.

Use Compress, not Convert, with the output set to MP3, bitrate 192 kbps and sample rate 44.1 kHz. Smol’s audio controls cover bitrate from 16 to 512 kbps and sample rate from 8 to 192 kHz.

Lossy encoding can nudge peaks, so measure the MP3 you will upload, not the project. If it is outside the levels, correct the master and re-export.

1. 1

   Master and name the chapters

   One chapter per file, section header spoken at the start, and room tone at both ends.

2. 2

   Drop the folder into Smol

   Every chapter goes in at once. Smol shows progress and savings per file.

3. 3

   Set Compress, MP3, 192 kbps, 44.1 kHz

   Send the output to a new folder so the masters stay as they are.

4. 4

   Check one chapter first

   Run the ACX levels check in your editor on the MP3, and confirm the bitrate and sample rate in your editor.

5. 5

   Check every length

   Anything over 120 minutes goes back to the editor to be split at a section break.

6. 6

   Upload the MP3 copies

   Add the retail sample (5 minutes or less) and the opening and closing credit files.

Read the rejection message first; it names the check that failed. Then match it to the table below: encoding failures are fixed by re-exporting, and level failures by remastering.

Converting an already compressed MP3 again stacks losses. Always encode from the WAV or FLAC master.

For the format side of the job, see converting WAV to MP3, FLAC to MP3 and batch audio conversion. A narrator who also records a show will find the host side in our podcaster page and the audition side in voice-over auditions.

• Your levels are out of range. Smol does not measure or change loudness, peaks or noise; use your editor.
• A chapter is over 120 minutes. Smol does not split or merge audio.
• You need proof the file is constant bit rate. Smol does not promise it, so verify with your editor or the platform’s own check.
• You need a one-off MP3 and already own an editor with a good exporter; the free route is enough.