Audacity M4A Bitrate Options: The Full Range, Officially

Audacity's M4A (AAC) export has one Quality slider, and the official manual states its ends in a single sentence: "The Maximum bit rate (best quality) is 320 kbps (stereo) 160 kbps (mono) and the minimum is 98 kbps (mono)."

Read that again, because it is doing more work than it looks. Those three numbers are not three points on one scale — two of them are tagged stereo and mono, and the third carries a channel tag as well. That is the whole reason this dialog confuses people: the range you actually get depends on what you are exporting, and the manual only tells you three of the four corners.

The limits, at a glance

Every figure below comes from the AAC Export Options page of the Audacity Manual unless noted otherwise.

Two structural facts to hold alongside that table. First, AAC export is not built in: "FFmpeg is required for AAC export but it is not shipped with Audacity." Second, the manual points to the Audacity support site for FFmpeg installation rather than listing steps, so for download links and version requirements, check the official documentation rather than any figure you find quoted elsewhere.

Why the numbers split by channel count

The manual explains the halving in the same breath as the range: "A stereo file is always produced, but a file exported from a mono track will have about half the bit rate (and hence half the file size) of one exported from a stereo track."

That is a clean mechanism. The slider is not applying a hard number to the encoder; a mono source lands at roughly half of what the same setting gives a stereo source. It also explains why the mono maximum is exactly half the stereo maximum in that quoted sentence — 160 and 320.

It does not explain the minimum. 98 kbps is not half of anything listed, and it is tagged mono. A stereo minimum simply is not stated on that page. Treat the bottom of the stereo range as unknown rather than assuming it is 196 kbps, or double 98, or anything else you can derive. Audacity's dialog is the only thing that will tell you, and the manual does not print the figure.

The odd bit the manual leaves standing

Here is the part that trips people up when they go looking for the answer themselves.

The same AAC page has a Channels section offering Mono, Stereo and Custom mapping — and a Quality section that says "A stereo file is always produced." Both sentences sit on one page. The manual does not reconcile them, and neither will this article: what it means in practice for a given build is not something the documentation settles.

There is a related default worth knowing about, documented on the Export Audio Dialog page and applying to exports generally: "Stereo: stereo pair with left and right channels - this is the default setting even if you only have mono tracks for exporting." So if your export came out at double the size you expected from a single mono track, the Channels control is the first thing to check, not the Quality slider.

What the slider does with the number you pick

One more limit does not appear in any range table, because it is not a range — it is a documented defect. Audacity's manual states that "there is a bug in Audacity whereby the bit rate setting that you make in an AAC export is not always honored by Audacity," and then splits the behaviour:

• "For stereo all selected bit rates of 196kbps or less export at 196-198kbps,"
• "Settings in the range 200kbps to 320kbps are reasonably closely honored on export."

So the usable range and the honored range are different things. The bottom half of the stereo range collapses onto roughly one value; the top half works approximately. Note that the 256 kbps default sits inside the band the manual calls reasonably closely honored — this bites people turning quality down, not people leaving the dialog alone. The full picture, including both official workarounds, is in Audacity AAC bitrate not working.

What to do when the range does not cover you

Three documented options, in the order worth trying.

1. Type the number instead of sliding it. The Custom FFmpeg Export Options page calls this "an alternative way to specify the bit rate of exported M4A (AAC files) rather than using the Quality slider." You select mp4 in the Formats list, libvo_aacenc in the Codecs List, and "enter a bit rate up to 320 kbps expressed in bits per second. For example, to export at 280 kbps, enter "280000" (without quotes)." The ceiling is the same 320 kbps; what changes is precision. One catch that catches everyone: "you must add the extension relevant to the format at the end of your chosen file name."
2. Leave AAC entirely. The manual's "strongly recommended workaround" for the bit rate bug is "to export as uncompressed WAV or AIFF and use iTunes/Music, where you have greater control of the settings, to make the AAC conversion."
3. Export MP3 instead if the destination does not require M4A. MP3 gives you four bit rate modes including a constant one, and Audacity's own comparison of the two formats is that a "quality setting of 160 typically produces a similar sized, but slightly higher quality file compared to an MP3 at Audacity's default 128 kbps constant bit rate."

Try it right here

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If you already have an M4A whose bit rate is not what you asked for and what you actually needed was a predictable, widely playable file, converting it in the browser takes one step and installs nothing — the M4A to MP3 converter is the same tool on its own page. Do it knowing it is a second lossy pass. Where you still have the Audacity project, re-exporting from the project is the better move.

Gotchas specific to M4A export

• Sample rate moves the bit rate. "Increasing the sample rate in Audio Settings Preferences will increase the bit rate and so the file size (and vice versa)." If your file is bigger than the slider implies, that preference is the next place to look.
• VBR means the number is a target. The manual describes AAC as VBR "as opposed to the CBR or constant bit rate encoding used by default when exporting MP3 files," and notes for VBR formats generally that "the achieved size will vary depending on the content."
• You have more extensions than .m4a. Audacity lists "Optional permitted extensions: .mp4, .m4r (ringtone) and .3gp (mobile)." Type one of those and no warning appears, because they are recognised for this format.
• Channel count above two is its own limit. The format table lists 32 channels for M4A, but the footnote on the same page says more than 2 channels "is not supported with the recommended FFmpeg 2.2.2 library" unless you export via the external program route with the native FFmpeg encoder, which "produces maximum 6 channels. For up to 8 channels, point the same command to the latest FFmpeg-git."

FAQ

What is the maximum bitrate for M4A export in Audacity? 320 kbps for stereo and 160 kbps for mono, per the AAC Export Options page. The same ceiling of 320 kbps applies if you set the rate through Custom FFmpeg Export instead.

What is the minimum AAC bitrate in Audacity? The manual gives one minimum: 98 kbps, and tags it mono. It does not print a stereo minimum on that page, so check the dialog in your own copy rather than inferring the number.

What bitrate does Audacity use for M4A by default? 256 kbps. The manual adds that this "is that quality that Apple now uses for audio files sold in iTunes/Apple Music."

Why is my mono M4A export half the bitrate I selected? That is documented behaviour, not a fault: "a file exported from a mono track will have about half the bit rate (and hence half the file size) of one exported from a stereo track."

Do I need FFmpeg to export M4A from Audacity? Yes — "FFmpeg is required for AAC export but it is not shipped with Audacity." The manual sends you to the Audacity support site for installation details rather than listing them, so check the official documentation for current versions and downloads.

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All figures quoted from the Audacity Manual pages linked above. For more format ceilings see every limits guide, or browse all Clapr tools.