AAC Export Options
From Audacity Manual
Adjust the audio quality setting, a higher quality setting usually gives better audio quality but produce larger files.
- Accessed by: File Export Dialog's Options button
AAC Export Setup
- Quality: Choose a quality setting from 10 to 500 (defaults to 100).
AAC is a lossy, compressed audio format. AAC files usually have M4A extension, with variants such as M4P (protected) and M4R (ringtones). AAC usually gives better quality for the same bit rate than the older MP3 format and is the default audio format for iTunes®, iPod® and iPhone®, and Sony PlayStation 3.
Advanced encoders don't take an argument of Bitrate; the best you can do is get in the fuzzy ballpark and you can't predict the output bitrate. The 256 ballpark for a stereo show is Excellent, you are never going to hit it exactly because Excellent quality changes depending on the complexity and bandwidth of the sound in the original audio.
One way of putting it is that if you leave the quality setting at 500 (maximum) a one minute flute solo will have a smaller file size (so have a lower averaged-out bit rate) than the same piece with full orchestra. The flute piece encoded at quality 500 will probably give you an averaged-out bit rate of about 200 kbps as compared to about 270 kbps for the orchestra.
Quality 400 might make that average 160 kbps for the flute, 240 kbps for the orchestra - but you may hear no difference between quality 400 and 500 for the flute audio; you may well notice a difference for the orchestra audio.
