Apple Logic Pro 8 - Manual (Page 608)

608 Chapter 27 Bouncing Your Project
Choose the MP3 name (and activate the checkbox) in the Destination pane to access
the following options:
Bit Rate (Mono/Stereo)
MP3 bit rates are selectable between 32 kbps and 320 kbps, but default to 80 kbps
mono, and 160 kbps stereo. These rates offer acceptable quality and good file
compression.
To enhance audio quality, provided you can afford the extra file size, choose:
 96 kbps for mono streams
 192 kbps for stereo streams
You can, of course, choose even higher rates, but the quality improvement afforded by
bit rates above 96/192 kbps is nominal.
Use Variable Bit Rate Encoding (VBR)
Variable Bit Rate encoding compresses simpler passages more heavily than passages
that are (more) harmonically rich, generally resulting in better quality MP3 files.
Unfortunately, not all MP3 players can accurately decode VBR-encoded MP3s, which is
why this option is turned off by default. If you know that your target listeners can
decode VBR-encoded MP3s, you can switch this option on.
Quality
Keep this set to Highest (the default) whenever possible. Reducing the quality
accelerates the conversion process, but at the expense of audio quality.
Use Best Encoding
Again, like the Quality parameter, if you uncheck this option, you will gain encoding
speed at the price of audio quality. This should always be kept on, unless conversion
time is an issue.