MP3 preferences are only available when Show Advanced Tools is selected in Advanced preferences.
Bit Rate (Mono/Stereo) pop-up menus: You can choose bit rates between 32 kbps and 320 kbps. The defaults are 80 kbps mono and 160 kbps stereo. These rates offer acceptable quality and good file compression. If you can afford the increased file size, you should choose 96 kbps for mono and 192 kbps for stereo streams. These settings will deliver better audio quality. You can choose even higher rates, but the quality of improvement in bit rates above 96/192 kbps is minimal.
Use Variable Bit Rate Encoding (VBR) checkbox: Variable Bit Rate (VBR) encoding compresses simpler passages more heavily than harmonically rich passages, generally resulting in better-quality MP3s. Unfortunately, not all MP3 players can accurately decode VBR-encoded MP3s, which is why this option is unselected by default. If you know that the audience for your MP3 files can decode VBR-encoded MP3s, you can select this option.
Quality pop-up menu: Keep this set to Highest whenever possible. Reducing the quality accelerates the conversion process, but at the expense of audio quality. This option is only accessible when the Use Variable Bit Rate Encoding (VBR) checkbox is selected.
Use Best Encoding checkbox: If you deselect this option, you will gain encoding speed at the price of audio quality. Keep selected, unless conversion time is an issue.
Filter Frequencies Below 10 Hz checkbox: Frequencies below 10 Hz are removed, leaving slightly more data bandwidth for the frequencies that humans can hear, resulting in an improvement in perceived quality. Such frequencies are usually not reproduced by speakers, and aren’t audible to human ears. Only deselect this option if you’re experimenting with subsonic test tones, or exporting MP3s for whales.
Stereo Mode pop-up menu: Choose between Joint Stereo and Normal Stereo. Depending on the original file, these settings may or may not offer any audible difference. Experiment with both settings to determine your preference.