-q n Algorithm quality selection
Bitrate is of course the main influence on quality. The higher the bitrate, the higher the quality. But for a given bitrate, we have a choice of algorithms to determine the best scalefactors and Huffman encoding (noise shaping).
n = 0
Use slowest and best possible version of all algorithms. "-q 0" and "-q 1" are slow and may not produce significantly higher quality.
n = 2
Recommended. Same as -h.
n = 5
Default value. Good speed, reasonable quality.
n = 7
Same as -f. Very fast, OK quality. (psychoacoustics are used for pre-echo & mid/side, but no noise shaping is done).
n = 9
Disables almost all algorithms including psychoacoustic model. Poor quality.