MOS is a system of grading the voice quality of telephone connections. With MOS, a wide range of listeners judge the quality of a voice sample on a scale of one (bad) to five (excellent). The scores are averaged to provide the MOS for the codec.

Score Quality Description of Quality Impairment
5
Excellent, Imperceptible

4
Good
Just perceptible, but not annoying

3
Fair
Perceptible and slightly annoying

2
Poor
Annoying but not objectionable

1
Bad
Very annoying and objectionable