additive noise

For a signal that can be assumed periodic (i.e., a sustained vowel), the signal-to-noise ratio equals the harmonics-to-noise ratio, which you get can get by selecting a Sound and choosing one of the "To Harmonicity..." commands from the Periodicity menu (for the algorithm, see Sound: To Harmonicity (ac)... or Sound: To Harmonicity (cc)...). These are the world's most sensitive HNR measurements (up to 90 dB). For more information, see the Harmonicity manual page.