A: Signal-to-Noise Ratio (SNR) is an important metric of whether a recording is worth further processing by other speech technologies, so it is part of our Speech Quality Estimation. However, calculating SNR automatically is not a trivial task.
We use the fact that the statistical distribution of the frequencies in the waveform of speech has Gamma distribution. In contrast, noise has Gaussian distribution. So we can estimate the SNR by looking at the frequency distribution in individual frames.
This approach to SNR estimation is based on the article by Kim Chanwoo, and Richard M. Stern, called “Robust Signal-to-Noise Ratio Estimation Based on Waveform Amplitude Distribution Analysis”, Interspeech 2008.