Signal-to-noise ratio

[moble]

The signal-to-noise ratio variables currently in the notebook are actually time-domain quantities based on the amplitude of the data. Because they need to be audible WAV files, the data can't easily have appropriate normalizations. (Though that might be fixable.) In the mean time, maybe there could be some way of evaluating the usual SNR for a given distance.

At the very least, this should be explained somewhere.