This is quite an interesting look at the basics of sound. Using a small explosion, it demonstrates that the speed of sound at sea level is around 760mph, but that varies as the altitude increases, the temperature falls and therefore can drop to c. 660 mph before entering the constancy of the stratosphere. What's that got to do with high speed aviation, I cry? Well, this well written and delivered documentary proceeds to explain the difference between sub sonic pressure waves and those travelling as fast or faster than sound allowing us to consider how it and speed are distinctly inter-related i... read the rest.