Wolfram|Alpha gives 343.2 m/s (1236 km/h) at 20 °C and 1 atm. http://www.wolframalpha.com/input/?i=speed+of+sound http://www.wolframalpha.com/input/?i=speed+of

Wolfram|Alpha gives 343.2 m/s (1236 km/h) at 20 °C and 1 atm. http://www.wolframalpha.com/input/?i=speed+of+sound http://www.wolframalpha.com/input/?i=speed+of+sound Of course, in a closed system that isn't directly touching humans, it doesn't necessarily have to use those conditions. Still, I don't think you're going to get anywhere near 6400km/h.