Speed of Sound in Dry Air at 20 °C

Sound speed in an ideal gas is c = √(γRT/M): for dry air with γ = 1.40, R = 287.05 J/(kg·K) and T = 293.15 K, that is 343.2 m/s. The striking feature is what is missing: pressure. Raising pressure raises both stiffness and density in the same proportion, so they cancel, and sound travels at the same speed at sea level and at 10 000 m for the same temperature. Altitude changes the speed only because it is colder up there, which is why the sound barrier is a lower true airspeed at altitude.

Temperature is everything: c ≈ 331.3 + 0.606 × T(°C) m/s, so a 0 °C winter morning gives 331 m/s and a 35 °C afternoon 352 m/s. Humidity adds a little (a percent or so at saturation) because water vapour lowers the mean molar mass. Use it for the 340 m/s round trip in ultrasonic level and distance sensors (which must be temperature-compensated or they read 6 % long across a seasonal swing) for duct acoustics, and for the classic five-seconds-per-mile lightning estimate.