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Speed of sound in gases
Intermediate
Depends on temperature and molar mass.
v = √(γRT/M)v = √(γRT/M). Heavier molecules are slower, hotter gases faster. Helium raises the speed to about 1000 m/s, giving a squeaky voice.
Air at 300 K: √(1.4·8.31·300/0.029) ≈ 347 m/s.
Use T in kelvin.
In more detail
In an ideal gas, the speed of sound is v = sqrt(γRT/M), where γ is the ratio of specific heats, R is the gas constant, T is the temperature and M is the molar mass.
Real-world uses
- Engineers use the speed of sound to calculate Mach numbers for aircraft.
- Gas sensors measure gas composition using sound speed.
- Engineers designing jet engines use the speed of sound in hot exhaust gases to calculate noise and performance.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
Why does helium make voices higher?
Helium has a lower molar mass, so sound travels faster in it, raising the resonant frequencies of the vocal tract.
Does pressure change the speed in gases?
Not at fixed temperature, because density and pressure change together.
Does sound travel faster in helium?
Yes. Helium's low molar mass gives a much higher speed of sound, which makes voices sound high-pitched.
Related topics in Waves & Sound
Wave speed on a string · Wave speed in a medium · Speed of sound · Reflection of waves · Phase change on reflection · Refraction of waves