Why does sound travel at different speeds in helium and carbon dioxide?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Watch a suspended vibrating spring to learn about the propagation of sound waves — sound Learn about the propagation of sound waves from a point source by observing the motion of a suspended vibrating spring.

The speed of sound in a gas is determined by its physical properties, specifically its molecular weight, absolute temperature, and the ratio of its specific heat capacities ( $γ$ ).1

The relationship is expressed by the formula:
$v = \sqrt{\frac{γ R θ}{M}}$

In this equation, $v$ is the velocity, $R$ is the gas constant, $θ$ is the absolute temperature, and $M$ is the molecular weight. Because helium has a much lower molecular weight than carbon dioxide, sound travels significantly faster through it (965 m/s) compared to carbon dioxide (259 m/s) at the same temperature.1

Essentially, lighter gas molecules respond more rapidly to the pressure variations of a sound wave, resulting in a higher propagation speed. While pressure and density also influence sound, for an ideal gas, these factors balance out such that the speed depends primarily on the gas's composition and the ambient temperature.1