Speed of Sound in Air

The speed of sound in dry air is given approximately by

where T_C is the celsius temperature,

This calculation is usually accurate enough, but for great precision one must examine the more general relationship for sound speed in gases. This sound speed does not apply to gases other than air, for example the helium from a balloon.

It is important to note that the sound speed in air is determined by the air itself. It is not dependent upon the sound amplitude, frequency or wavelength.

Selected Sound Speeds in Gases

Sound Speed in Helium

The speed of sound in helium at 0°C is about 972 m/s, compared to 331 m/s in air. This is consistent with the general relationship for sound speed in gases since the density of helium is so much less than that of air.

The high speed of sound is responsible for the amusing "Donald Duck" voice which occurs when someone has breathed in helium from a balloon. Note that if the vibration frequency of the vocal folds does not change, the actual pitch of the voice is not higher. The cavity resonances which determine the vocal formants would be raised by the higher sound speed, so the timbre of the voice would be different. It is possible for the pitch of the voice to change since gas dynamics ( i.e., Bernoulli effect) is partially responsible for the closing frequency of the vocal folds, but I haven't been able to find any data which demonstrates such a change.