Speed Of Sound

What is the speed of sound ?

The speed of sound is how fast sound waves travel in any medium. Our interest in aerodynamics is in how fast sound travels in Air.
The speed of sound changes dependant upon local temperature.
The local speed of sound is known as Mach 1.
Any velocity can be related to the local speed of sound by diving by the local speed of sound.
A speed less than Mach 1 is Subsonic.
A speed equal to the speed of sound is Sonic.
A speed greater than the speed of sound is Supersonic.
There is a strange case which involves airflow where some is subsonic and some is supersonic, this is known as Transonic.

Air at speeds greater than about .3M is compressible, and calculations involving these speeds must take this into account.

Calculations involving the speed of sound.

Calculating the local speed of sound.

We know that the local speed of sound is related to Temperature. It can be calculated using the following formula.

a = √ γ R T / M

a: Local Speed Of Sound [m s^-1]
γ: Adiabatic Constant [this is a ratio, so it's dimensionless]
R: Gas Constant [8.41 J Mol^-1 K^-1]
T: Local Temperature [° Kelvin]
M: Molecular Weight of the Gas [kg Mol^-1]

Example Content

Calculating The Local Speed of Sound in Air

The definition above equation applies to just about any gas.
We can shortcut some of the above by plugging in known figures for the properties of air.
R =8.41 J Mol^-1 K^-1, and for air γ =1.4 , M = .02895 kg Mol^-1.
This reduces the above equation down to
a =√406.7 * T

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© Copyright 2000-2009 Gary Spender

Temperature		° c
T		° K

a		m s^-1
M		kts