# Supersonic aircrafts speed is often measured in Mach, what is that?

Supersonic aircrafts speed is often measured in Mach, what is that?
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The Mach number is defined as the ratio of the flow velocity (V) to the speed of sound (c):

M = V/c

This number defines whether a flow or vehicle is moving at subsonic (M<1), sonic (M=1), or supersonic (M>1) speed. Namely, whether is traveling slower, at the same speed, or faster than the speed of sound.

If a vehicle travels at Mach 2 it means that is moving at twice the speed of sound.

• Hi @prajput  ,

How much is the speed of sound?

The speed of sound does not have a specific value but it depends on the medium. Indeed, it represents the velocity of propagation of a sound wave through a medium. Its more general expression, valid for all gases, is:

a = sqrt[ (dp/drho)_s ]

where s indicates an isentropic process.

A more commonly used expression, for the speed of sound, is the one for calorically perfect gases:

a = sqrt( gamma*R*T)

where

gamma is the ratio of the specific heats

R is the specific gas constant

T is the medium temperature