Mach number : It is the ratio of velocity of source to the velocity of sound.
Mach Number = Velocity of sound / Velocity of source.
Read more about Supersonic speed
Related Posts:
What is Ultrasonic sound waves Velocity and Acceleration in Cylindrical Coordinates MPH to Mach Speed Converter What is Wave ? Defination of wave Reflection and Refraction of Sound Waves. 
Velocity and Acceleration in Cartesian Coordinate System 