Mach 2 is a measurement of an individual object as it moves through a fluid substance, especially air, at faster than the speed of sound, but there are variations as to how Mach 2 is measured.
Recording local conditions plays a key part in the calculation of Mach 2. Both temperature and pressure are important criteria, as is what the surrounding medium consists of. Determining what is Mach 2 on aircraft, for instance, involves flight instruments that rely on pressure differential. A Mach number, relating to the flight of an aircraft is, in basic terms, calculated by a moving aircraft’s velocity, and what the speed of sound is at the relevant altitude. Sound will normally decrease the higher an aircraft ascends.
When the sound barrier is broken by an aircraft there is a very noticeable shift in pressure. A shock wave then develops and a consequence of this is a sonic boom. A shock wave increases in power as a Mach number increases, so a Mach 2 shock wave would be significantly more powerful than a Mach 1, and both temperature and pressure will increase as a result. Mach numbers are used to measure not only ultra fast movement of objects through a fluid, but also fast movement of fluids when they are contained, such as inside nozzles, and even in wind turbines.
The name Mach 2 has its origins from Ernst Mach, an Austrian physicist and philosopher, who was famous for his work studying shock waves and the speed of sound. In France, a Mach number is also referred to as a Sarrau number, being so-called in memory of Emile Sarrau, who researched explosions in the 19th Century.
Supersonic transport, also known as SST, can move faster than the speed of sound, which is Mach 1. Twice the speed of sound is Mach 2. The Russian Tupolev Tu-144 was the first commercial plane to reach Mach 2 in 1969, with the Anglo-French built Concorde the only other commercial SST to ever have flown.