What is supersonic flight?

  The term "supersonic" refers primarily to the flight or motion of any solid body through a gas at a velocity that exceeds the velocity of sound in that gas. When we speak of sound in this connection, we have in mind the pressure variations that move outward in all directions, in the form of waves, from a "disturbance" such as the tooting of a horn or the roar of a lion. The waves in question are usually very weak. When they strike our eardrums, they set up vibrations that are recorded in the brain so that we "hear" the disturbance that set off the waves. Of course, these pressure variations, or sound waves, as they are generally called, exist whether or not our eardrums and result in the sensation of hearing.
  Sound waves traveling through the atmosphere near the Earth's surface have a velocity that decreases continually with altitude. It ranges from 1,120 feet per (762 miles per hour) at sea level. standard conditions of temperature and pressure, to 970 feet per second (660 per hour) at an altitude of 36,000. Then it remains constant to 82,000 feet altitude, after which it starts to increase.
  Suppose that the velocity of sound in the atmosphere at a given altitude is 700 miles per hour. A plane traveling at 1,400 miles per hour is moving twice as fast as the velocity of sound; we would say that it has attained a supersonic velocity of twice the speed of sound. Supersonic velocities are not restricted to solids traveling through gases. Strong sound waves, such as those set up by an explosion, travel at a velocity greater than the very weak sound waves that we described above.
  The speed of any supersonic flow is usually given in terms of the Mach number, named after the Austrian physicist: Ernst Mach (1838-1916). The Mach number represents the ratio of the speed of a body to the speed of sound in the surrounding atmosphere. For example, the expression "Mach 2" applied to a supersonic aiscraft means that its velocity relative to the atmosphere is twice the velocity of sound passing through the atmosphere.
  The Mach number can also be applied to subsonic speeds — that is, speeds less that the speed of sound. In this case, also, it represents the ratio of the speed of a body to the speed of sound in the atmosphere. The Mach number, in the case of subsonic speeds, is always less than unity (that it is, less than 1). For instance the Mach number .9 refers to flight at nine-tenths the speed of sound.
  A plane traveling at subsonic speed is preceded by the sounds or pressure waves that are caused by its flight. But since a plane in supersonic flight travels faster than sound, it gives no advance warning of its coming. In other words, it is preceded by a "zone of silence." In World War II, the German jet-propelled device called the V-1, traveling at the subsonic speed of some 300 miles per hour, was preceded by the sound waves caused by its passing, and it could therefore be heard before it arrived. But the terrible weapon called the V-2 traveled at supersonic speed. It could not be heard until after it passed by, and therefore it struck without warning.