Sonic Speed

Sonic speed . It is the speed of propagation of a disturbance within a substance and its value depends on both the elastic properties and the density of the substance.


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  • 1 Features
  • 2 Cut with a knife
  • 3 flywheel body
  • 4 supersonic aircraft
  • 5 Sources
  • 6 external links


In the air, the value of the sonic speed is 340 m/s or 1224 km/h at 15 degrees Celsius, and it varies depending on the temperature and the height, that is, the density of the air. While a body moves through the air at less than sonic speed, the air behaves like a soft substance, deflecting as it passes in all directions. On the other hand, when the sonic speed is reached, the air begins to behave with respect to the body as if it were a hard substance that must be cut and emits elastic waves at the cut points that, due to their reduced speed, are exceeded by the body. The transition from the supersonic current (without disturbances) to the infrasonic current, originates a pressure shock (shock wave). Next, the phenomenon described is compared with others that occur for better understanding.

knife cut

When cutting any solid substance with a knife , for example, a sheet of paper: when the cut is made, the edge of the knife then emits sound waves that are perceived in the form of a hissing noise.

flying body

When a flying body cuts through the air at supersonic speeds, a phenomenon with similar characteristics originates. In this case, the air behaves like a solid substance and the tip of the projectile behaves in turn like the edge of a knife, emitting a sound wave called head wave when cutting the air, at the same time a sound wave is also created. sound wave at the trailing edge of the projectile (tail wave), since the surrounding air then enters in a turbulent manner into the void that forms behind the projectile and produces a eddy trail there.

supersonic aircraft

In supersonic aircraft, the head wave has the shape of a cone (noise cone), which, when it comes into contact with the Earth’s surface, originates there an extensive noise carpet. Sonic pressures measured on the carpet have values ​​between 25 kp/m2 and 150 kp/m2, enough to cause glass to break.


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