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What is transonic? Definition and examples - glossary -

What is transonic? Definition and examples

Transonic (also ‘transsonic’) relates to travel at speeds just below the speed of sound in air (typically between 768 mph or 1,236 kph). Exceeding this speed means that the sound barrier is broken; doing so creates a sonic boom.

Though the sound barrier has been broken on land, travel at this speed mainly relates to flight, which can be divided into four categories. In ascending order:

  1. Subsonic (below the speed of sound in air)
  2. Transonic (“across the speed of sound”)
  3. Supersonic (faster than the speed of sound)
  4. Hypersonic (more than five times the speed of sound).

Transonic speed typically spans Mach 0.8 to 1.2 (because the speed of sound varies according to a range of factors), Mach numbers being units of measurement which express the speed of a moving object in relation to the speed of sound. The speed of sound is expressed as Mach 1; hypersonic speed is Mach 5. 

It was only during WW2 that difficulties related to transonic speed became apparent: because air acts very differently at supersonic speeds than at subsonic speeds, approaching the sound barrier means that drag increases rapidly and airflow around aircraft becomes asymmetric and unsteady (because airflow around the object in motion will move at subsonic or supersonic at different points along its surface). 

Aircraft cause ripples in air akin to those caused by a boat in water; however, when an aircraft passes the speed of sound, the sound waves that would usually be dispersed in front of it pile up and form shock waves. By combining into a single shock wave, this effect creates the a sonic boom. These thunderous sounds fan out behind the aircraft like the wake behind a boat (but aren’t audible to the pilots themselves). During steady supersonic flight, the sonic boom moves with the aircraft and is known as a carpet boom.

In addition to aircraft, sonic booms can also be caused by natural phenomena such as earthquakes, volcanic eruptions, and meteor showers, and – on a smaller scale – whips and bullets.

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