Introduction

  • There was a ‘loud sound’ heard in Bengaluru on 20th May 2020 afternoon, which puzzled lakhs of city dwellers- It was revealed to have emanated from an IAF test flight involving a supersonic profile.
  • The sound effect caused due to such high-speed flights is known as ‘sonic boom’.
  • Considering the atmospheric conditions and reduced noise levels in the city during these times, the aircraft sound may become clearly audible even if it happened way out from the city.

What is a ‘sonic boom’?

  • Sound travels in the form of waves which are emitted outwards from its source. In air, the speed of these waves depends on a number of factors, such as the temperature of the air and altitude.
  • From a stationary source, such as a television set, sound waves travel outwards in concentric spheres of growing radii.
  • When the source of sound is moving – e.g, a truck– the successive waves in front of the truck get closer together, and the ones behind it spread out. This is also the cause of the Doppler effect– in which bunched waves at the front appear at a higher frequency to a stationary observer, and spread out waves that are behind are observed at a lower frequency.
  • As long as the source of the sound keeps moving slower than the speed of sound itself, this source– say a truck or a plane – remains nested within the sound waves that are travelling in all directions.
  • When an aircraft travels at supersonic speed – meaning faster than sound (>1225 kmph at sea level) – the field of sound waves moves to the rear of the craft. A stationary observer thus hears no sound when a supersonic flight approaches, since the sound waves are at the rear of the latter.
  • At such speeds, both newly created as well as old waves, are forced into a region at the aircraft’s rear called a ‘Mach cone’, which extends from the craft and intercepts the Earth in a hyperbola-shaped curve, and leaves a trail called the ‘boom carpet’. The loud sound that is heard on the Earth when this happens is called a ‘sonic boom’.

-Source: Indian Express

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