Context: The ‘loud sound’ heard in Bengaluru recently, which puzzled lakhs of city dwellers, was revealed to have emanated from an IAF test flight involving a supersonic profile.

More about the news:

  • These test flights were done well beyond the city limits in specified sectors. 
  • But considering the current atmospheric conditions and reduced noise levels in the city, the aircraft’s sound may become clearly audible even if it happened way out from the city.

Understanding concept of 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.
  • Propagation of sound waves:
    • From a stationary source
      • Sound waves from stationary sources such as a television set, travel outwards in concentric spheres of growing radii.
    • From a moving source:
      • When the source of sound is moving the successive waves in front of the source 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.
        • The Doppler effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.


  • Moving with supersonic speed(faster than sound ie >1225 kmph at sea level)
    • As long as the source of the sound keeps moving slower than the speed of sound itself, this moving source remains nested within the sound waves that are traveling in all directions.
    • However when an object travels at supersonic speed the field of sound waves moves to the rear of the object. 
    • That is the reason why a stationary observer hears no sound when a supersonic flight approaches, since the sound waves are at the rear of the latter.



  • ‘Mach cone’ and ‘boom carpet’
    • At supersonic speeds, both newly created as well as old waves, are forced into a region at the aircraft’s rear called a ‘Mach cone’
    • This region extends from the aircraft 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’.
  • After effects:
    • When such aircraft fly at a low altitude, the sonic boom can become intense enough to cause the glass to crack or cause health hazards. 
    • Overland supersonic flights have thus been banned in many countries.

Sound waves

  • Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving.
  • The motion of the particles is parallel (and anti-parallel) to the direction of the energy transport. 
  • This is what characterizes sound waves in air as longitudinal waves.
  • Sound waves need to travel through a medium such as a solid, liquid, or gas. The sound waves move through each of these mediums by vibrating the molecules in the matter.

Additional Information

Supersonic flights

  • In 1947, the American military pilot Chuck Yeager became the first to breach the sound barrier, flying the Bell X-1 aircraft at 1127 mph. 
  • After that, many supersonic flights have followed worldwide, with advanced designs allowing speeds of over Mach 3, or three times the speed of sound.
  • India’s fastest jets include the Sukhoi SU-30 MKI (Mach 2.35) and the Mirage-2000 (Mach 2.3).
Image Source: IE