Indian Air Force Updates

What is the ‘Sonic Boom’ that rattled Bengaluru city?


From UPSC perspective, the following things are important :

Prelims level: Sonic Boom, Mach Number

Mains level: India's missile programme

The ‘loud sound’ heard in Bengaluru a few days back, which puzzled lakhs of city dwellers, was revealed to have emanated from an IAF test flight involving a supersonic profile. The sonic boom was probably heard while the IAF aircraft was decelerating from supersonic to subsonic speed between 36,000 and 40000 feet altitude.


We often get to hear about updates in  India’s missile programme. UPSC may ask a basic physics question asking fundamental differences between various Mach number and its differences.

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.
  • When an aircraft travels at supersonic speed – meaning faster than sound (>1225 kmph at sea level) – the field of sound waves moves to the back 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’ (resembles bomb-blast sound).


  • 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.

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 kmph.
  • Since then, many supersonic flights have followed, with advanced designs allowing speeds of over Mach 3, or three times the speed of sound.
  • According to the IAF website, India’s fastest jets include the Sukhoi SU-30 MKI (Mach 2.35) and the Mirage-2000 (Mach 2.3).

Back2Basics: Traverse of sound

  • 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.

Mach number

  • The ratio of the speed of the aircraft to the speed of sound in the gas determines the magnitude of many of the compressibility effects.
  • Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number in honour of Ernst Mach, a late 19th-century physicist who studied gas dynamics.
  • Subsonic conditions occur for Mach numbers less than one, M < 1.
  • As the speed of the object approaches the speed of sound, the flight Mach number is nearly equal to one, M = 1, and the flow is said to be transonic.
  • Supersonic conditions occur for Mach numbers greater than one, 1 < M < 3.
  • For speeds greater than five times the speed of sound, M > 5, the flow is said to be hypersonic.
  • The Space Shuttle re-enters the atmosphere at high hypersonic speeds, M ~ 25. Under these conditions, the heated air becomes ionized plasma of gas and the spacecraft must be insulated from the high temperatures.

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