4 min readNew DelhiMay 21, 2020 08:22 PM IST
First published on: May 21, 2020 at 07:40 PM IST
A stationary observer hears no sound when a supersonic flight approaches, since the sound waves are at the rear of the latter. (Photo: Wikimedia Commons)
The ‘loud sound’ heard in Bengaluru on Wednesday afternoon, which puzzled lakhs of city dwellers, 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’.
In a statement, the Ministry of Defence’s PRO in Bengaluru said, “The sonic boom was probably heard while the aircraft was decelerating from supersonic to subsonic speed between 36,000 and 40000 feet altitude.” It confirmed that the aircraft belonged to the Aircraft Systems and Testing Establishment (ASTE) and had flown in allocated airspace outside city limits.
Explaining the unusual sound heard in the city, the Training Command headquarters of the Indian Air Force, said in a separate statement, “These (test flights) are done well beyond the city limits in specified sectors. However, 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’.
When such aircraft fly at a low altitude, the sonic boom can become intense enough to cause 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 Indian Air Force website, India’s fastest jets include the Sukhoi SU-30 MKI (Mach 2.35) and the Mirage-2000 (Mach 2.3).
A stationary observer hears no sound when a supersonic flight approaches, since the sound waves are at the rear of the latter. (Photo: Wikimedia Commons)
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