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.

Note:

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).

Impacts

• 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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