Article by Sgt. Jiban Krishna Haldar (Retd.)
What is Sonic Boom?
Sonic boom is a loud sound caused by shock waves created by any object or moving aircrafts that travels through the air faster than the speed of sound (above 1 Mach). Sonic booms create huge amount of energy producing huge sound something similar to an explosion or a thunderclap to the human ears.
Causes of Sonic Boom
- When an aircraft travels at subsonic speed, the pressure disturbances so generated extend in all directions and this pressure disturbance is transmitted earthward continuously to every point along the path and so there is no sharp change of pressure causing any boom.
- However, at supersonic speeds, the pressure field is confined to a region extending mostly to the rear and extending from the aircraft in a restricted widening cone (called a Mach cone). As the aircraft proceeds, the trailing parabolic edge of that cone of disturbance intercepts the atmosphere producing a sound of a sharp bang or boom.
- When aircraft travels at supersonic speed, a rise in pressure occurs at the nose, and steadily decreases to a negative pressure at the tail, followed by sudden return to normal pressure after the object passes. This causes a “overpressure profile” known as an N-wave. The “boom” is experienced when there is a sudden change in pressure; therefore, an N-wave causes two booms – one when the initial pressure-rise reaches to the observer, and another when the pressure returns to normal. This leads to a distinctive “double boom” from a supersonic aircraft. If the aircraft is especially long, double sonic booms might be detected, one emanating from the leading edge of the plane and one from the trailing edge.
- For steady supersonic flight, the boom is described as a carpet boom since it moves with the aircraft as it maintains supersonic speed and altitude and the boom is continuous for the entire supersonic flight as pressure waves continuously move along the boom carpet.
Intensity of Sonic boom
Intensity of Sonic boom may be huge. A vehicle flying at greater altitude will generate lower pressures on the ground, because the shock wave reduces in intensity as it spreads out away from the vehicle, but the sonic booms are less affected by vehicle speed. When an aircraft flies at a low altitude with supersonic speed, the shock wave may be of sufficient intensity to cause breakage of window glasses of buildings and other damage. The ground also can influence the sound of a sonic boom. Hard surfaces such as concrete, pavement, and large buildings can cause reflections which may amplify the sound of a sonic boom. The intensity of the sonic boom is determined mostly by following factors:
- distance between the aircraft and the ground
- the size and shape of the aircraft
- the types of manoeuvres that it makes
- the atmospheric pressure and temperature
- Wind flow and direction
Yes: Pilots Cannot Hear Sonic Boom
One might be wondering how pilots handle sonic booms. Both pilots and passengers actually don’t hear them. They can only see the pressure waves (boom carpet) around the plane. But it affects only observers that are positioned at a point that intersects a region in the shape of a geometrical cone behind the object. As the object moves, this conical region also moves behind it and when the cone passes over the observer, they will briefly experience the boom. Depending on the aircraft’s altitude, sonic booms need some time to reach the ground after flyover. The speed of sound at any altitude is a function of air temperature. Under standard atmospheric conditions, air temperature decreases with increased altitude. This temperature gradient helps bend the sound waves upward. Therefore, for a boom to reach the ground, the aircraft speed relative to the ground must be greater than the speed of sound at the ground. So the man at ground can hear the boom sound but the pilot can’t as he has already crossed the range of sonic speed (speed of sound) which is needed to strike the ear drum to produce sound for listening.
How to control sonic boom?
Due to huge amount of sound intensity, sonic boom can cause huge noise pollution which may result in crack or damage of buildings, ability of hearing in humans and animals and other issues. Several research works are conducted which can help to contain the issue of sonic boom including design modification of flying vehicles, changing characteristics of altitudes etc. In 1964, NASA and the Federal Aviation Administration (FAA) began the Oklahoma City sonic boom tests, ultimately entangled the government in lawsuit, and lost on appeal in 1969. So far it is understood that the sonic booms cannot be completely prevented but research suggests that with careful shaping of the vehicle, the nuisance due to the sonic booms may be reduced to some extent.
- “Review of Sonic Boom Theory” by K.J. Plotkin