Sonic Boom

Since the dawn of human civilisation, the race for faster speed has always been there. With the invention of wheels, humans have always strived for faster speed. One of the most critical developments in this historical race was breaking the sound speed threshold.

Since the first successful launch of aeroplane flights, pilots have been trying to push the speed limits to go faster and faster. In 1947, significant design improvements, such as an adjustable horizontal stabiliser paved the way for an American military pilot Chuck Yeager to launch the Bell X one aircraft at 313m/s, becoming the first person to break the sound threshold, and travel faster than the speed of sound.

With greater power, there come greater consequences. When the speed of an object crosses the speed of sound, a tremendous amount of shock waves will be produced. It is generally called Sonic Boom. Sonic booms produce immense sound energy, similar to an explosion or a thunderclap. In this article, let us acquaint ourselves with the concept of Sonic Boom.

What is Sonic Boom?

A sonic boom is a thunderous noise caused by compressed moving sound waves. It is generated by an object moving faster than the speed of sound (about 335m/s at sea level).

When an aircraft breaks the sound speed, pressure waves merge and form shock waves that move forward from the generation or “release” area.

A sonic boom is very powerful, but it cannot be heard or detected in all directions. A sonic boom can be heard in the area of an imaginary three-dimensional cone at the back of the moving object. The sonic boom can be heard inside that geometrical zone as the object passes you. On the other hand, the sonic boom will be inaudible outside the imaginary zone.

In fact, the imaginary conical region also moves with the vehicle. As the object passes nearby, the observer experiences the boom for a very brief period.

Sonic Boom Principle

The basic physical principle behind the sonic boom is the superposition of pressure waves induced by extreme speed.

Throw a pebble in a pond, small waves will be created in concentric circles, and move away from the point of contact. If a boat moves on a water body at 2m/s to 3m/s, small waves will travel in the same manner, both front and behind the boat.

When the boat moves faster than the speed of the waves, the waves will not be able to get out of the trajectory of the boat, and they will form a wake. A wake is a single big wave. It is created out of all the small waves that would have moved in front of the moving object, but failed to.

Air acts like a fluid to supersonic objects. Whenever an object travels through the air, sound waves are produced. In the case of fast-moving aircraft, air disturbance will be large and powerful. As speed increases, the characteristics of the created sound waves also change. If the aircraft is moving less than the speed of sound, then sound waves can travel ahead of the aircraft. When the aircraft crosses the sound threshold and travels faster than the sound speed, it creates a sonic boom. ‘Wake’ of the sound wave is the sonic boom. Every sound wave that might have usually travelled ahead of the aircraft is superimposed together; within a fraction of seconds, you will hear the thunderous boom.

Factors that Control Sonic Boom

Many variables can influence sonic booms: weight, size, and shape of the aircraft or object. Besides physical attributes, the nature of the trajectory of the movement is also very crucial. Factors like weather conditions, altitude, flight path and attitude play vital roles in generating a sonic boom.

Sonic Boom Measurement

Sonic booms are calculated in pounds per square foot (overpressure). It is the amount of the increase over the usual atmospheric pressure (2,116 psf/14.7 psi).

Generally, the pressure caused by a sonic boom is around a few pounds per square foot.

With higher altitudes, there will be less pressure on the ground. The shock wave’s strength diminishes as it spreads away from the aircraft.

Effects of Sonic Booms

• A sonic boom creates a tremendous amount of noise and pressure variations.
• Anything in the path of its trajectory will surely experience the sonic boom. After-effects are strictly dependent on the source’s height (from the receiver) and other atmosphere variables.
• A mighty sonic boom might damage delicate building parts with large surface areas and low tensile strength.
  • No structural damage is caused by one pound overpressure.
    Minor damage might be generated with 2 to 5 pounds of overpressure.
  • Good condition structures can easily withstand overpressures up to 11 pounds.
  • Sonic boom generating overpressures around 20-144 pounds have not caused human injuries.
  • When overpressure hits 720 pounds, eardrums can be damaged.Lung damage is expected above 2160 pounds of overpressures.
• Sonic booms have caused serious effects on human behaviour and living experience, as noted by abnormal reactions and feelings of fear. It can disrupt sleep and relaxation.
• Unique cases such as insomniacs and people sensitive to noise should be given special consideration.
• In other cases, complaints to sonic booms are due to the belief that property has or can be damaged by such disturbances.

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