How the Astounding Sonic Boom Phenomenon Actually Works?

Understanding the phenomenon behind the loud shockwave and how it is restricting the testing of supersonic jets.

Humans have been obsessed with speed for ages. They are finding out ways to travel faster which have led to many inventions that we see today including the high-speed aircraft.

It was this fascination for speed for humans which triggered an American military pilot, Chuck Yeager to speed test an aircraft in which he managed to travel 428 m/s, breaking the sound barrier for the very first time and travel faster than the speed of sound.

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When an object travels equal to or faster than the speed of sound, it produces a shock wave called Sonic Boom which can be heard as thunder or a big clap by the people on the ground.

How Is Sonic Boom Generated?

When an aircraft is traveling through the air, it produces sound waves. The sound waves produced in front of the air is compressed as the aircraft moves faster.

The faster the aircraft move, the closer these waves get. As long as the object's velocity does not cross the speed of sound which is 340.29m/s, the waves will not collide with each other.

But when the object is traveling faster than the speed of sound, the sound waves produced cannot move from one another since the speed is beyond that of sound — thus colliding with each other. That is the object emitting the wave is traveling faster than the waves themselves.

This cause the waves to force themselves or combine into a single shock wave traveling at a critical speed known as 'Mach 1' and has an approximate value of 1,235 km/h. So a "boom" is heard due to this compression of the sound waves. These are called sonic booms.

Supersonic Speed
Source: Chabacano/Wikimedia Commons

This generates a great amount of sound energy thus making a sound similar to that of thunderclap to our ears. According to NASA, a sonic boom happens when the air reacts like a fluid to supersonic objects and the force created by objects pushing aside air molecules as they are traveling through the air forming a shock wave, which is like a boat breaking the water.

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The booms can continue as far as the object is moving in supersonic speed. The waves are formed in a conical shape behind the object.

When the observer intersects in the position of this conical region, they get to experience the boom. To the person inside the aircraft, the sound seems to come from behind the plane because the sound heard was emitted by the plane many seconds earlier. The plane is flying ahead of its sound.

There is also a pressure variation in the nose and tail of the aircraft. Overpressure profile is experienced due to this, known as N-wave because of its shape. So, a supersonic aircraft can also have “double boom.”

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The boom is generated continuously as long as the aircraft is supersonic. A narrow path on the ground is generated along the flight path of the aircraft known as “boom carpet.”

How Large Can A Sonic Boom Be?

The size of a sonic boom depends on the size and weight of the aircraft. The intensity of the sonic boom is based on the aircraft’s length and its cross-sectional area, whereas its shape depends on the local air turbulence near the ground.

The direction in which the sonic boom travel and the strength of shock waves generated by the compression of sound waves are influenced by wind, speed, direction, and also the air temperature and pressure.

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Can It Cause Damage?

The intensity of the boom can be measured in pounds per square foot (psf) of air pressure. It is the amount of pressure that is increased from the normal pressure around us (2,116 psf/14.7 psi).

And at the measurement of one pound overpressure, there is no expected damage to any structures. Supersonic aircraft flying at normal operating altitude has overpressures measured from of 1 to 2 psf.

The booms caused due to large supersonic aircraft can be loud which catches people's attention, and animals can be annoyed with its sound. Strong booms may also cause minor damage to the building structures.

Buildings in good condition can withstand shockwaves up to 11 psf without causing any damage. However, a shockwave of less than two psf will have a minor chance for affecting historical structures and those buildings of bad condition.

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If the overpressure increases, the chances for structural damage and stronger public reaction are also increased. Tests have shown that structures which are in good condition have been undamaged by overpressures of up to 11 psf, meaning that it depends on the condition of the building structures whether it gets affected or not.

Future of supersonic flights

A sonic boom is considered as a problem for supersonic flights. To mitigate the problems associated with a sonic boom, there’s significant research going on. Recently, NASA signed a contract with Lockheed Martin Aeronautics Company of Palmdale for testing an airplane that can travel with a quiet sonic boom.

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This can let them have the permission for flying the supersonic flights over the land without causing any disturbance to the people. The first flight is scheduled to be on 2021.

According to the contract, the work will be beginning from April 2 until December 31 of 2021. The experimental aircraft is known as X-plane.

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NASA says that the plane will be traveling at high speeds of about 940 mph and instead of the sonic boom, the sound generated will only be that of a car door closing which is of 75 (PLdB), which will not be noticeable to the people.

The demonstration is named Low Boom Flight Demonstration (LBFD) and the design is called as QueSST (Quiet Supersonic Transport) aircraft design. The LBFD flight will be using the existing General Electric F414 engine and there will not be any major changes from other supersonic flights.

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In the mid of 2020, the design will be ready according to the contract, and NASA will be flying the X-plane above the U.S. cities. They aim for collecting the responses of people and use this data to make necessary changes in the rules of flying supersonic flights over the land.

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