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.
Kashyap Vyas

Some people are obsessed with speed, while others prefer a more sedate pace. But there is no denying that travelling faster saves time, and that for much of human history we have been working on ways to speed up. 

It was American military pilot, Chuck Yeager who first managed to travel faster than the speed of sound (343 m/s), and break the sound barrier.

SEE ALSO: 7 of the Deadliest Fighter Jets Still in Active Service

Yeager and those who worked with speeding objects before him, noticed that, when an object travels equal to or faster than the speed of sound, it produces a shock wave, commonly called a Sonic Boom, a thunder-like noise that can be heard for some distance. 

How Is Sonic Boom Generated?

When an aircraft is traveling through the air, molecules are pushed aside with great force, and this forms a shock wave,  like the way a boat creates a wake in water. The bigger and heavier the aircraft, the more air it displaces.

The waves produced in front of the aircraft are 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 shock waves produced cannot move away from one another fast enough and collide with each other. That is, the object emitting the wave is traveling faster than the waves themselves.

The shock wave forms a “cone” of pressurized or built-up air molecules, which move outward and rearward in all directions and extend all the way to the ground. As this cone spreads across the landscape along the flight path, it creates a continu­ous sonic boom along the full width of the cone's base. The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom.

Supersonic Speed
Source: Chabacano/Wikimedia Commons

 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.

The booms can continue as long as the object is moving in supersonic speed. 

When the observer intersects with the 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 between the nose and tail of the aircraft. This means that all aircraft generate two cones, one at the nose and one at the tail. They usually have a similar strength. The time interval between when each of the two cones reach the ground depends largely on the size of the aircraft and its altitude.

A large aircraft, or one flying at a high altitude can produce two sonic booms. For example, many sonic booms produced from NASA research flights or Space Shuttles are heard as  “double” booms. This is the result of the generation of two separate cones, at the nose and the tail of the aircraft.

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 intensity 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 travels and the strength of the shock waves generated by the compression of sound waves are influenced by wind, speed, direction, and also the air temperature and pressure.

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 have overpressures measured from 1 to 2 psf.

The booms caused by large supersonic aircraft can be loud, which certainly catches people's attention. Particularly strong booms may also cause minor damage to the building structures.

Most buildings in good condition can generally withstand shockwaves up to 11 psf without causing any damage. However, a shockwave of less than two psf still has a minor chance of affecting historical structures and buildings that are not structurally sound.

If the overpressure increases, the chances for structural damage is, of course, increased. 

Future of supersonic flights

The nuisance of the sonic boom is considered a problem for the future of supersonic flights. To mitigate the problems associated with a sonic boom, there’s significant research taking place. NASA has even signed a contract with Lockheed Martin Aeronautics Company for testing an airplane that can travel with a quiet sonic boom.

NASA says that the plane will be traveling at high speeds of about 940 mph and instead of the loud clao from a sonic boom, the sound generated will be similar to that of a car door closing, around 75 PLdB, which will not be noticeable to most people.

This could allow supersonic flights over populated areas without causing any disturbances to people. The plane, the Lockheed Martin X-59 QueSST (Quiet SuperSonic Technology) is part of NASA's Low-Boom Flight Demonstrator program. The X-59 is scheduled for delivery in late 2021, with test flights set to being from 2022. The plane is expected to cruise at Mach 1.42 (1,510 km/h; 937 mph) and 55,000 ft (16,800 m).

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