What is a sonic boom? Is it dangerous?

A sonic boom produced by a US Air Force F-22 Raptor stealth fighter in flight in 2009. © RONALD DEJARNETT, US NAVY/HANDOUT/REUTERS/REDUX

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There seems to be a common misunderstanding about "sonic boom", that is, people think that sonic boom only occurs once, but in fact, during the entire supersonic flight, the sonic boom continues in the landing area. Wherever the aircraft flies, it will drag the noise shock wave behind it. Therefore, supersonic aircraft can only fly over the sea or fly to the upper atmosphere above 25 kilometers, because there is almost a vacuum there, and the atmospheric density is only less than 5% of that on the ground. Otherwise, this route will inevitably cause residents to protest against noise. The Concorde mentioned in the article was suspended, and noise was one of the factors.

One key to eliminating sonic booms is the design of the fuselage structure. In 2018, Lockheed Martin built an aircraft with a low sonic boom. The aircraft, called X-59 QueSST, uses quiet supersonic technology and will reach a speed of Mach 1.42. According to NASA, the noise of this aircraft is about the same as the sound of closing a car door.

If you hear a sound like thunder and there are no clouds in the sky, it is most likely a sonic boom.

A sonic boom is the noise created when an aircraft or other object exceeds the speed of sound. It's literally as loud as thunder or an explosion, often reaching 110 decibels. They're becoming more common along Florida's Space Coast as SpaceX continues to test rockets.

So what causes a sonic boom, and is it dangerous to be near one? To understand these questions, we first need to think of air as a liquid.

What causes a sonic boom?

Sonic booms can be produced by small objects like bullets and bullwhips, but they can also be caused by natural phenomena like volcanic eruptions, meteor showers and earthquakes. However, they are most commonly associated with airplanes. Why is that?

This NASA image depicts two US aircraft flying at supersonic speeds, about 30 feet apart, creating the shock waves that interact and are heard as sonic booms. © NASA, NAT GEO IMAGE COLLECTION

When something (like a spacecraft) accelerates, it generates sound waves that radiate out in all directions, like ripples from a pebble thrown into a lake. However, as the object speeds up, the ripples at the front of the spacecraft begin to pile up and compress, rather than ripple outward. Meanwhile, the sound waves continue to push outward from the sides and rear of the craft, like the wake left by a passing speedboat.

If the spacecraft continues to accelerate, eventually it will exceed the speed of the sound wave in front of it, which means it has reached 1,225 km/h.

At this point, so much pressure builds up in front of the spacecraft that it causes a huge sound wave to be released, which sounds like an explosion. It is this "sudden release of pressure" that we hear that is called a sonic boom, according to NASA.

The moment a U.S. F/A-18 Hornet fighter jet creates a sonic boom. © Techshots's Newsletter/Substack

What is the sonic boom touchdown zone?

If the spacecraft continues to exceed the speed of sound, it will create a cone-shaped area of ​​compressed air molecules. So the sonic boom actually moves with the spacecraft, directing some of its sound waves toward the ground where we can hear it. This is called the "sonic boom touchdown zone."

Interestingly, the size of the sonic boom area is proportional to the altitude of the object, with the touchdown area increasing by about 1.6 kilometers for every 1,000 feet of altitude increase. For example, NASA calculated that an aircraft flying at an altitude of 15.24 kilometers will produce a sonic boom touchdown area with a diameter of 80.5 kilometers. Those directly under the aircraft's flight path will hear the loudest sonic boom, while those at the edge of the touchdown area will hear a quieter sound.

© Tenor

The intensity of the sound waves generated can also be affected by the movement of the aircraft or the geographical conditions of the area. For example, sudden acceleration, S-turns, hills and valleys can amplify the effect of a sonic boom.

Are sonic booms dangerous?

The strength of a sonic boom is measured in pounds-force per square foot (psf), a value that increases with the size and speed of the object producing the boom.

The good news is that in almost all cases where sonic booms are commonly heard, the sound or overpressure produced (on the order of 1 to 2 pounds) is too small to cause physical injury.[1]

Still from Top Gun: Maverick (2022). © The Mirror

According to NASA, some damage to buildings may occur when overpressure reaches 2 to 5 pounds. This is most likely to occur with large aircraft or when flying at very low altitudes. However, NASA estimates that most buildings can withstand overpressures of up to 11 pounds per square foot without damage.[2]

By these standards, human eardrums are safe: To cause damage to an eardrum, the overpressure would have to be 600 times greater than that generated by a space shuttle landing.

History of Sonic Booms

The space and aerospace industries must carefully consider the effects of sonic booms in certain operations.

The first aircraft capable of producing a sonic boom was the Bell X-1 rocket plane, which broke the sound barrier for the first time in 1947 while being flown by test pilot Charles “Chuck” Yeager. The moment was immortalized in literature and film by Tom Wolfe’s book and film of the same name, The Right Stuff.

Bell X-1 rocket plane. © Wikipedia

In the years following that achievement, aircraft manufacturers hoped to usher in a new era of supersonic flight, and by 1976, an Anglo-French jetliner known as the Concorde began regular commercial flights across the Atlantic. With its sound-barrier-breaking speeds, the Concorde was able to carry passengers from the East Coast of the United States to Europe in just three and a half hours, a journey that would normally take eight hours.

Concorde first flew in 1969 and entered service in 1976, mainly used to carry out scheduled transatlantic routes from London Heathrow Airport (British Airways) and Paris Charles de Gaulle International Airport (Air France) to and from New York John F. Kennedy International Airport. The aircraft can cruise at 2.02 times the speed of sound at an altitude of 18,000 meters, and it only takes about 3 hours and 20 minutes to fly from Paris to New York. © Steam Community

However, the sonic booms produced by Concorde flights were too unfriendly to residents below the route, and the flights were terminated in 2003. In the United States, the Federal Aviation Administration has banned supersonic flights over land since 1973, although there are exceptions for certain circumstances such as the military and space agencies.

References:

[1]www.nasa.gov/centers/dryden/pdf/120274main_FS-016-DFRC.pdf

[2]www.nasa.gov/centers/armstrong/news/FactSheets/FS-016-DFRC.html

By Jason Bittel

Translated by tim

Proofreading/Rabbit's Light Footsteps

Original article/www.nationalgeographic.com/science/article/sonic-boom-cause-shock-wave-physics

This article is based on the Creative Commons License (BY-NC) and is published by tim on Leviathan

The article only reflects the author's views and does not necessarily represent the position of Leviathan