If you hear what sounds like a thunderclap, but there isn’t a cloud in the sky, the source of the noise might just be a sonic boom.
A sonic boom is the noise created by an aircraft or some other object when it surpasses the speed of sound. Typically clocking in at 110 decibels, sonic booms are indeed about as loud as thunder or an explosion. And they’re increasingly common along central Florida’s Space Coast as SpaceX continues to test its rockets.
But what exactly causes a sonic boom—and is it dangerous to be near one? To understand, you first need to think about air a bit like liquid.
What causes a sonic boom?
Sonic booms can be made by objects as small as bullets and whips or by natural phenomena like volcanic eruptions, meteor showers, and earthquakes. But they’re most associated with aircraft. Why is that?
When something—a space shuttle, say—accelerates to high speeds, it creates ripples of air molecules and sound that radiate outward in every direction, like a pebble dropped into a lake. However, as that object speeds up, the waves at the nose of the spacecraft start to pile up and compress rather than ripple outward. Meanwhile, waves of air and sound continue to push away from the sides and back of the ship, like the wake coming off a passing speed boat.
(Tonga's volcanic eruption produced a sonic boom heard more than 1,300 miles away.)
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Now, if the vehicle’s velocity continues to increase, eventually it will surpass the speed at which the waves in front of it can travel—meaning it has started to exceed the speed of sound, usually about 761 miles an hour at sea level.
At that point, so much pressure builds up in the front of the vehicle that it releases a large sound wave, also known as a shock wave, that sounds like an explosion. It’s this “sharp release of pressure” that we hear as a sonic boom, according to NASA.
What is a sonic boom carpet?
If the space shuttle continues to exceed the speed of sound, it will keep creating a cone of pressurized air molecules. So a sonic boom actually travels with an object as it moves, directing some of its sound waves toward the ground where we can hear them. This is known as the “sonic boom carpet.”
Interestingly, the size of the sonic boom carpet increases with the height of the object, to the tune of one mile per 1,000 feet of altitude. For example, NASA calculates that a plane traveling at 50,000 feet will create a sonic boom carpet that is 50 miles wide. Those directly beneath the path of the aircraft will experience the loudest parts of the boom, while those on the edges of the carpet will hear a much softer sound.
(Weird 'boomerang' earthquake created geologic version of a sonic boom.)
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At the same time, the aircraft’s movements or the area’s geography can affect the intensity of the sound waves it produces. Sudden accelerations, “S” turns, hills, and valleys can all amplify the sonic boom, for instance.
Are sonic booms dangerous?
The intensity of a sonic boom is measured in pounds per square foot of overpressure, and this increases with the size and speed of the object that created it.
The good news is that in almost all instances in which sonic booms are normally heard, the sound or overpressure created—around one to two pounds—is too weak to cause physical injury to people.
At two to five pounds of overpressure, some damage to structures may occur, according to NASA. This is most likely to occur with exceptionally large aircraft or those flying at unusually low altitudes. However, the agency estimates that most normal buildings can withstand overpressures of up to 11 pounds per square foot without damage.
By these metrics, human eardrums are safe: For a sonic boom to do damage, it would have to be nearly 600 times stronger than the overpressure created by a typical space shuttle landing.
History of the sonic boom
The space and aviation industries have had to carefully consider the impact of sonic booms as a result of certain operations.
The first aircraft capable of a sonic boom was the Bell X-1 rocket plane, which officially broke the sound barrier for the first time in 1947 while carrying test pilot Charles “Chuck” Yeager. The moment was immortalized in both literature and film through Tom Wolfe’s book and movie by the same name—The Right Stuff.
(The future of spaceflight—from orbital vacations to humans on Mars.)
In the years following that achievement, aircraft manufacturers hoped to usher in a new age of supersonic flights, and by 1976, an Anglo-French jetliner known as the Concorde was making regular commercial flights across the Atlantic Ocean. With sound-barrier-breaking speed, the Concorde was able to transport passengers from the U.S. East Coast to Europe in just three and a half hours—instead of the eight hours it typically takes.
However, the sonic booms that accompanied the flights proved unpopular with people in the Concorde’s flight path, and by 2003, the service was discontinued.
In the United States, supersonic flights over land have been banned by the Federal Aviation Administration since 1973, though exceptions such as those for the military and space agencies do still occur.
