Have you ever seen a plane fly overhead at a supersonic speed? If so, you may have heard a loud “boom" as it passed by. Did it explode? Nope! You can still see it flying. Then what was that sound? It was a sonic boom.
A sonic boom is a loud sound kind of like an explosion. It's caused by shock waves created by any object that travels through the air faster than the speed of sound. Sonic booms create huge amounts of sound energy.
When an object moves through the air, it makes pressure waves in front of and behind it. Have you ever seen a boat move through water? The bow waves (front) and stern waves (back) are similar to the invisible pressure waves created by an object as it moves through the air.
At sea level and 68° F, the speed of sound through air is about 761 miles per hour. At an altitude of about 20,000 feet where the atmosphere is thinner and colder, sound travels at about 660 miles per hour.
Austrian physicist Ernst Mach developed a method of measuring airspeed relative to the speed of sound. If a plane if flying at the speed of sound, it is said to be going Mach 1. A speed of Mach 2 would be twice the speed of sound.
As an object, such as an airplane, travels faster and faster, the pressure waves can't get out of the way of each other. They build up and are compressed together. Eventually, they will form a single shock wave at the speed of sound.
The sonic boom we hear caused by an airplane flying at Mach 1 usually takes the form of a “double boom." The first boom is caused by the change in air pressure as the nose of the plane reaches Mach 1, and the second boom is caused by the change in pressure that occurs when the tail of the plane passes and air pressure returns to normal.
As long as an airplane travels at Mach 1 or faster, it will generate a continuous sonic boom. All those in a narrow path below the airplane's flight path will be able to hear the sonic boom as it passes overhead. This path is known as the “boom carpet."
If you're WONDERing about how pilots handle sonic booms, they actually don't hear them. They can see the pressure waves around the plane, but people on board the airplane can't hear the sonic boom. Like the wake of a ship, the boom carpet unrolls behind the airplane.