What Is a Sonic Boom?

Have you ever witnessed an aircraft flying above you at a speed faster than the speed of sound? If you have, you might have heard a loud noise resembling an explosion. Did the aircraft blow up? No! It was still flying. So, what was that sound? It was a sonic boom.

A sonic boom is a loud noise that is similar to an explosion. It is caused by shock waves produced by any object traveling through the air at a speed faster than the speed of sound. Sonic booms generate a significant amount of sound energy.

When an object moves through the air, it creates pressure waves in front of and behind it. Have you ever observed a boat moving through water? The bow waves (front) and stern waves (back) are comparable to the invisible pressure waves formed by an object as it moves through the air.

These pressure waves travel at the speed of sound. How fast is that? Pretty fast! The speed of sound varies depending on the material it travels through. It also changes with altitude and temperature.

At sea level and a temperature of 68° F, the speed of sound through the air is roughly 761 miles per hour. At an altitude of approximately 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 is traveling at the speed of sound, it is referred to as Mach 1. A speed of Mach 2 would be twice the speed of sound.

As an object, such as an airplane, accelerates, the pressure waves are unable to avoid each other. They accumulate and compress together. Eventually, they form a single shock wave at the speed of sound.

The sonic boom that we hear caused by an airplane flying at Mach 1 typically 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 flies at Mach 1 or faster, it will produce a continuous sonic boom. All those within a narrow path below the airplane’s flight path will be able to hear the sonic boom as it flies overhead. This path is known as the “boom carpet.”

If you’re curious about how pilots handle sonic booms, they actually cannot hear them. They can see the pressure waves around the plane, but passengers on the airplane cannot hear the sonic boom. Similar to the wake of a ship, the boom carpet unfolds behind the airplane.

Try It Out

Are you ready to take flight? Find a friend or family member to assist you in exploring the following activities:

  • Is it necessary to have an airplane to create a sonic boom? Not necessarily! Go online and watch a video of Chris Giorni from Tree Frog Treks as he creates a “sonic boom” using ordinary household items.
  • If you have been to a rodeo recently, you might even be able to create your very own sonic boom in your yard. Do you own a bullwhip? Bullwhips are commonly found on farms and are often sold as souvenirs at rodeos. If you have ever cracked a bullwhip, you are likely familiar with the loud cracking sound it produces. That sound is actually a mini sonic boom. When a whip is swung, energy travels from your hand through the whip to its end. By the time the energy reaches the end of the whip, known as the “cracker,” it is moving faster than the speed of sound. Similar to a supersonic airplane, the whip surpasses the speed of sound and creates the cracking sound you hear due to the compressed sound waves. Scientists believe that the bullwhip may have been the first human invention to break the sound barrier. So, if you happen to have a bullwhip at home, give it a crack. You probably never realized that you could create a sonic boom right in your own yard!
  • Have you ever wondered what it would be like to fly faster than the speed of sound? Let your imagination run wild. Picture yourself in the cockpit of a fighter jet. You push the throttle to its maximum and shoot up into the sky. What do you see? What do you hear? Write a brief story about your imaginary journey to Mach 1 and beyond. Share your story with a friend or family member. Would they like to experience flying faster than the speed of sound? Why or why not?

Sources of Wonder

  • http://www.howstuffworks.com/question73.htm
  • https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-016-DFRC.html

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