Breaking the Sound Barrier and Sonic Boom

In aerodynamics, the sound barrier usually refers to flight at transonic speeds, between subsonic and supersonic speeds. The term came into use during World War II when a number of aircraft started to encounter the effects of compressibility, a grab-bag of unrelated aerodynamic effects. The term fell out of use in the 1950s when aircraft started to routinely "break" the sound barrier.


U.S. Navy F/A-18 at transonic speed. The origin of the cloud is debated; the leading theory claims: "a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets."

• What causes a sonic boom?

You can learn a lot about sonic booms by looking at the wakes boats leave in the water.

If you toss a pebble in a pond, little waves will form in concentric circles and propagate away from the point of impact. If a boat travels through the pond at 3 to 5 miles per hour, little waves will propagate in the same way both ahead of and behind the boat, and the boat will travel through them.

If a boat travels faster than the waves can propagate through water, then the waves "can't get out of the way" of the boat fast enough, and they form a wake. A wake is a larger single wave. It is formed out of all the little waves that would have propagated ahead of the boat but could not.

When an airplane travels through the air, it produces sound waves. If the plane is traveling slower than the speed of sound (the speed of sound varies, but 700 mph is typical through air), then sound waves can propagate ahead of the plane. If the plane breaks the sound barrier and flies faster than the speed of sound, it produces a sonic boom when it flies past. The boom is the "wake" of the plane's sound waves. All of the sound waves that would have normally propagated ahead of the plane are combined together so at first you hear nothing, and then you hear the boom they create.

It is just like being on the shore of a smooth lake when a boat speeds past. There is no disturbance in the water as the boat comes by, but eventually a large wave from the wake rolls onto shore. When a plane flies past at supersonic speeds the exact same thing happens, but instead of the large wake wave, you get a sonic boom.

• Jet-F-18 Breaking Sound Barrier

• Sonic Booms

• Breaking The Sound Barrier