About Shock Waves

Have you ever heard a sonic boom? Have you ever seen the shock waves that cause one? The first image shows shock waves from an F-18—a Navy jet—moving at supersonic speed. Shockwaves are regions of increased air pressure and temperature, and when these waves reach the ground, we hear the sonic boom. For another example, look at the “shadowgram” image of the speeding bullet. Here the first shock wave is curved and is actually slightly ahead of the bullet (it’s “detached”), and other shocks emanate from the bullet itself and even from the turbulent region of disturbed air directly behind.

Shock waves from an F-18 in supersonic flight. (image courtesy of NASA).

A bullet moving faster than the speed of sound. In this kind of image, called a shadowgraph, the air within the shock waves, which has a different density from the surrounding air, bends light passing through to cast shadows on a screen (image courtesy of Andrew Davidhazy, Rochester Institute of Technology).

The rocket-powered Bell X-1, the first plane to break the sound barrier. (image courtesy of NASA)

The drawings show waves produced at regular time intervals by an object moving through a fluid. When the object speed is less than the wave speed (top), the waves run out ahead. When the object speed equals the wave speed (middle), the waves pile up right in front of the object. When the object speed is greater than the wave speed (bottom), the waves form a wake.

To understand these shock waves, take a look at the diagrams below, which show waves spreading out from an object moving through a fluid. Such waves are produced continually as the object moves. Each diagram shows a series snapshots taken at regular time intervals. These waves spread out in circles, like ripples from a stone dropped into a pond, and move at the speed of the waves in the fluid. Notice that if the object moves faster than the speed of the waves, the circles form a “V”-shaped wake, like the wake of a ship or the shock waves from the nose of the F-18 in the image above.

As the diagram shows, when the object’s speed equals the wave speed, the waves pile up right in front of the object. In the 1940s, engineers feared that as an aircraft approached the speed of sound, this accumulation of waves could produce loss of control, and indeed, early attempts to break the sound barrier failed, with the loss of test aircraft and pilots. Finally, in 1947 Air Force pilot Chuck Yeager smoothly accelerated the Bell X-1 (see photo) to Mach 1.2, or 1.2 times the speed of sound, which at the X-1’s altitude is about 300 meters per second, (660 miles per hour).