Tobias Rossmann, a research engineer with Advanced Projects Research and a visiting researcher at the California Institute of Technology, provides the following explanation.
Image: U.S. Navy photo by ENSIGN JOHN GAY
Any discussion of what happens when an object breaks the sound barrier must begin with the physical description of sound as a wave with a finite propagation speed. Anyone who has heard an echo (sound waves reflecting off a distant surface) or been far enough away from an event to see it first and then hear it is familiar with the relatively slow propagation of sound waves. At sea level and standard atmospheric conditions of 22 degrees Celsius, sound waves travel at 345 meters per second (770 miles per hour). As the local temperature decreases, the sound speed also decreases, so for a plane flying at 35,000 feet¿where the ambient temperature is ¿54 C¿the local speed of sound is 295 meters per second (660 miles per hour).
Because the propagation speed of sound waves is finite, sources of sound that are moving can begin to catch up with the sound waves they emit. As the speed of the object increases to the sonic velocity (the local velocity of sound waves), these sound waves begin to pile up in front of the object. If the object has sufficient acceleration, it can burst through this barrier of sound waves and move ahead of the radiated sound. The change in pressure as the object outruns all the pressure and sound waves in front of it is heard on the ground as an explosion, or sonic boom.
At supersonic speeds (those greater than the local sound speed), there is no sound heard as an object approaches an observer because the object is traveling faster than the sound it produces. Only after the object has passed will the observer be able to hear the sound waves emitted from the object. These time periods are often referred to as the zone of silence and the zone of action. When the object has passed over the observer, the pressure disturbance waves (Mach waves) radiate toward the ground, causing a sonic boom. The region in which someone can hear the boom is called the boom carpet. The intensity of the boom is greatest directly below the flight path and decreases on either side of it.
U.S. Navy Ensign John Gay captured one of the best images ever taken of a sonic boom (the breaking of the sound barrier) in 1999. He snapped a photo of an F/A-18 Hornet on a humid day from the weather deck of the USS Constellation in the Pacific Ocean (see image). Because aircraft wings generate both low-pressure regions (because of lift) and amplified low-pressure disturbances, large low-pressure regions exist near the aircraft, especially under sonic flight conditions. The lowered pressure condenses the water in the air, creating a vapor cloud. As the jet produces these pressure waves and propagates ahead of them, the regions of lower pressure are usually strongest behind the nose of the jet, on the wings and body. As the aircraft continues to speed up, the vapor cloud will appear farther toward the rear of the aircraft. Then, just as the aircraft bursts through the sound barrier, the air is locally disturbed by the resulting shock wave and the condensation/vapor cloud disappears. Ensign Gay snapped his photo at the moment he heard the boom, just before the cloud vanished. Thus, it literally appears as if the F-18 is pushing through the sound barrier at the instant the photo was taken.