Original source : http://www.scientificamerican.com
Posted : March 2002
Author : Tobias Rossmann
What is the sound barrier?
The sound barrier, in aerodynamics, is the point at which an
object moves from transonic to supersonic speed. The term, which occasionally
has other meanings, came into use during World War II, when a number of
aircraft started to encounter the effects of compressibility, a collection of
several unrelated aerodynamic effects that "struck" their aircraft
like an impediment to further acceleration. By the 1950s, new aircraft designs
routinely "broke" the sound barrier
(http://en.wikipedia.org)
Tobias Rossmann, a research engineer with Advanced Projects
Research and a visiting researcher at the California Institute of Technology,
provides the following explanation.
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 54oC 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.
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