Why "Sound Barrier"?
Before 1947, it was believed that the speed of sound created a physical
barrier for aircraft and 
pilots.
As airplanes approach the speed of sound, a shock wave forms and the aircraft
encounters sharply increased drag, violent shaking, loss of lift, and loss
of control. In attempting to break the barrier, several planes went out
of control and crashed, injuring many pilots and killing some.
Eventually, the barrier proved to be mythical. Capt. Chuck Yeager,
who punched through the barrier in the X-1, later wrote in his autobiography:
"I thought I was seeing things! We were flying supersonic! And it was as
smooth as a baby's bottom. Grandma could be up there sipping lemonade."
What Is a "Sonic Boom"?
Sonic booms are created by air pressure. Much like a boat pushes up
a wave as it travels through water, a vehicle pushes air molecules aside
in such a way they are compressed to the point where shock waves are formed.
The shock waves move outward and rearward in all directions and usually
extend to the ground. As the shock cones spread across the landscape along
the flightpath, they create a continuous sonic boom along the full width
of the cone's base. The sharp release of pressure, after the buildup by
the shock wave, is heard on the ground as the sonic boom.
Fifty years ago, aircraft encountered serious turbulence from the accumulating
shock wave. As the first to successfully ?punch through? the sound barrier,
Col. Yeager was the first to report that smooth flight resumed ?on the
other side.? Trying to break the sound barrier had already killed
several pilots who lost control when they hit the shock wave. The 24-year-old
Yeager encountered the same turbulence as other pilots, but tried something
new -- he slammed the throttle forward and literally punched his way through
the previously impenetrable barrier. On the other side, the flight returned
to its routine smoothness.