Breaking the Sound Barrier
What is the Speed of Sound?
The speed of sound is not a set number, but varies depending on air conditions -- temperature, humidity, dew point, and so forth. It is also less the higher you fly. Generally, the speed of sound is defined as approximately 750 mph at sea level on a 60-degree day. When Chuck Yeager, flying the Bell X-1, became the first man to travel faster than the speed of sound, he was flying at about 700 mph at an altitude of 43,000 feet -- or Mach 1.06. http://www.achievement.org/autodoc/page/yea0int-1
Because of the variables in measuring the speed of sound, we measure it at Mach 1. Mach 2 is twice the speed of sound, etc. The fastest any airplane has ever flown is Mach 6.7 -- a record set by the X-15, flown by Air Force Capt. Peter Knight on Oct. 3, 1967. The X-15  was built by North American.

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.