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Sound travels as a wave through mediums such as air, liquid and plasma. In the air, the speed that the waves travel is determined by atmospheric conditions, so the speed of sound can vary depending on temperature. The sound barrier is a concept developed in the early 20th century, when many scientists believed that the drag on aircraft caused by approaching the speed of sound made it impossible for any aircraft to reach or exceed the speed of sound without being destroyed. Thanks to the brilliant and often daredevil aviators of the day, the concept of a prohibitive sound barrier was eventually disproved, and aircraft now routinely break it as they reach supersonic speed.
Since the speed of sound is based on the temperature of the medium it travels through, there is no constant speed at which the sound barrier will be broken. To give a general idea, many scientific publications list the speed of sound at 742 miles per hour (1194 kilometers per hour) based on a standard temperature of 32 degrees Fahrenheit (0 degrees Celsius.) But as the temperature shifts upward, so the sound barrier speed increases.
The original issue leading to the concept of a sound barrier is a result of propeller technology. As an aircraft approaches the speed of sound, the movement of the propeller becomes disruptive to sound waves, causing turbulence and reducing the performance of the aircraft. It seemed clear to many early aviation innovators an engine powerful enough to counteract the propeller's problems and continue to let the aircraft gain speed would be too large and heavy to work in the first place.
Yet, adventurous aviators didn't like the concept that there was a sound barrier preventing even greater speeds, and many attempted to prove that the barrier could be safely broken by achieving the speed of sound through extremely risky dives that would boost speed dramatically. Many early attempts ended in fatal crashes, as airplane technology couldn't handle the speed dives, and pilots could suffer illness from the steep dive and lose control of the plane.
Advances in aircraft design led to considerable improvement in the controls and performance of planes at high speed. By the 1940s, engineers had developed an improved tail that included a broad horizontal fin to allow greater lift and control. The first officially recognized sound barrier break was accomplished on 14 October 1947, by Air Force Capt. Chuck Yeager. Yeager, a well-known test pilot, used a highly advanced aircraft called the Bell X-1 to achieve a speed of 807.2 mph (1299 kph) and break the sound barrier at last. Other sources credit another test pilot, George Welch, with breaking the barrier days before, but as no US officials were present, his flight is officially discounted.
In modern times, supersonic speed is a common ability in many varieties of aircraft. Breaking the barrier has ceased to be a momentous event, although interest was again stirred in the late 1990s when a land vehicle broke the barrier 50 years after Yeager's flight. Still the concept of breaking the barrier was, for a time, one of the major focal points of aviation, leading to modern inventions such as the jet engine. The success of innovation and daring over what seemed like scientific certainty proved inspirational for many, and renewed the idea that any barrier may be broken by the bold.
One of the trickier things for our brains to wrap around is that sound actually is something that moves. Going faster than sound or breaking the sound barrier means passing it, much like passing a truck while driving a car. Although the noise of the truck can still be heard by us in our cars, if we were truly passing sound we would not be able to hear it at all unless we slowed down and allowed the sound to catch up.