BREAK THE SAOUND BARRIER
Not long after the first flights in the early 20th century, several pilots tried to break the sound barrier and fly supersonically but faced great resistance from the air, several times with tragic results. The first pilot to make it was American Chuck Yeager in 1947.
So began a race between America, Europe and the former Soviet Union to create the first ultrasonic plane. Concorde, the result of Anglo-French cooperation, has operated transatlantic flights since 1976, travelling twice as fast as the sound, but stopped in 2003 because it was unprofitable. The Soviet Union created the supersonic Tu-144 that flew only for a few years.
Supersonic planes, however, have one serious drawback: when they fly faster than the sound they produce, they create shock waves that reach Earth as sonic explosions known as "sonic booms." These noises disturb people and animals and can damage buildings and are the main reason that the ultrasonic flight program was cancelled in America in 1971. Since 1973 ultrasonic flights have not been allowed over land.
Scientists around the world have been studying sonic booms for decades, trying to predict their path to the atmosphere, landing and how strong they will be so that they can contain them.
To understand how sonic booms are created, we need to start with the basics. When you throw a stone into a quiet lake, what do you notice? The disturbance of the water from the stone is transferred to the surrounding water in the shape of circles that grow. These circles are called wave fronts. In a similar way, even though we don't see it, an audio source, like your phone, disrupts the air around it by creating sound waves that reach your ear and so you hear it. Wave fronts are spheres that grow inside each other. Sound travels on "sound rays" as light travels on rays but is about a million times slower – that's why we first see a lightning bolt and then hear the thunder. The speed of sound changes with height, air temperature, etc. – at sea level it is about 1225 km/h.
Now think of a plane which is an animated sound source. As the plane moves more slowly than the sound that produces the wave fronts they crowd in front of the plane but are still inside each other. When the plane moves ultrasonically, the image changes. The plane moves faster than the sound it produces and all the waves crowd, in the form of a cane, behind the plane creating strong shock waves that reach the ground. Simulations with mathematical models on the computer use the above basic principles for sonic booms to adjust the routes of airplanes accordingly.
Sonic booms are not created only by supersonic planes! They are produced by every object that moves faster than sound – such as spaceships, whips and spheres. It is believed that even some dinosaurs were creating sonic explosions by quickly platforming their tails to scare their opponents!
