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Union University Department of Physics

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Science Guys > April 2001

April 2001

What happens when something breaks the sound barrier?

On October 14, 1946, a small, almost rocket type plane called the Bell X-1 was dropped from a large B-29. Capt. Chuck Yeager fired the X-1 engine and was accelerated past the sound barrier becoming the first man to travel faster than the speed of sound. The speed at which sound travels is known as the sound barrier. The speed of a sound wave actually varies with temperature and air density, increasing about 0.6 m/s for every Centigrade degree temperature increase. At 68° F the speed of sound is about 343 m/s or 767 mph at sea level. Exactly why is this speed called the sound barrier?

A plane produces sound that radiates out from the plane in all directions. The waves propagating in front of the plane get crowded together by the motion of the plane. As the plane approaches the speed of sound, the sound pressure "waves" pile up on each other compressing the air. The air in front of the plane exerts a force on the plane impeding its motion. As the plane approaches the speed of sound, it approaches this invisible pressure barrier set up by the sound waves just ahead of the plane. The compressed air in front of the plane exerts a much larger than usual force on the plane. There is a noticeable increase in the aerodynamic drag on the plane at this point, hence the notion of breaking through the "sound barrier." When a plane exceeds the speed of sound it is said to be supersonic. Often supersonic speeds are referred to in terms of a Mach number. The Mach number is the speed of the object divided by the speed of sound. Thus Mach 3 means three times the speed of sound.

Imagine a boat traveling through the water. The boat pushes the water and a wave crest goes out from the bow of the boat and spreads across the lake. This conical bow-wave visible on the surface of water, called a wave-front, is similar to an airplane’s sonic boom. When an aircraft is flying at supersonic speeds the sound pressure forms a cone whose vertex is at the nose of the plane. Consider a supersonic aircraft flying toward you while you look up at it from the ground. Initially, you hear nothing because the plane is moving faster than the sound itself but when the sound pressure cone arrives at your ear you hear a boom. An object traveling through the air causes sound wave energy (air) to pile up along a conical line (like the bow wave of a boat) called a wave-front. As these waves pile up, a very large pressure difference exists across the wave-front, which is called a shock wave. As this wave-front passes an individual, the sudden pressure differential or change in pressure creates the "sonic boom" that we hear.

Anything exceeding the speed of sound creates a "sonic boom", not just airplanes. An airplane, a bullet, or the tip of a bullwhip can create this effect; they all produce a crack. This pressure change created by the sonic boom can be quite damaging. In the case of airplanes, shock waves have been known to break windows in buildings. Shock waves have applications outside of aviation. Kidney and gallstones are broken up with a technique called extracorporeal shock-wave lithotripsy. This technique uses waves that are outside our normal hearing range but nevertheless are still waves. A shock wave is produced outside the body and focused by a reflector so that it converges on the stones. The stress created by the shock waves causes the stones to be broken into small pieces that can then be eliminated.