If the plane is traveling slower then the speed of sound, then sound waves can spread out ahead of the plane. If it breaks the sound barrier and flies faster than the speed of sound, it produces a sonic boom when it flies past. The boom is the wake of the plane's sound waves. All the sound waves that would have normally spread out ahead of the plane are combined together, and you hear the boom. When you're on the shore of the ocean and a boat zooms past, at first there is no disturbance in the water, but shortly after, a large wave from the wake crashes up to the shore. When a plane flies past at supersonic speeds, the same thing happens. Instead of the large wake wave, you'll hear a sonic boom. Another way to think of sonic booms is to imagine all the molecules that make up our air. When planes fly through the air at moderate speeds, the molecules have time to move aside to let the plane through.
If the aircraft goes too fast, though, the molecules can't move aside, and the plane slams right into them--boom! A plane traveling below the speed of sound is going at subsonic speeds. Traveling at the speed of sound is transonic; speeds one times the speed of sound are supersonic, and hypersonic speeds are more than five times the sound barrier. Mach is another way of referring to the speed of sound. Flying at Mach 2, for instance, means you're flying at twice the speed of sound. How fast is the speed of sound? The answer depends on several factors, including how high the airplane may be flying--air becomes less dense at higher elevations, and it's easier for sound waves to travel. The generally accepted figure for the speed of sound is 1,220 kilometers (760 miles) per hour, which is the speed of sound at sea level.