Boom

Perhaps one of the most exciting developments in aviation in the 20th century was the innovation of hyper-sonic flight.  In 1947 Chuck Yeager broke the sound barrier for the first time, flying an X-1 at Mach 1.07 (or 7% faster than the speed of sound) and as a result produced the first aircraft-related sonic boom.
 
The explosion-like sound of a sonic boom is caused by a quick change in air pressure associated with hyper-sonic travel.  During sub-sonic travel (like a car) sound waves are propagated in front of the vehicle and behind it.  This means that an observer can hear a car both approaching them and driving away from them.  When a car is approaching a person, the sound waves can only travel at a finite speed, and the speed of the vehicle causes the sound waves to stack on top of one another and gives it a high pitched sound.  When the car is driving away, the sound waves are stretched out due to the car propagating sound in the opposite direction of its travel, which gives it a low-pitched sound.  This is called the Doppler effect.

When a vehicle is traveling faster than the speed of sound, the Doppler effect is distorted because the vehicle can not propagate sound waves in front of the vehicle, and as a result both the sound of its approach and leaving are propagated behind the vehicle.  Additionally, the speed of the vehicle allows it to displace air around the shape of its body as it passes, leaving a partial vacuum behind it which the air can not travel fast enough to fill immediately.  This has the effect of creating a large and short sound wave with both high and low frequencies, which to an observer sounds very similar to an explosion.

The first truly hyper-sonic machine was actually the bull whip, which allows the tip of the whip to reach speeds large enough to break the sound barrier and create a sonic boom, which sounds a lot like a firecracker.