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SONIC BOOM [sonic boom] shock wave produced by an object moving through the air at supersonic speed, i.e., faster than the speed of sound. Since sound is a mechanical disturbance that propagates through the air, there is a limit to its speed. An object such as an airplane, moving through the air, generates sound. When the speed of the object reaches or exceeds the speed of sound, the object catches up with its own noise; at higher speeds, it forces the sound ahead of itself faster than the noise would ordinarily travel. The piled-up sound takes the form of a violent shock wave called a sonic boom propagating behind the object. Sonic booms occasionally have mechanically destructive effects in addition to their role as noise pollutants.
Originally posted by Jeddyhi
I've seen some of those photos you refer to. Thanks for the offer to upload them. I didn't realize that a sonic boom is heard as an object passes a particular location relative to the viewer. I thought it only happened at the moment of crossing the threshold.