A sonic boom occurs when an object, such as a plane, exceeds the speed of sound, which is about 740mph in the air at sea level. The speed of sound way up in the air is slower though which makes it easier for a plane to make a sonic boom. WHat happens is that circular waves go in all directions around the plane. When the plane exceeds the speed of sound though, the waves cannot keep up with it, which leaves all of the sound generated by the plane behind it. I'm not sure what exactly what causes a 'sonic boom' or what it really is.
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