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What is the cause of the sonic boom?
Answer 1:

I'm enclosing 3 pictures to help explain this.
Imagine a jet flying along. It's a noisy jet, so it's emitting sound waves all the time.
These waves travel out from the jet at the speed of sound. If the jet is traveling slower than the speed of sound, this will look like the picture"subsonic.jpeg". Let me explain this picture. The circles around the jet are the sound waves. The outer-most circle is sound that was produced the earliest, since it's expanded the most since it was emitted. The innermost circle was produced last, just a moment before the picture was drawn, since it hasn't gotten very big yet. (Imagine dropping pebbles in a pond and watching the ripples. The biggest ripple is from the pebble you dropped the earliest.) The important thing to notice about this picture is that the waves are getting bigger faster than the jet is going.
See -- right ahead of the jet, are waves that are going even faster than the jet itself -- they're going to arrive where the jet is heading before the jet!

Now look at the next picture, "sonic.jpeg".
This jet is going exactly the speed of sound. There are no waves out ahead of the jet anymore. They're right with the jet, all built up at the jet's nose. This should make sense. The jet is moving at the speed of sound. So are the sound waves.
So they will all travel along together, at the same speed.

To exceed the speed of sound, the jet must break the "sound barrier" --
which is all those waves that are built up right around the jet's nose.
When the jet breaks through, and flies faster than the speed of sound, it will appear as in the last picture, "supersonic.jpeg." Now the jet is going faster than the waves. There are no waves out in front of the nose like in the first picture. But look what's happened. The circles aren't inside of each other like in the first picture. They're overlapping! The"V" lines show the shock front that has now formed. If you're standing on the ground, when that "V" reaches you, that means all of that sound is reaching you at the same time. All of the circles (the sound waves) will reach your ear at the same time and it will be really LOUD! That's the sonic boom.

FYI, Here's where I got the pictures from:

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