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Here's a picture I found.
Picture the engine being located at the point of this cone.
If you are in front of the point, you will never hear it, if you are beside the point, you will never hear it, but in some places behind the point, you will hear it.
The cone get pointier the faster you go, and at 2 times the speed of sound I'm pretty sure it would be 67.5 degrees behind perpendicular.
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