The loudest boom I've ever heard was just recently while at home in Tucson. The Thunderbirds were practicing for an airshow at Davis-Monthan AFB when one of the team went supersonic in straight and level flight. Plenty of broken windows south of my house, but no injuires.
Years ago when I lived well south of Tucson it wasn't that rare to hear sonic booms in the distance.
When I lived in Florida, north of Tampa, we heard booms from the space shuttle when it returned from orbit quite often.
There are designated areas in Nevada and California where military are actually permitted to go supersonic during testing and training. Some good information can be found here:
Civilian aircraft are banned from supersonic flight over the US, but it is permitted beyond the ADIZ.
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