The best objective study seems to have been during "Operation Bongo II" (apparently) when 1,253 sonic booms were generated over Oklahoma City during the first half of 1964.
The Wikipedia article on the tests is fairly good:
Oklahoma City sonic boom tests - Wikipedia, the free encyclopedia
Perhaps the best quote is: "An FAA-hired camera crew, filming a group of construction workers, were surprised to find that the booms signalled their lunch break."