The boom is caused by the plane hitting the air.
The intensity of the boom is higher or lower depending on (in ELI5 language) the slickness and length of the aircraft.
One feature the boom SST will have, is a very long nose to increase the time between the air compressing at the start of the plane, and where it mixes with the more slower air around the wings and such.
This will reduce the intensity of the boom to a thud.
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