“When the experimental XB-1 aircraft broke the sound barrier three times during its first supersonic flight on 28 January, it did not produce a sonic boom audible from the ground…

the XB-1 took advantage of a physics phenomenon called the Mach cutoff. Because sound moves more slowly at higher altitudes, an aircraft breaching the sound barrier at those heights will produce a boom that cannot reach the ground – if the boom moves downward, the increasing speed of sound will deflect it, pushing its shock waves upward instead.”

From New Scientist.