Fuzing
The traditional fuze is the same as a classic armor-piercing bomb: a combination of timer and a sturdy dynamic propeller on the rear of the bomb. The fuze is armed when the bomb is released, and detonates when the propeller stops turning and the timer has expired.
Modern bunker busters may use the traditional fuze, but some also include a microphone and microcontroller. The microphone listens, and the micro controller counts floors until the bomb breaks through the desired numbers of floors.
ATK is working on a Hard Target Void Sensing Fuze (HTVSF) for 2000 and 5000 pound weapons to explode when they reach an open space in a deeply buried bunker.
Read more about this topic: Bunker Buster