Neighbors of bin Laden in Abbottabad, Pakistan, told ABC News they didn’t hear the helicopters the night of the Sunday raid until they were directly overhead. The rotor covering, along with a special rotor design, suppressed the choppers noise while inbound, Bill Sweetman, editor and chief of Defense Technology International, said.

“Helicopters make a very distinctive percussive rotor sound which is caused by their rotor blades and if you can blend that down, of course that makes a noise that is much less likely to be heard and much more likely to blend into any background noise that there is,” Sweetman said.