http://archive.armytimes.com/article/20090908/NEWS/909080313...
> Soldiers can adjust the rocker with a quick "click" depending on the amount of protection they need. When it's in the open or "weapons fire" position, sound can travel through the sound channel filter into the ear.
> For noisy environments that don't require an acute listening capability, such as around helicopters, troop carriers or generators, the rocker can be switched to the closed or "constant protection" position.
There's also larger over-ear kinds that are integrated with comms:
http://www.bosssafety.com/p-10356-peltor-comtac-iii-a-c-h-he...
This style has both active and passive configurations.