The APH-5 Flight Helmet was introduced in the late 50's as a replacement for the US Navy H-4 series for use on fast jets.
Here's an Air Cav Huey pilot's
comment on the APH-5 flight helmet. "This standard Army issue helmet proved
to be grossly inadequate, and the high-pitched whine of the engines caused all
Vietnam helicopter pilots to lose high frequency hearing. The Army had to lower
their flight-physical hearing standards just to keep combat instructor pilots."