During the process of introduction of the AFH-1 flight helmet, the US ARMY developed the new generation of crew helmets for specific use on helicopters.
 
Starting from from the US NAVY SPH-3 model as a basis, a new helmet was designed giving priority to crash and acoustic protection using Epoxy One Resin and a new ear cup design.
 
The new flight helmet, named SPH-4, was equipped with a new M87 mike, an improvement over the old M33 used on both APH-5 and AFH-1. 
 
The first SPH-4 helmets reached SEA in July 1969, but spare parts were delayed until beginning 1970.
 
By 1972, all units in combat zone were thus equipped, and APH-5 and AFH-1 were declared obsolete.
The clear or dark single visor had initially distortion problems, and this issue was fixed only by 1972.
 
The SPH-4 is still in use today , and interestingly was adopted by several Formula 1 drivers for a short period around 1970.