NSN# 8415-00-144-4178 by Gentex
Size'Regular' is Medium and fits a circumference of 20 3/8 to 22 1/4" and a hat size between 6 1/2 to 7 1/8
Safetech earphones
Dynamic Mic M-87/A1C
All pictures are of the helmet for sale here
$399

SPH-4 Helmet Details:

SPH-4 - Sound Protective Helmet is a derivative of the US Navy SPH-3 and was used by the US Army since 1970. It is a single-visor lighter-weight version of the SPH-3 and it replaced the two army aircrew helmet then in use (which was the APH-5). The SPH-4 was specifically designed for sound protection, provided superior sound attenuation but the earlier models (1970 version) provided no more impact protection than the old APH-5A. The SPH-4 was modied in 1974 with improvements to counter act and prevent head injuries. These improvements included a thicker energy absorbing liner to reduce the risk of concussions. The new liner was 1.27 cm thick and with the same density as the original liner (which was 97mm thick with a density of 5.2lb/ft3). In 1982 the risk of concussions was reduced even further by manufacturing the energy absorbing liner with a lower density 4.5lb/ft3. All in all the impact protection of this helmet (which is the one for sale here) was improved about 33% over the original SPH-4 from 1970.

Helmet retention was also improved. The original 1970 helmet had a chinstrap with a single snap fasteners on each side and was designed to withstand a load of 150 lbs. In 1978 a double-Y chinstrap with two snap fasteners was incorporated to reduce failures. This chinstrap had a failure limit of 250 lbs based on the adjustment buckle strength. In 1980 a third chinstrap was instroduced. It was fastened to the ear cup assembly on one side with a small screw and T-nut and the other side with two snap fasteners. This chinstrap had a failure limit of 300 lbs and it is this fastener that you see on the helmet for sale here.

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