Difference between Civil spec headsets and Military types? There are 3 main types of headset impedance specifications - none are interchangeable with each other without specific adapters / converters etc (available from Headset Services Ltd) The USA Mil headset is difficult to convert to civil spec, the NATO headset can be converted to civil spec with an adapter box. You cannot economically convert civil spec headsets to military spec. Even after conversion you may still encounter compatibility problems 1.Civil Specification headset - 600ohm total earphone impedance. Microphone is "100 ohm carbon level" which is achieved in a variety of ways - old style carbon mic. Amplified dynamic mic or amplified electret mic - as per David Clark headsets. 1.75m straight cable with twin plugs. 2.NATO spec Military headset - 150 ohm total earphone impedance. Microphone is a 150-ohm dynamic microphone. UK NATO plug. 3.USA Military Specification - 9.5-ohm total earphone impedance. Microphone is a 5-ohm dynamic microphone. USA U174/U NATO plug.