The Comtac headset is a double-earpiece communications interface with output sound in both ear and a microphone on an adjustable boom. Despite offering well padded earpieces on both sides, the headset also has additional booster microphones on the outside to pipe in sound through the headset digitally As such, both the radio communication sound and the environmental sound is coming through the earpiece. By adjusting the volume of your radio and the volume control on the headset booster you can balance the two sound levels so that you can hear both the comms traffic and your surroundings without obstruction. You can even crank up the volume to actually enhance the surrounding sounds to increase your auditory perception or turn it down if your surroundings are drowning out your comms or your concentration Although the Airsoft versions booster mics are not as good as the real thing they are still not bad. Both Comtac Headsets I and II are similar, both running on a pair of AA batteries (not included). With no batteries, the booster mics will not be turned on but the headset will still function with your radio. The Comtac I has a screw-sealed panel to reveal the batteries on the left side where as the Comtac II has an individual compartment for each battery on each side Please do make note that these are just headsets and need to be combined with Z-Tac PTT units and radios to form a complete set. This headset will fit any Z-Tac PTT unit but the PTT unit has a plug type for each specific kind of radio so be sure to pick up the correct one if you do not have one already