David Clark M-7/DC 150 Ohms Civilian Impedance Boom Microphone, new condition. The M-7/DC Electret Microphone is a noise canceling type designed to operate in place of carbon and amplified dynamic microphones. The microphone provides clear, crisp voice transmission while dramatically reducing omni-directional ambient noise. It will not operate in circuits that do not supply a DC bias voltage. Most modern aviation communications equipment do supply this required voltage. If unsure, consult with your radio technical manual or with your local avionics shop.