222 CHARTRES A¥E » ESTONIAN, iLL » CABLE UREMIC) MICROPHONES AND ELECTRONIC COMPONENTS EXPANSION PHONE DA 85300 + CHIDES STONE SH 31600 General: Model 440 and Mods! 440SL microphones are pressure operated units using the balanced armature controlled magnetic principle and especially designed to have high intelligibility for radii communications and are especially designed for the narrow audio frequency range required for single side band transmission.