i'll try to draw a circuit for this later, but i have seen professional microphones that use two elements and use some method of subtraction, either direct wired in opposite phase (you can't do this with electrets) or an op amp with the two microphones going into the differential inputs (one mic to the inverting input, and one going into the noninverting input). the microphones are then mounted a couple of inches apart, but you only speak into one of them. since the in phase signals (the ambient noise) are equal in both microphones, they cancel. you speaking into one of the microphones and not the other creates a signal that doesn't exist in the other microphone, and the signal passes through rather than cancelling.