I'm reading about constant-charge capacitor microphones, and they are essentially made up of a parallel-plate capacitor (one plate fixed, the other moving to sound pressure), in a circuit with a series resistor and DC source to provide polarising voltage. The circuit is basically an RC filter, with a cut-off frequency down to 10-20Hz (so from f = 1/2piRC it requires a large R if C is small). The polarising voltage from the DC source is often around 48V, but can be as large as 200V for some microphones. My question now is: The distance between the parallel plates in the capacitor is often around 20 micrometers. Now, if the dielectric strength of dry air is 3*10^6 V/m, there should be arcing at 3*10^6 V/m * 20 * 10^-6 m = 60 V. So from my theory it seems that most of these mics should arc due to the polarising voltage, just by turning them on! Clearly I must have a gap in my understanding some place? Thanks!