Pressure Gradient / Dual Backplate
What is so special about a dual backplate / "pressure gradient" capsule design? Let's first define the concept of "pressure gradient" and it should make sense by the end.
A pressure gradient-operated capsule is one in which the difference in pressure between two physical points provides the driving force for the diaphragm. It is a good bit more complex than is possible to define here, but let us begin with the rudiments of the concept before we get into advanced physics.
If your house has a front door and a back door you have probably experienced the phenomenon without knowing you've had the experience. If you stand in front of your house on a warm summer day and have been cooled by a light breeze you will feel this breeze as something gentle and pleasant. If you open your front door you will get a bit of that breeze to enter the house.
If you leave the front door open and then open the back door, you will find that gentle breeze will be magnified (and most likely the mail you had stacked up in the kitchen will have rearranged itself on your kitchen floor). This amplification of the breeze is caused because there is a higher level of air pressure on one side of the house than the other.
The house is actually an obstacle to the wind. The side of the house where the wind is blowing will have a higher level of air pressure than the other side of the house (which creates a "shadow"). This obstacle causes the side of the house where the wind isn't blowing directly to have a lower level of air pressure. When you open both doors you're enabling the higher air pressure from the front of the house to rush to the area of lower air pressure behind the house which amplifies the wind (and blows your mail across the kitchen in the process). This is called a "pressure gradient", which couples with the increase in velocity that occurs when the stream encounters a restricting agent (the door is obviously smaller than the house).
In electronics this is a very similar concept to an increase in voltage. The wind would be the "current", the air pressure would be the "voltage" and the "power" would be a combination of the "current" and the "voltage". Just as you can make 100w light bulb light with 100 volts and 1 ampere of current, that bulb can also achieve its 100w with 200 volts and .5 amps of current. By increasing the pressure (voltage) we require less current, which could be electric current or in the previous analogy, wind current.
The condition holds true for wind moving in either direction, as this is what constitutes the AC component of the sound pressure. Essentially, this is how the pressure gradient microphone works. Vents in the back plate approximate the doors in the house, as the capsule itself is the house. For all intents and purposes, sound pressure is wind, and the gradient drives the stretched membrane of the microphone capsule. Add a second backplate with different sized holes (doors in our house example) and you now have the basis for the "pressure gradient" to amplify the sound pressure level within our microphone capsule.
As mentioned at the beginning of this explanation, this is a very basic primer and overview. The size of each hole and the number of holes in the backplates, as well as the spacing and pattern of the holes in the backplates are also highly critical factors for the sound of the capsule. They affect the phase response of the sound, which is captured by the capsule and transduced to electrical pulses that can be recorded, but that is really well outside the scope of this basic explanation.
