Click here to purchase the entire book in PDF format. Electrostatic Loudspeakers When we learned how capacitors worked, we talked a bit about electrostatic attraction. This is the tendency for oppositely charged particles to attract each other, and similarly charged particles to repel each other. This is what causes electrons in the plates of a capacitor to bunch up when there's a voltage applied to the device. Let's now take this principle a step farther. Figure 6.134: Put three conductive plates side by side (just like in a capacitor, but with an extra plate). Have the two outside plates fixed, but the middle one suspended so that it can move (shown by the little arrow at the bottom of the diagram. Charge the middle plate with a DC polarizing voltage. It is now equally attracted to the two outside plates (assuming that their charges are equal). If we change the charge on the outside plates such that they are different from each other, the inside plate will move towards the plate of more opposite polarity. Figure 6.135: If the middle plate is very positive and the outsite plates are slightly positive and slightly negative relative to each other, the middle plate will be attracted to (and be pulled towards) the more negative plate while it is repelled (and therefore pushed away from) the more positive plate. This system is known as push-pull for obvious reasons. If we then perforate the plates (or better yet, make them out of a metal mesh) the movement of the inside plate will cause air pressure changes that radiate through the holes into the room. There are (as expected) advantages and disadvantages to this system.