Next, The Electrostatic Speaker
Completely different from the dynamic speaker is the electrostatic. This type of speaker still relies on a diaphragm that moves back and forth. The cause of that motion, however, is different. Both dynamic and electrostatic speakers rely on electric current to move the membrane that moves the air, but implementation is different.
The underlying theory of electrostatic speakers is really quite simple. Basic physics say that like charges repel, while opposites attract. Using this principle, we can construct a conceptual electrostatic speaker. First, we stretch a thin plastic membrane on a rigid frame of some sort. Then, we can coat it with a low-mass electrically-conductive substance like fine graphite power or metal flake. We then fabricate two stiff, flat electrically conductive structures, called the stators. These stators are connected to your electrical outlet to provide the current to "charge" the stators. They are insulated to prevent electrical discharge. Each stator has the same area as the thin membrane. The stators are mounted in front of and behind the diaphragm. Lastly, the membrane is placed exactly equidistant between the two stators.
Now to get the electrostatics to work, we have to get a dense population of electrons onto the membrane using a power supply. The audio signal voltage from the amplifier is connected to the two stators, but in a special manner: the signal applied to each stator is identical with the exception of phase; one signal is 180 degrees out of phase with the other. Thus, when the signal voltage on one stator increases positively, it attracts the negative electron charge on the membrane. At the same time, the signal voltage on the other stator is increasing negatively and, hence, repels the charge on the membrane. This "push-pull" force is exerted on the membrane. When the audio signal reverses, the push-pull force also reverses. Since the membrane is compliant and lightweight, the push-pull electrostatic force applied to it causes it to move. Thus, air is moved and sound is created representing electrical driving force.