The Electrostatic Speaker

Author: N.H. Crowhurst

The electrostatic speaker diaphragm

The dynamic or moving-coil transducer has had a very long period of popularity. Much has been done in a great variety of ways to perfect it. The reason for its popularity has been that it has given extremely good results at quite a low cost. For various reasons, however, it is impossible to make it a perfect transducer. Therefore some workers in the field have been trying to find a substitute that may do better. One of these is the electrostatic unit.

When a voltage is applied between any two surfaces, it sets up an electric field. This field causes attraction to take place between the surfaces. By varying the voltage, the force of attraction is varied. This is the basis of the electrostatic loudspeaker.

This system has its problems - to get a large force from the applied voltage, the spacing between the surfaces must be very small; making the spaces small means the moving surface has little room in which to move. This means that the electrostatic loudspeaker is either limited to use at high frequencies, where only small movement is required, or else a very large diaphragm size is needed to move enough air for the low frequencies. Some have even suggested building the loudspeaker into a room and having it occupy one entire wall. Between these extremes, some quite useful units have been made, which radiate frequencies above about 800 cycles, using a moving coil woofer for the lower frequencies.

Unlike the moving-coil speaker's diaphragm, the electrostatic speaker's diaphragm is driven almost uniformly.

The big thing in favor of the electrostatic speaker is the fact that the driving force is distributed over the diaphragm surface. The dynamic loudspeaker is driven from a separate coiL The disadvantage of the coil drive (as any loudspeaker manufacturer knows only too well) is the problem of getting an absolutely rigid mechanical coupling between the drive coil and the diaphragm that is not so heavy that it prevents movement at high frequencies. Choice of material and construction is always a compromise for this reason. The electrostatic units avoid this problem by driving the diaphragm over its entire surface in addition to producing sound from the entire surface. This means that the mechanical properties of the diaphragm (so critical in the moving-coil speaker) become relatively unimportant.

Another problem involved with the use of the electrostatic unit will be better understood in a later section of this course; it concerns matching from the output of the amplifier. The moving-coil speaker is much easier to use from this standpoint.