Meeting Topic: Electrostatic Drivers for Earphones: Navigating the Potholes
Moderator Name: Ken Platz
Speaker Name: Roger Grinnip, Principal Acoustical Engineer at Shure Incorporated
Meeting Location: Shure Incorporated, 5800 W. Touhy Ave, Niles, IL 60714
Electrostatic transducers are used in a variety of applications in audio including speaker systems, headphones, and in ear monitors (IEM's). Compared to moving coil drivers or balanced armature receivers, electrostatic drivers offer several benefits. Electrostatic drivers have significantly less moving mass (no coil or magnetic structure), the diaphragm is not as prone to modal breakup, high bandwidth can be achieved through one device (rather than multiple drivers and a crossover network), and they are generally more linear in actuation (primarily a concern with balanced armatures).
To start developing electrostatic IEM's, Roger first looked at a single side electrostatic transduction mechanism. This is similar to how condenser and electret microphones operate where there is always a pulling force on one side of the diaphragm, referred to as a negative stiffness. The negative stiffness term requires a large spacing between the diaphragm and stators, which requires a large actuation, which requires a larger bias voltage. This is a viable design for condenser/electret microphones since the diaphragm undergoes a small actuation. However, this is not a viable solution for an electrostatic speaker.
Next, Roger evaluated a push-pull transduction mechanism. In push-pull actuation the diaphragm is centered between the stators and the charge on the diaphragm is fixed (which may or may not be a realistic assumption). He approximate a fixed charge on the diaphragm by using a slightly conductive coating. An external bias through a large resistor keeps the diaphragm charged. The shape of the response is controlled by carefully designing the final assembly cavities and nozzles to achieve the target response.
In an idealized electrostatic transducer there is no mechanical instability. However, a realized product is subject to real world manufacturing limitations and part tolerances which create mechanical instability. Variation in the diaphragm spacer, glue processes (placement and amount of glue), and diaphragm thickness, among other variances, combine to create asymmetries within a given assembled transducer and variation in performance between transducers. Additionally, there are physics based limitations on the system such as Paschen's law.
Roger developed some general design guidelines to help address these instabilities. The mechanical stiffness of the diaphragm must be at least three times higher than the negative stiffness. The bias voltage must not exceed the Paschen breakdown voltage. Lastly, the AC actuation voltage must be less than two times the bias voltage to prevent diaphragm collapse.
A high voltage bias is needed to create the fixed diaphragm charge. The IEM cable can be ten times the capacitance as the capacitance of the electrostatic driver. Using a transformer to drive this load would require a transformer that is larger than the transducer, which is not ideal for a portable application. Solid state amplifiers are better suited to portable electrostatic designs, which is ultimately what Roger used in the amplifier for the electrostatic IEM's.
Written By: Emily Wigley