Senior Thesis: Electrostatic Speaker
An electrostatic speaker operates by the forces caused by static attraction and repulsion - easily demonstrated in a dry climate with a balloon and a head of hair. These forces, however minute, can be applied across a diaphragm of thin material and modulated at audio frequencies, causing the diaphragm to oscillate with the input signal and produce sound waves that are detectable by human ears. Electrostatic loudspeakers, or ESLs, are typically found only in high- end audiophile systems or as homebrew ‘project’ speakers. Traditional designs need to be quite large to be effective at producing a respectable range of frequencies. Still, they are often accompanied by an electrodynamic driver to provide supplemental low frequency response.
I became interested in the design and performance enhancement of ESLs after reading a paper published by researchers at the University of California at Berkeley (Qin Zhou and A. Zettl. “Electrostatic Graphene Loudspeaker.”). The researchers used a material called graphene, an allotrope of carbon composed of a single layer of graphite, to create the diaphragm for the speaker. The material is incomprehensibly thin, stronger than steel for its weight, and conducts electricity well - all extremely important properties for an ESL diaphragm.
Many production and DIY electrostats use a thin plastic mylar sheet as the diaphragm, coated in a conductive material such as aluminum or powdered graphite. Mylar works well as it is resilient and maintains its tension over time. Compared to graphene, it is heavy. Its mass forms a mechanical resonator, dampening its movement and causing resonances in the speaker’s frequency response. Graphene, nearly massless, suffers from reduced dampening, resulting in resonances that are attenuated and shifted out of the range of human hearing.
The paper released from UC Berkeley outlines the implementation of an earbud-sized graphene-diaphragm ESL. The researchers manufactured the diaphragm, installed it into a housing designed to mimic an in-ear headphone, and measured its performance. They discovered that the graphene performed as well, if not better, than current production earbuds, producing a flatter frequency response. With graphene, less engineering is required to design the acoustics of an earbud to overcome the faults of a traditional magnetic driver.
My thesis concept is to create a similar testbed for an over-the-ear electrostatic headphone with a carbon diaphragm. From January to May 2015, I researched, designed, and built a device that would allow the comparison of various diaphragm material. Each parameter of the speaker, including diaphragm tension, spacing, and input and modulation voltages are completely adjustable to optimize the performance of the ESL.
I performed tests with diaphragms of aluminized mylar 1.00 mil, 0.50 mil, and 0.25 mil in thickness. I installed the 0.25 mil mylar into a frame and pretensioned it, then mounted the frame into the testbed, tightening the diaphragm further so its fundamental resonance was 321Hz. I used the program FuzzMeasure to generate a sine sweep, and measured the output of the speaker with an Earthworks omnidirectional microphone. I repeated the measurement with the 0.50 mil and 1.00 mil mylar. The data collected was used to generate a frequency response curve for the three materials.
The graph shows poor response below the clearly visible resonance at 321Hz. Above the fundamental, the performance of the speaker can be judged by the ‘smoothness’ of the curve. The red line - data from the 1.00 mil diaphragm - is the most jagged and has the sharpest corners in its response. The blue line - data from the 0.25 mil diaphragm - is the least jagged, and has rounder corners.
I was unable to obtain graphene for testing. I instead found that of the three diaphragms, 0.25 mil mylar, the thinnest material available at the time of testing, proved to be the most sonically accurate. I extrapolated that the use of the thinner material graphene in my testbed would yield a even smoother response and more impressive sonic performance.
A short compiliation of machining of the speaker frame:
Reference audio (Ballad of Hugo Chavez - Arkells; Sky full of Stars - Coldplay) played through the speaker and recorded with Earthworks microphones. The close microphone is the one used for the response tests; the far microphone is a few feet away.