The plasma speaker was something I designed for the SPIE club at the University of Alberta. I wanted to use it as a tool to help inspire other engineers and show them they too can build cool projects if they put their mind to it. The club focused on providing funding for club member projects. This gave people the opportunity to build whatever they wanted, whether they had a team or just wanted to work on something individually. The plasma speaker was created to show others that building cool projects is achievable. In addition to club promotion, I thought it would be a great project to help advance my analog circuit design knowledge. It proved to be a very challenging task and took me quite some time to get right.
How it works
I have made a couple versions of the plasma speaker but they both use very similar techniques. They all modulate the audio signal using pulse width modulation. Then, take that new signal and amplify it using a class D amplifier circuit. Then, feed the amplified signal into a low pass filter to demodulate the signal. The demodulated signal is similar to the original signal (well close to it, unfortunately, a lot of noise was introduced in my design) but is larger in magnitude. Then a flyback transformer is used to substantially increase the voltage of the signal. The breakdown voltage of air is 3 kV/mm so high voltage is required in order to form the plasma arc. The best way I found to conceptualize what is happening is to imagine the plasma arc as a cylinder in space. The plasma cylinder increases and decreases its diameter depending on the voltage across the arc. If we change the diameter in a similar way to the way we move a speaker back and forth, we can create the same vibrations in the air, and thus create sound. It's a pretty amazing and creative way to create sound.
Plasma Speaker Components
My Desk + Schematic