In a previous post, I talked about how plasma can be used to build high-fidelity speakers. Plasma isn’t limited to only producing sound– it can also be used to record it. Being made up of physical particles, plasma can be affected by vibrations through the air. This means it is possible to build a plasma arc microphone using the proper circuit. This application is far less common than using a plasma arc for a speaker, but research has still been done on the subject.
The picture above depicts a simple circuit that uses a plasma arc to measure sound vibrations. The design comes from short research report published in 1969. In the report, the plasma microphone is referred as an “Ionophone” due to the ionic nature of plasma. The researchers demonstrated plasma can be used as a microphone, as seen in the image below.
Although it is difficult to see, audio recordings of a regular microphone and Ionophone are compared on an oscilloscope. Just like with the arc speaker, the properties of plasma make it an ideal candidate for certain acoustic applications. The researchers purposed using the Ionophone as part of a speech recognition circuit that could Identify vowels. Because the microphone is made of plasma it does not introduce distortion or attenuation when recording low frequencies, like some traditional microphones do. So this meant low frequency patterns present in different vowels could be recorded and then identified when fed through an inverse filter. Today we take for granted speech advanced speech recognition algorithms that enable us to talk to our phones. Back in the early 70s, speech recognition was pretty cutting-edge stuff. Obviously today plasma microphones are not common place. This is probably due to the same limitations holding back plasma arc speakers. Even so, the fact that sound can be recorded with nothing more than electric spark is pretty amazing.