In Reverse

Massless Microphones

Ion and Plasma Microphones

As with most audio transducers, massless devices can not only reproduce sound but also convert sound back to an electrical signal.

Existing microphone technologies such as dynamic, condenser and electret already have significant advantages with respect to mass than a loudspeaker does.  The diaphragms are much smaller and don't need to move air, rather reacting to it which means they are much lighter by default.

With most of the massless designs, e.g. ion wind and plasma, the large amount of bias required makes it harder to discern the sound signal from the supply and noise voltages.  As there is no physical diaphragm the sensitivity is very low making successful retrieval of the microphonic information even more difficult.  But the effect is there and it has been successfully exploited in niche applications.

Commercially the use of plasma microphones was way ahead of the technology's use as a speaker. In 1918 two scientists Vogt, Engl and engineer Massolle created a plasma microphone for use in recording sound for movies, known as the Kathodophone (or Kathodofon). This became the company Tri-Ergon in 1923.

There have been patents, which in the early days were more of a proposed idea than anything in actual production. These include Blondel 1903, Hermanus 1906, Stille 1915 and Thomas 1923.  It isn’t until much later that Yu Akino 2013 patented a plasma microphone although there was other successful work recorded in articles and papers.

Practical experiments are shown in the 1969 paper on the STL-Ionophone being used as a microphone.  Although somewhat confusingly called an Ionophone it is a plasma microphone and the paper includes some simple circuitry and test results.  In 2012 a JAES paper on the study of Ionic Microphone also demonstrates a massless design, this time based on a large single corona discharge, close to a glow discharge.  Again this includes decent test results - although it shows how much background noise can be created.  In 2013 a plasma microphone paper, called Modeling of a corona discharge microphone, experimentally showed a reasonable response up to 4kHz.

In 2018 an ultra high frequency unit is mentioned from a Princeton University document for use with structures.  It suggests a frequency response from the plasma microphone from 100kHz to 1MHz.  It is patented in 2020 revealing that practical use is not in open air but in a tube attached to the structure to be listened to.  In 2021 a device using "microplasmas" was demonstrated in an Acta Acustica paper, involving people from the 2013 earlier work.

So from the work so far it looks like there are some practical applications taking advantage of the extended ultrasonic response.  Massless microphones have not been worked on as much as loudspeakers but it looks like there is promise in this field as well.

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