Diaphragm-less ionic loudspeakers using both low-temperature and high-temperature plasma methods have already been studied and developed for practical use. This study examined using similar methods to create a diaphragm-less ionic microphone. Although the low-temperature method was not practical due to high noise levels in the discharges, the high-temperature method exhibited a useful shifting of the oscillation frequency. By performing FM detection on this oscillation frequency shift, audio signals were obtained. Accordingly, an ionic microphone was tested in which the frequency response level using high-temperature plasma increased as the sound wave frequency decreased. Maintaining performance proved difficult as discharges in the air led to wear of the needle electrode tip and adhesion of products of the discharge. Study results showed that the stability of the discharge corresponded to the non-uniform electric field that was dependent on the formation shape of the high-temperature plasma, the shape of the discharge electrode, and the use of inert gas that protected the needle electrode. This paper reviews the experimental outcome of the two ionic methods, and considerations given to resolve the tip and discharge product and stability problems.
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