ThatвЂ™s when I decided to focus on gas-discharge tubes instead of atmospheric discharges. I was barely able to get a signal out of the very noisy background. I operated a plasma speaker in reverse, that is as a plasma mic, a couple of years ago. I think itвЂ™s fair to characterize them as a novelty, rather than a viable speaker technology.

DuddellвЂ™s вЂњsinging arcвЂќ was introduced in 1900 and plasma speakers are still commercially available. Yes, plasma speakers (including flame speakers) are more common than plasma mics (not to mention the plasma pickup). IвЂ™m working on a peer-reviewed publication that will have all the details. A directional tube would be useful, as it could differentiate between shear and compression waves. I also thought the gas-discharge tube transducers would be quite directional, but initial testing says otherwise! My current hypothesis is that thereвЂ™s a lot of diffraction inside the tube. ThatвЂ™s why elastic waves in solids and fluids are a more suitable target for this tech. Gas-discharge tubes could be used as microphones (to detect sound propagating in air), but they have poor acoustic-impedance matching (most of the acoustic energy is reflected by the glass envelope). Some bats vocalize as high as 200 kHz, but I reckon youвЂ™d have to get pretty close to detect that. But as you state, air is quite viscous at frequencies above 100 kHz or so.

Your questions are all excellent! An atmospheric discharge could detect high frequencies, as long as the electrode dimensions resolve half the wavelength. Posted in Misc Hacks Tagged NE-2, neon, transducer, ultrasound Post navigation To further bestow the virtues of this indicator device of a bygone age, we have a neon lamp based Nixie clock, an NE-2 not-LED matrix display unit, and finally using an array of them to visualise an electric field. Take the current signal through the lamp and high-pass filter it, and youвЂ™ve got yourself a really cheap transducer. The operating principle is simple enough, once the neon is ionised and the device is operating in the breakdown condition, pressure variations in the surrounding air are conducted through the glass, which causes pressure variations in the neon next to the electrodes. Traditionally the only way to do this is with a piezoelectric acoustic sensor, but those are short-lived in harsh environments, annoyingly resonant in frequency response, not to mention, expensive.Д®nter the humble NE2 with its almost flat frequency response in the 200 kHz to 1 MHz region of interest, its ability to withstand operating temperatures of up to 400 degrees Celsius and being a sealed glass container, quite impervious to conditions if you can protect the connecting leads. The new work concentrates on the idea of detecting the ultrasonic emissions due to initial stress relief during metal fatigue failure of large structures such as bridges and oil drilling platforms. The idea is not new at all, with some previous attempts at using electric discharge in a gas to detect audio, going back to the early part of last century, but those attempts either used atmospheric pressure air or other non-sealed devices that exhibited quite a lot of electrical noise as well as producing noxious gases. Now, due to some recent research by and the team at Princeton University, the humble NE-2 tube has a new trick up its sleeve: acoustic transduction. Capable of many other tricks such as the ability to protect RF circuits from HV transients, its negative resistance operating region after it illuminates gives us usable hysteresis which can used to form a switching element and the way the pair of electrodes are arranged give it the ability to indicate whether a voltage source is AC or DC. The familiar warm orange glow has graced the decks of many a DIY timepiece, sometimes in a purely indicating duty, and sometimes forming a memory element in place of a more conventional semiconductor device.

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