Listening to radio without a speaker?

While this may not be a surprise to those with electrical engineering backgrounds, this is a first for me — never seen this sort of thing before. While the effect is very cool, it’s potentially very dangerous and you shouldn’t go jumping the fence at the neighborhood 50kW shortwave station to try it out.

Wikipedia on plasma speakers:

Plasma speakers are a form of loudspeaker which varies air pressure via a high-energy electrical plasma instead of a solid diaphragm. Connected to the output of an audio amplifier, plasma speakers vary the size of a plasma glow discharge, corona discharge or electric arc which then acts as a massless radiating element, creating the compression waves in air that listeners perceive as sound. The technique is an evolution of William Duddell‘s “singing arc” of 1900, and an innovation related to ion thruster spacecraft propulsion.

The effect takes advantage of two unique principles. Firstly, ionization of gases causes their electrical resistance to drop significantly, making them extremely efficient conductors, which allows them to vibrate sympathetically with magnetic fields. Secondly, the involved plasma, itself a field of ions, has a relatively negligible mass. Thus as current frequency varies, more-resistant air remains mechanically coupled with and is driven by vibration of the more conductive and essentially massless plasma, radiating a potentially ideal reproduction of the sound source.

After doing some looking around, I realize that I’ve seen a similar phenomenon in the past while visiting the Boston Museum of Science — the singing Tesla coil!  Here is a fun example:

(via Reddit)

Matt Thomas, W1MST, is the managing editor of Contact him at [email protected].

5 Responses to “Listening to radio without a speaker?”

  • Richard KWØU:

    Pretty wild, like some of those stories of people hearing radio with fillings in their teeth. Think I’d rather invest in a cheap speaker than run umpteen thousand Watts in the house….

  • Stuart WB6RXG:

    It doesn’t take 50,000 watts. The Chief Engineer at a 5kw AM station I worked for did this using just a screwdriver to close the gap on the static/lightning discharge balls at the base of the antenna for the arc to start. The arc also modulates the transmitted signal. 13KV at the base if I remember right.

  • Larry Trail KD7KBG:

    Several religious documents have stated the fact “Just because you can do something doesn’t mean its a smart thing”. Or as the Apostle Paul said “All things are permissible but not all things are good”

    All we need is some newbie who has a 50/50 chance of guessing which end of a D cell is + try to make one of these things and get killed. ADULT SUPERVISION REQUIRED!

  • Ken Akin AC0HO:

    Ion or plasma tweeters have been a niche market in the world of “high end” audio for a number of years. As I recall the early versions bothered uses because of high ozone levels in the room.

    And, playing around high power RF is dangerous. To drastically shorten a long story, I was working as a broadcast engineer for a radio station in Washington, D.C. Out chief engineer decided to measure the height of our tower sections, climbing inside a tower with a tape measure in his hip pocket and the end of the tape measure taped (loosely)to the bottom rung of the tower. The tape at the bottom broke free as he was climbing and dropped hitting the base plate (grounded) which supported 1/2 of the lightning protection ball gap.

    This was a 450 foot tower, 5KW AM. Fortunately, I was standing at the base of the tower, and as he yelled I jumped into the dog house and pulled the switch. He was inside (not climbing on the outside, a no-no even before OSHA) the tower, and was able to with difficulty climb back down the 40 feet or so that he had reached. I got him to the hospital. A very hasty burn, he came into work the following day, but did not sit down for perhaps 2 weeks.

    Yes, this should never have happened –

  • Peter kg5wy:

    Real Smart?
    Just get a speaker.

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