Plasma speaker

Plasma speakers or ionophones 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 term ionophone can also be used to describe a transducer for converting acoustic vibrations in plasma into an electrical signal.
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.
Comparison to conventional loudspeakers.
"Conventional" loudspeaker transducer designs use input electrical frequencies to vibrate a significant mass: This driver is coupled to a stiff speaker cone — a diaphragm which pushes air at respective frequencies. But the inertia inherent in its mass resists acceleration — and all changes in cone position. Additionally, speaker cones will eventually suffer tensile fatigue from the repeated shaking of sonic vibration.
Thus conventional speaker output, or the fidelity of the device, is distorted by physical limitations inherent in its design. These distortions have long been the limiting factor in commercial reproduction of strong high frequencies. To a lesser extent square wave characteristics are also problematic; the reproduction of square waves most stress a speaker cone.
In a plasma speaker, as member of the family of massless speakers, these limitations do not exist. The low-inertia driver has exceptional transient response compared to other designs. The result is an even, linear output, accurate even at extreme frequencies beyond any audible range. Such speakers are notable for accuracy and clarity, but not tremendous power because plasmas composed of tiny particles are unable to move large volumes of air. So these designs are more effective as tweeters.
Practical considerations.
Early plasma-speaker designs ionized ambient air containing the gases nitrogen and oxygen. In an intense electrical field these gases can produce reactive by-products, and in closed rooms these can reach a hazardous level.
Plasmatronics produced a commercial plasma speaker that used a helium tank to provide the ionization gas. In 1978 Alan E. Hill of the Air Force Weapons Laboratory in Albuquerque, NM, designed the "Plasmatronics Hill Type I", a commercial helium-plasma tweeter. This avoided the ozone and nitrogen oxides produced by radio frequency decomposition of air in an earlier generation of plasma tweeters. Theirs is also the only design relying on the quieter glow discharge mode instead of the more common arc and corona discharges. But the operation of such speakers requires a continuous supply of helium.
In the 1950s, the pioneering DuKane Corporation produced the air-ionizing "Ionovac", marketed in the UK as the "Ionophone". Currently there remain manufacturers in Germany who use this design, as well as a do-it-yourself design available on the Internet.
To make the plasma speaker a more widely available product, ExcelPhysics, a Seattle-based company, and Images Scientific Instruments, a New York-based company, both offered their own variant of the plasma speaker as a DIY kit. The ExcelPhysics variant uses a flyback transformer to step up voltage, a 555 timing chip to provide modulation and a 44 kHz carrier signal, and an audio amplifier.
A flame speaker uses a flame for the driver. Some designs dating to the 1950s use combustion of natural gas or candles to produce a plasma through which current is then passed. These combustion designs do not require high voltages to generate a plasma field.
A similar effect is occasionally observed in the vicinity of high-power amplitude-modulated radio transmitters when a corona discharge (inadvertently) occurs from the transmitting antenna, where voltages in the tens of thousands are involved. The ionized air is heated in direct relationship to the modulating signal with surprisingly high fidelity over a wide area. Due to the destructive effects of the (self-sustaining) discharge this cannot be permitted to persist, and automatic systems momentarily shut down transmission within a few seconds to quench the "flame".
References.
[1]-Severinsen, Daniel & Sen Gupta, Gourab. (2013). Design and Evaluation of Electronic Circuit for Plasma Speaker. Lecture Notes in Engineering and Computer Science. 2. 1111–1116.