Types - Massless - plasma tweeter corona loudspeakers

I use the term massless speaker on this site as it encompasses all of the different types, including the more popular (if you could call something so rare that) plasma tweeter. Here I try and describe the different types of speaker possible without using conventional solid high mass diaphragms.

It is difficult to strictly define some types of massless speaker. Of the many articles I have read there are contradictory definitions throughout them all. Here I define what I mean when discussing the different types on this site.

These can all be considered electrohydrodynamic or electro-fluid-dynamic (the fluid being air).

For a full detailed explanation of electric discharges please view this excellent video https://www.youtube.com/watch?v=0Pl5qBajPLk from See the Pattern.

Basic Types of Massless Speaker

Corona Discharge

Also known as Corona Wind, Ion Wind, Ion Cloud and Ionic Loudspeakers.*
A typically high voltage (high kilovolts) low current (microamps) form of discharge.
An arc is not quite formed, but a glow will be seen at the tips called the corona discharge of pointed/wire electrodes and sometimes weakly between them.
This type can run cooler than most others as power is dissipated over a wider area.
The mechanism of sound output is by modulating the streams of ions/electrons released from the electrodes that collide with air particles (the corona wind). Operated over a wide enough area this creates a virtual diaphragm of air. It was recently shown that electrons are involved in the process of moving air as well as ions (see paper Sanghoo Park et al, 2018), if that was the dominant mode the speakers could simply be called electric.

Glow Discharge

Also known as a Plasma Tweeter, Plasma Arc, Ionophone and Ionic.*
Uses a high voltage (low kilovolts), but lower than the above and with some current (milliamps). This is typically created at radio frequencies with RF amplification or high frequency switching.
A glowing plasma thread - a glow discharge - is visible either linking electrodes sometimes contained by an RF field in an insulating vessel, typically quartz. This can be several centimetres long.
Plasma can also be modulated using a magnetic field.
The mechanism of sound output is that by modulating the plasma, the superheated air around it expands and contracts producing sound waves. These are often called ionic and although the plasma is ionised gas the actual physical principle creating the sound is thermal and these are sometimes referred to as electrothermal.

Brush Discharge

Also known as a plasma speaker, singing Tesla coil, zeusaphone or thoramin.
Uses a very high voltage, kilovolts or even megavolt. Often created at radio frequencies.
A large glowing plasma, typically with many uncontrolled threads, is visible from a single electrode. It uses the air as the ground potential.
The plasma discharge is typically created using a Tesla coil based power supply. These are typically controlled with a class D or E amplifier connected to the tesla coil primary. The mechanism of output is the same as the glow discharge.

Arc Discharge

Also known as a plasma arc or carbon arc.
A low voltage (10s of volts) high current (amps) discharge.
A large but short arc is created. This is typically a very hot plasma arc with hot electrodes, similar to the arc created when welding metals. Sound output is similar to the plasma discharge mechanism. Due to the intense heat electrodes are often made from expendable carbon rods. This was one of the first electric arcs known to produce sound, called the singing arc by William Duddell.

Flame

A real flame is produced by burning a gas.
This can then be modulated using physical (vary the flow of gas with a valve) electrical (electrodes near the flame) or magnetic (electromagnets near the flame) means.
A flame contains plasma and can be modulated in similar ways to a plasma arc. Early organs based on resonant flames were known as pyrophones or plasmaphones.

Thermal

In a thermophone a solid is heated and cooled rapidly enough, usually by applying an AC voltage, to heat/cool the air local to it. As the air expands when it heats, the heat/cool cycle makes the air expand/contract creating sound waves. The solid generally needs to be a conductive very low mass (thin wire or film, graphite, graphene, silver nano-wire etc.) and as such is not massless but does not move so is included here for completeness. The limiting factor is the thermal lag in the solid, so this may negate the benefit of not having a large moving mass. A low voltage higher current technology.

Categorising

It is sometimes difficult to determine the predominant effect that is producing the sound in some of the designs above. This is made harder by the commercial marketing names of some speakers and by the misnaming of earlier designs. Also the difference between a weak plasma and strong corona discharge can be hard to separate.

In general, if there is a steady visible plasma produced it is a glow discharge loudspeaker. The actual physical principle with which the sound is made is by the rapid changing of the intensity of the plasma creating hotter/cooler air which in turn makes it expand/contract which creates pressure differences and therefore sound. This really makes it a thermal loudspeaker, but the heat is created with the plasma in this case.

If there is no visible plasma beyond the corona the major physical principle will be the production of ions/electrons in the air, flowing from a high to low potential. As the ions/electrons move through the air they collide with air molecules causing them to move. Varying the voltage in turn varies the amount of air moved and the speed of it creating sound, so this would be called an ion or corona wind loudspeaker. Recent research suggests that the electron flow is as much involved in moving the air as the ions (see paper Sanghoo Park et al, 2018), so it could be called electric but the name ionic or just ion seems to fit better.

What is Plasma?

Simply put plasma is the fourth state of matter (after solid, liquid and gas). It is the most abundant form of matter in the universe and makes up over 99% of the visible matter. It is an ionized gas and behaves differently to a gas.

Sometimes these plasmas can be described as cold plasma. The definition of a non-themal cold plasma is not consistent and depends on which article you read, which physics area you work in or even which country you are in. DYOR, it is impossible to get in to it here. Suffice to say the term is not helpful with respect to loudspeakers.

Try searching for the question “Is flame a plasma?” and you will go down quite a rabbit hole. In addition to that the use of the terms hot or cold plasma can further confuse things. This site has a pretty good go at a comprehensive definition of plasma: https://www.plasma-universe.com/plasma-classification-types-of-plasma/#Cold.2C_warm_and_hot_plasmas

Again the video https://www.youtube.com/watch?v=0Pl5qBajPLk will help to explain the various discharge types.

*There is also an Ionic loudspeaker based on an elastomer gel - it is not massless https://www.independent.co.uk/news/science/transparent-rubber-ionic-speaker-paves-the-way-for-soft-machines-8796789.html

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