Types of Massless Speaker
Plasma arc


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 (the corona) 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.

Plasma Discharge (or Glow Discharge)

Also known as a Plasma Tweeter, Plasma Arc 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 is visible either linking electrodes sometimes contained by an RF field in an insulating vessel, typically quartz, or sometimes this is a burst corona or brush discharge, typically from a tesla coil. 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.

A plasma discharge can also be created using a high kilovolt or even megavolt discharge typically from the output of a tesla coil.  These have been know as a singing tesla coil, zeusaphone or thoramin and are typically controlled with a class D or E amplifier connected to the tesla coil primary.

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 as 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 visible plasma produced it is a plasma 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.

Most of these massless speakers can be placed into either of those two categories, plasma or ion.  Many plasma loudspeakers have names relating to ions, such as ionic or ionophone.  Fair enough, if we were going to be very strict then they would either be thermal or electric wind.  I'm going to try and stick with the simple principle that if it looks like plasma it is plasma and if you can't see anything it's ion.

So it would be nice to have these as clear definitions but historically there is such a mix of terms that it seems impossible.

What is Plasma?

Simply put plasma is the fourth state of matter (after solid, liquid and gas).  It is an ionized gas and behaves differently to a gas.  This makes it sound straight forward but the subject is quite complex. 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 You will find some speakers referred to as cold plasma, you decide if that is correct.

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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