The term massless speaker is used 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. The different types of speaker that are possible without using conventional solid high mass diaphragms are described here.
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.
Massless speakers can be classified into 6 basic types. The first 4 are plasma speakers.
1. Corona Discharge
Also known as Corona Wind, Ion Wind, Ion Cloud and Ionic Loudspeakers*. Also Dark Discharge. 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 very 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.
2. Glow Discharge
Also known as a Plasma Tweeter, Plasma Speaker, Plasma Arc, Ionophone and Ionic.* Uses a high voltage (low kilovolts), but lower than the above and with some current (milliamps). This can be created at radio frequencies with RF amplification or high frequency switching, or with DC. A glowing plasma thread - a glow discharge - is visible either linking electrodes or sometimes contained by an RF field in an insulating vessel, typically quartz. This can be several centimetres long. The plasma can also be modulated using a magnetic field. The mechanism of sound output is that by modulating the plasma with an electric signal, the superheated air around it expands and contracts producing pressure changes and therefore sound waves. These can be called ionic and although the plasma is ionised gas the actual physical principle creating the sound is thermal.
3. Brush Discharge
A type of glow discharge but needs to be identified separately. Also known as a plasma speaker, singing Tesla coil, zeusaphone or thoramin. Uses a very high voltage, kilovolts or even megavolts. Often created at radio frequencies. A large glow discharge 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 PWM 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.
4. 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.
Alternate Plasma Creation
Plasma can be created in more ways than just with an electrical arc. For example a laser can be used to create a plasma (closest resembling a glow or arc discharge) by targeting a solid with a pulsed laser or in air by using a femto-second pulsed laser. Once the plasma is created it can be modulated by an additional laser.
5. 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.
6. 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. The limiting factor is the thermal inertia in the solid, so this may negate the benefit of not having a large moving mass. A low voltage higher current technology.
A laser can also be used to heat the solid and when modulated can modulate the surface temperature, an alternative method of creating a thermal loudspeaker.
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 glow discharge plasma and strong corona discharge can be hard to separate sometimes.
In general, if there is a steady visible plasma produced it is a glow discharge loudspeaker.
If there is no visible plasma beyond the edges of the electrodes it is a corona discharge loudspeaker.
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.
Hot plasma, cold plasma - what's the deal?
Sometimes these plasmas can be described as hot or cold plasma. A physics definition of these types of plasma is a non-thermal plasma or a non-thermal cold plasma. Sometimes a corona discharge is called cold and a glow discharge hot. Unfortunately, the definition of a non-thermal cold/hot 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. Plasma Universe has a good go at a comprehensive definition of plasma temperatures and shows how difficult it is: https://www.plasma-universe.com/plasma-classification-types-of-plasma/#Cold.2C_warm_and_hot_plasmas
So this site tends to classify the plasma types of speaker by the discharge type as it is the most definitive and useful way to identify them to others. The terms cold plasma or hot plasma are not helpful with respect to loudspeakers, but they sound cool for marketing.
