Methods of Modulation
An essential part of a massless speaker system is taking an electrical audio signal as an input to convert into sound. The massless speaker itself is usually powered by a DC or AC power source which can either be modulated or the speaker affected in a different method depending on its assembly. There are a few basic methods of superimposing this audio signal onto the speaker. Here we are mainly considering plasma type massless speakers.
RF
Build a radio frequency circuit to create a plasma and power the massless speaker via a flyback transformer or Tesla coil. Use AM or FM to modulate the high frequency signal that drives the plasma. Using an RF arc increases the stability and can lower the audible noise of the system so using this radio method can be complimentary to that.
Switching
Use a switching power supply and modulate its switching frequency or pulse width (PWM) with an audio signal. The switching power supply uses a high voltage step-up transformer, typically a flyback transformer or tesla coil to create a high voltage plasma. A noisy method often used in brush discharge systems.
DC
Create a glow, arc or corona discharge using a high DC voltage and superimpose the AC voltage audio signal onto it via a transformer or directly with a valve (tube) or transistor.
Magnetic
Create a stable plasma arc (DC or RF) and surround it with electromagnets that are modulated with the audio signal. Plasma is quite easily effected by a magnetic field, its direction, flow and shape can be altered.
Laser
Use one high power laser to create a plasma by striking a target - or two lasers creating plasma where they cross. Use another audio frequency modulated laser (or digitally modulated with the signal from a 1-bit ADC) aimed at that plasma to modulate it and in turn create sound.
Methods of Assembly
With respect to ion wind or plasma designs the arrangement of the electrodes can be varied quite widely depending on the target sound quality, volume or frequencies that are most important.
Triode or Point/Grid/Plane
Two planes of points/wires, separated to form a corona with a grid (mesh or wires) in between them to modulate the ion stream, much like an electronic triode valve without its glass case. This generally lends itself to an ionic speaker design rather than a plasma design.
Point to plane
A sharp electrode (anode), pointing at a blunt electrode (cathode). The sharp electrode can be a needle point, thin wire or razor thin edge. The blunt electrode can be a dome or a mesh or even another point, insulated from the anode by air or a solid such as quartz or ceramic. Commonly used in single pair arrangements for plasma tweeters with more electrodes to increase the lower frequency response and volume. Used in multiple pair arrangements with ion wind based speakers.
Point to air
A point in free air with a very high voltage typically creating a brush discharge. This can also have a grid surrounding the point electrode but at some distance and is typically there for ground safety.
Electrode Design
The electrodes used in plasma massless speakers are one of the most important components.
Grids
A grid may be used to modulate the air stream - the grid is electrically conductive but in this case also needs to allow air to flow through or over it. In all cases, if a grid is used, the grid can take the form of:
A ring around each point
A mesh screen between the points
Wires between the points
Number
For all of these methods the speaker can be constructed as a single point/wire/edge or many points/wires/edges.
When using a single or few points/wires the emitting surface is small and therefore large volumes of air will be difficult to move. This restricts smaller devices to higher frequencies - unless they are driving a restricted volume of air as in a headphone. It may also be difficult to create enough audible volume.
This can be compensated for with the use of a horn or with brute force (more voltage...).
Using multiple points/wires can help to increase the amount of air operated on. This can help improve the volume and also extend the effective frequency down.
Electrode Materials
When operating at high energies, especially with plasma and arc discharges, heat from the plasma can conduct into the electrodes. In addition the plasma can ablate solid materials directly leading to erosion of the electrodes. Because these devices need to operate in air the increased temperature can also encourage more oxidation.
So when selecting materials for electrodes they need to be electrically conductive, heat resistant, oxidation resistant and ablation resistant. This usually results in some exotic metal alloys being used to give the electrode as much life as possible. One other odd property is that some materials create a noisier arc than others.
Some materials commonly used include carbon (including graphite), platinum group metals (including iridium), tungsten, nickel alloys (e.g. Inconel) or steel alloys (e.g. Kanthal). The first four can be noisy however. In short term or lower power use cases, or for demonstration purposes, most metals work well especially stainless steel or brass.
Corona based systems can be gentler on their electrodes, although the corona at the tip or along fine wires can slowly ablate and oxidise them. A wider range of materials will last longer as the temperatures are a lot lower.
Over time all of the above technologies can suffer from build up of soot on or around the electrodes - either from dirt in the air or from the combustion of the gases. This soot can be conductive and reduce the performance of the system by tracking voltage down the wrong paths.
