What’s in a name?


Is it a plasma arc speaker?
Not one you can hear
Problem

The naming of massless transducers is often inaccurate, or at least not accurate enough.

Examples

Massless speakers that are currently available:

Vaugn Loudspeakers’ Plasma Signature
Lansche Audio Corona
Acapella Ion TW-1

The plasma, corona and ion speakers in these designs are all the same type – but with completely different names!  They use a glow discharge plasma.  Historically the naming is worse. Is a corona wind the same as an ion cloud?  Could be, but maybe they are force fields as well. Is my Ionofone like your Plasmatronic?  Perhaps they both are like an Waveion.


Basic Definitions

Let’s start with some basics.  An electroacoustic transducer, that which takes an electrical signal and converts it into sound waves, is called a loudspeaker.  The word speaker is synonymous with loudspeaker – although older references may write ‘speaker as if shortening the word loudspeaker.

A conventional loudspeaker with a cone, coil and magnet is typically known as a dynamic speaker.  An electrodynamic loudspeaker to use its full name, also electromagnetic, but these names are less often used because everyone knows what a dynamic loudspeaker is.  It is after all the most common type of speaker by far and as a result it is what most people know as a speaker or loudspeaker without going any further.

There are a small variety of other speakers that might be put into a category of “moving mass”.  These can be related to the dynamic speaker but have different names to distinguish them from the cone/coil/magnet construction. They are often electrodynamic but have names such as ribbon, balanced armature, air motion transformer (AMT), planar magnetic, orthodynamic, isodynamic, magnetostatic, distributed mode (DML), MEMS etc.  In addition to dynamic moving mass speakers there are most notably the electrostatic, piezo ceramic and magnetostrictive types.

There has been little regulation or standardisation of naming as they typically get named by the inventors, related to marketing names, what they are made of or the physics employed.  However, in general most of those names describe their specific technology.

This doesn't apply to massless loudspeakers.


Massless Types

So in the massless loudspeaker category we will divide it into six distinct types.  Not all of these have made it into commercial production.

Type one – a plasma speaker that uses a glow discharge to create sound. The mechanism is actually thermal as the temperature of the plasma varies with the input electrical signal which in turn varies the heat of the air which in turn varies in expansion creating pressure waves.

Type two - a plasma speaker that uses a brush discharge to create sound. This also creates sound with the thermal mechanism above, but with higher voltages, more noise and a single electrode.

Type three – a plasma speaker that uses a corona discharge to create sound. The mechanism is ionic or electric where ions and/or electrons move through the air creating a wind which varies with the electric current. The corona discharge plasma is present on the edges or tips of the electrodes but contributes little if anything to the output sound.

Type four - a plasma speaker that uses an arc discharge to create sound. The mechanism is thermal as per type one. This is a hotter arc that runs at high current similar to an arc welder or arc lamp.

Type five – a flame speaker that uses a flame (typically from burning a gas) modulated by an electric or magnetic field to create sound as per the type one. A flame contains some plasma which behaves as per type one.

Type six - a thermal speaker or thermodynamic type – a mass is heated by an electric signal. The heat of the mass (typically a thin wire) is varied and this varies the temperature of the air around it causing it to expand/contract as per the type one.  So although a solid mass is involved the heat of the air is the active part of the speaker.



Principles of Operation

The main point of discussion later is around the type one and three, glow and corona discharge as these are the most common commercial designs.  First we will discuss the other less common types.

The type two brush discharge is often used for physics demonstrations and public displays and is often found in self build kits where a Tesla coil power supply is used to create very high voltages which are acoustically and electrically noisy.  Modulating the operating frequency of the Tesla coil in the audio range produces audible artefacts. This type doesn't lend itself to the use for accurate sound reproduction. Most of what applies to type one also applies to this type. Most often referred to as a singing Tesla coil or plasma speaker.

The type four arc discharge was derived from arc lamps in the late 1800s after observing the noise that it made. It would still be viable but is has not been used since the early 1900s musical instruments for accurate sound reproduction. Sound from arcs tends to be by-products from arc welders and radio transmitters.  Its mode of operation is similar to the type one and is generally referred to simply as an arc speaker.

The type five flame speaker is also seen in physics demonstrations and also doesn't lend itself to use beyond a curiosity, but it was the first of these types to be found and very popular in the 1800s followed by a small resurgence in interest in the 1960s. It has almost always been referred to as flame, with words prefixed with pyro-.

The type six thermal speaker did have some early research but has only more recently been better investigated, the technology to produce the thin wires and drive them hasn't been available until now. So there is potential and naming wise it is largely referred to as thermal and words prefixed with thermo-.

The major commercial and experimental types are the type one and three when it relates to the accurate reproduction of sound, their naming is the main issue.

Type one – a plasma arc is created in the air strong enough that it is in its glow discharge form. This is typically a high voltage plasma arc.  Sometimes the plasma is created between two electrodes, sometimes as a dielectric discharge.  It can also be created with DC and RF electric supplies.

It can be modulated in a number of ways and a number of different methods of supplying the high voltage can be used.  In all cases a distinct area of glowing plasma can be seen. Plasma is ionised gas.  The ions (electrically charged atoms or particles) and electrons in the plasma both create it and allow electricity to flow through it.  The plasma is very hot, heating the air around it.  If the intensity of the plasma is varied, e.g. at audio frequencies, the air around it varies in temperature which in turn causes it to expand/contract at the audio frequencies creating sound.

You see the plasma arc.

Most commonly called a plasma speaker and because of the limited volume and frequency response the are often called plasma tweeters.

Type three – a corona discharge is created on sharp electrodes.  When a high voltage is present on a sharp point, on a sharp edge or a fine wire, the electrons will fly off it creating a corona discharge.  The tip or edge will faintly glow with a corona (a class of plasma), only visible in low light.  If presented with a nearby ground point the electrons and ions charged by the electrons will flow through the air to that point, this effect is likely dominated by the electrons.  As they do so they collide with air molecules imparting motion and creating wind.  If you modulate the supply in some way the resulting air speed will vary.  If you do that at audio frequencies it produces sound.

You don’t see a plasma arc.

Most commonly called a corona wind loudspeaker or ion wind loudspeaker.


The Use of Temperature to Describe Plasmas

Sometimes these plasmas can be described as “cold plasma” or “hot plasma”. The definition of a non-thermal 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.

As the definition of a cold, warm or hot plasma is so vague it is not helpful to use the terms in relation to plasma speakers.  Keeping with the type of discharge is much clearer as the terms are well defined.


The Name

When it comes to naming, the type one and three are a mess.  They are both types of plasma and contain ions but they create sound using different physical principles.  However the same names are used interchangeably and as a result people often misunderstand the underlying principle at work for a specific speaker.  The terms are used for marketing but also in the scientific descriptions and by the inventors and designers of the speakers.

They have both been referred to as the following (at least), not even including most brand names:

Plasma
Ion
Arc
Ionic
Corona
Ionophone
Plasma arc
Ion wind
Ion cloud
Corona wind
Corona discharge
Ion-plasma
Singing arc
Talking flame
Blue flame
Force field (!)

The most commonly used term over all is plasma speaker and the most common type of that is a plasma tweeter. This can be extended to plasma arc loudspeaker. This is most often used to describe type one, where there is a visible glow plasma, and this is by far the most common commercial design.

Even near its early conception, this type of speaker was described as ionic and called an ionophone (later Ionofone, Ionofane etc. used as brand names).  The use of ions in the naming of type one is also common.  Yes plasma is an ionised gas and so it is not inaccurate, but the mechanism creating the sound is the heating of the air by the plasma.  In a way they are more thermal than ionic.

Then there is the type three.  This type has only once been seen in a commercial product, the Plasmasonic, otherwise in prototypes and demonstrations.  It is rarely described as a plasma speaker, although it has been and it is in the first commercial name.  Early on the term corona wind was used but most often now it can be referred to as ion wind, ion cloud or just ionic or corona.  Electrostatic has on occasion been used as a description, but the basic mode of operation does not involve a charge at rest even though there may be some static charge on or around the discharge points.

Further to that, testing has revealed that in the type three it is electrons that are the predominant cause of the air flow making it an electric speaker and only slightly ionic.  Electric just seems to be too vague a description though, aren’t all loudspeakers electric in some way? 

The use of the words corona and/or discharge nowadays has quite a different negative meaning to most people, so the type three can have a harder time finding a name.


A Solution?

This doesn’t pose a problem in most peoples lives but it is in the remarkably narrow field of solid state massless speakers.  “That’s a nice ion speaker.”, “It’s not, I think you’ll find it’s a plasma arc wind transducer.”

So the following simple names could be used with their easily recognisable forms.  Taking into account the historical usage, even though they may not be totally scientifically accurate.

If the plasma is too bright to look at, call it an arc speaker.
Physically an arc discharge plasma electrohydrodynamic loudspeaker. (type four)

If you can see a steady plasma, call it a plasma speaker.
Alternatives, plasma arc speaker, ionophone.
Physically a glow discharge plasma electrohydrodynamic loudspeaker. (type one)

If the plasma looks like lightning call it a singing Tesla coil.
Alternatives, it will still be called a plasma speaker too.
Physically a brush discharge plasma electrohydrodynamic loudspeaker. (type two)

If you can’t see any plasma, call it an ionic speaker.
Alternatives, ion or corona wind.
Physically a corona discharge plasma electrohydrodynamic loudspeaker (type three).

If you set something on fire to make it then it’s a flame speaker.
Alternatives, pyrophone.
Physically a flame thermodynamic loudspeaker (type five).

If you make something warm to make sound then it's a thermal speaker.
Alternative thermodynamic speaker, thermophone.
Physically a solid state thermodynamic loudspeaker (type six).


Conclusion

I’m not sure after all of that it is any clearer. The naming is at least categorised.

If the usage of the types of discharge were more common it would be better.  They clearly define the speaker type with little extra clarification needed.


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