Plasma Speaker
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Plasma speakers or ionophones are a form of
loudspeaker A loudspeaker (commonly referred to as a speaker or, more fully, a speaker system) is a combination of one or more speaker drivers, an enclosure, and electrical connections (possibly including a crossover network). The speaker driver is an ...
which varies air pressure via an electrical plasma instead of a solid diaphragm. The plasma arc heats the surrounding air causing it to expand. Varying the electrical signal that drives the plasma and connected to the output of an
audio amplifier An audio power amplifier (or power amp) electronic amplifier, amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup (music technology), pickup, to a level that is high enough for dr ...
, the plasma size varies which in turn varies the expansion of the surrounding air creating sound waves. The plasma is typically in the form of a
glow discharge A glow discharge is a Plasma (physics), plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a va ...
and acts as a massless radiating element. The technique is a much later development of physics principles demonstrated by William Duddell's "singing arc" of 1900, and Hermann Theodor Simon published the same phenomenon in 1898. The term ionophone was used by Dr. Siegfried Klein who developed a plasma tweeter that was licensed for commercial production by DuKane with the Ionovac and Fane Acoustics with the Ionofane in the late 1940s and 1950s. The effect takes advantage of several physical principles: First,
ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...
of a gas creates a highly
conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of Electric charge, charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow ...
plasma, which responds to alternating
electric Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
and
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
s. Second, this low-
density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
plasma has a negligibly small mass. Thus, the air remains mechanically coupled with the essentially massless plasma, allowing it to radiate a nearly ideal reproduction of the sound source when the electric or magnetic field is modulated with the audio signal.


Comparison to conventional loudspeakers

''Conventional'' loudspeaker
transducer A transducer is a device that Energy transformation, converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, M ...
designs use the input electrical audio frequency signal to vibrate a significant mass: In a dynamic loudspeaker this driver is coupled to a stiff speaker cone—a diaphragm which pushes air at audio frequencies. But the
inertia Inertia is the natural tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes the velocity to change. It is one of the fundamental principles in classical physics, and described by Isaac Newto ...
inherent in its mass resists
acceleration In mechanics, acceleration is the Rate (mathematics), rate of change of the velocity of an object with respect to time. Acceleration is one of several components of kinematics, the study of motion. Accelerations are Euclidean vector, vector ...
—and all changes in cone position. Additionally, speaker cones will eventually suffer tensile
fatigue Fatigue is a state of tiredness (which is not sleepiness), exhaustion or loss of energy. It is a signs and symptoms, symptom of any of various diseases; it is not a disease in itself. Fatigue (in the medical sense) is sometimes associated wit ...
from the repeated shaking of sonic vibration. Thus conventional speaker output, or the
fidelity Fidelity is the quality of faithfulness or loyalty. Its original meaning regarded duty in a broader sense than the related concept of '' fealty''. Both derive from the Latin word , meaning "faithful or loyal". In the City of London financial m ...
of the device, is distorted by physical limitations inherent in its design. These distortions have long been the limiting factor in commercial reproduction of strong high frequencies. To a lesser extent
square wave Square wave may refer to: *Square wave (waveform) A square wave is a non-sinusoidal waveform, non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same ...
characteristics are also problematic; the reproduction of square waves most stress a speaker cone. In a plasma speaker, as member of the family of massless speakers, these limitations do not exist. The low-inertia driver has exceptional transient response compared to other designs. The result is an even output, accurate even at higher frequencies beyond the human audible range. Such speakers are notable for accuracy and clarity, but not lower frequencies because plasma is composed of tiny molecules and with such low mass are unable to move large volumes of air unless the plasma are in large number. So these designs are more effective as tweeters.


Practical considerations

Plasma speaker designs ionize ambient
air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
which contains the gases
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
and
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
. In an intense electrical field these gases can produce reactive by-products, and in closed rooms these can reach a hazardous level. The two predominant gases produced are
ozone Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...
and
nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, is an intermediate in the s ...
.
Plasmatronics Plasmatronics is a company, founded by former Air Force Weapons Laboratory (now Phillips Laboratory) scientist Dr. Alan E. Hill, which produced a plasma speaker design. This was first demonstrated at the 1978 Winter Consumer Electronics Show. Th ...
produced a commercial plasma speaker that used a
helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
tank to provide the ionization gas. In 1978 Alan E. Hill of the Air Force Weapons Laboratory in Albuquerque, NM, designed the ''Plasmatronics Hill Type I'', a commercial helium-plasma tweeter. This avoided the ozone and nitrogen oxides produced by
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
decomposition of air in earlier generations of plasma tweeters. But the operation of such speakers requires a continuous supply of helium. In the 1950s, the pioneering DuKane Corporation produced the air-ionizing ''Ionovac'', marketed in the UK as the ''Ionophone''. Currently there remain manufacturers in Germany who use this design, as well as many do-it-yourself designs available on the Internet. To make the plasma speaker a more widely available product, ExcelPhysics, a Seattle-based company, and Images Scientific Instruments, a New York-based company, both offered their own variant of the plasma speaker as a DIY kit. The ExcelPhysics variant used a
flyback transformer A flyback transformer (FBT), also called a line output transformer (LOPT), is a special type of electrical transformer. It was initially designed to generate high-voltage sawtooth signals at a relatively high frequency. In modern applications ...
to step up voltage, a 555 timing chip to provide modulation and a 44 kHz
carrier signal In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or frequ ...
, and an audio amplifier. The kit is no longer marketed. A flame speaker uses a modulated flame for the driver and could be considered related to the plasma loudspeaker. This was explored using the combustion of natural gas or candles to produce a plasma through which current is then passed. These combustion designs do not require high voltages to generate a plasma field, but there has been no commercial products using them. A similar effect is occasionally observed in the vicinity of high-power amplitude-modulated radio transmitters when a
corona discharge A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor (material), conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone ...
(inadvertently) occurs from the transmitting antenna, where voltages in the tens of thousands
volt The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...
s are involved. The ionized air is heated in direct relationship to the modulating signal with surprisingly high fidelity over a wide area. Due to the destructive effects of the (self-sustaining) discharge this cannot be permitted to persist, and automatic systems momentarily shut down transmission within a few seconds to quench the "flame".


See also

* Singing Tesla coil


References


External links

{{Commons category, Plasma speaker
William Duddell


Magnetic propulsion devices speaker Loudspeakers Transducers