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"Plasma-Dynamic Speaker"
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Preliminary Patent Application

December 7, 1969
Menlo Park, Calif.

Copyrighted © by The Townsend Brown family. All rights reserved.

Definition:

A device by which electrical signals are converted into high fidelity sound without the use of a moving cone or diaphragm and without (concurrently) producing a flow of air.

Scientific Background:

The plasma-dynamic loudspeaker results from extensive research in electric propulsion for spacecraft. More particularly, it represents a specialization in the filed of electrogasdynamics concerned with the phenomenon of corona pressure rather than the propulsion (movement) of gaseous dielectrics.

The physical principles underlying the plasma-dynamic loudspeaker are not new. Although complex, and not yet wholly understood, corona pressure (present in high voltage glow discharge) has been observed and reported in scientific literature for over 100 years.

Exists in Nature:

In thunderstorms, the powerful effect of corona pressure in producing sound is dramatically shown in lightning and the resultant thunder. The lightning flash, ionizing and heating a relatively thin channel of air, produces a rapidly-expanding envelope of air which subsequently collapses into a quasi-vacuum. This electrical "explosion" produces an intense pressure pulse and, with subsequent reverberation, causes thunder. No one can question the enormous sonic energy represented in thunder. Numerous references to the pressure of sound waves generated by electrical discharge can be found in technical literature.

It is interesting to note that this appears to be the only method by which Nature produces sound from electrical energy. Mankind has never made practical application of this method, until now.

Flame Ionization:

The inventor of the plasma-dynamic loudspeaker recalls, while a student at CalTech, he watched a mountainside brush fire sweep under a high tension line supplying power to the city of Los Angeles. As the flames swept up between the high voltage wires, the resulting discharge of half-million volt, high-current energy, created a roar which actually shook the earth. The roar was characteristic of the 60 cycle frequency carried by the line. Townsend Brown never forgot this experience, and it may account in part for his interest in this development.

Electromagnetic and Electrostatic Loudspeakers:

Up to now, the industry has directed its attention largely to the electromagnetic methods for the production of sound, wherein a paper-like vibrating cone is used. Even in the newer electrostatic loudspeakers, a thin electrically-conducting membrane is used which moves back and forth in a modulated electrostatic field. In every case, a moving cone or diaphragm is employed.

Inertial Distortion:

It has long been recognized in the loudspeaker art that the mass of the moving diaphragm introduces "inertial distortion." The cone or diaphragm overshoots and does not follow the input electrical signal faithfully. This overshooting is normally reduced by making the moving part as light as possible, but there is a limit in the weight-to-strength ratio beyond which designers cannot go. Loudspeaker manufacturers have long dreamed of weightless cones.

The plasma-dynamic loudspeaker has no cone or diaphragm whatever, and therefore suffers no inertial distortion. The electrical signals are converted into sound waves with almost perfect compliance.

Doppler Distortion:

This is another form of distortion present more or less in all loudspeakers commercially available today, resulting from the to-and-fro movement of the cone. It appears as a periodic shift in the higher frequencies. Like a train whistle, as the train approaches, the pitch is high. It then drops suddenly to a lower frequency as the train passes and moves into the distance.

This compression and extension of sound waves, in the case of a loudspeaker, causes a frequency spread which the mind of the listener tries to average out, but does so ineffectually. The impression that the listener receives is that the voice or music, so distorted, lacks realism. He is not aware of this deficiency until he compares the performance with that of the plasma-dynamic speaker which is not so distorted. The listener's reaction is that the plasma-dynamic speaker is more "life-like."

The elimination of these two causes of distortion puts the plasma-dynamic loudspeaker in a class by itself. The loudspeaker provides life-like realism, and "listener fatigue: is notably reduced.

Listener Reaction:

A critical listener recently reported his impression during one of the first demonstrations of the plasma-dynamic speaker: - "The unit was attached to the TV set in my living room. I personally have never particularly enjoyed watching TV and seldom watch for any length of time. With the new loudspeaker, however, I discovered my reaction was quite different. For the first time, I could "sense" the action. I even enjoyed the realism of the Westerns - the distant beating of horses' hoofs, even the crickets and gunfire. I actually sat for three hours listening and enjoying the late, late shows. I feel that every consideration should be given promptly to adapting this new type of speaker to TV reception. It would seem to me to provide a whole new parameter in TV realism and enjoyment."

The "Ionvac":

In only one instance, to the best of our knowledge, has a system been proposed which ever disposed of the diaphragm. This device was known as the "Ionvac." When it was introduced in France about 15 years ago, it was heralded as the long sought solution. The "Ionvac" was essentially a high frequency "tweeter," actually only effective only in the higher frequencies. The device was marketed for a short time, but ran into practical difficulties because it operated at high temperature. Sound waves were generated by the thermal expansion and contraction of a modulated RF arc which was maintained within a thimble-like enclosure. The temperature of the arc was so high that it gradually vaporized the electrodes and eroded the ceramic thimble. Spikes of metal in time condensed on the walls of the thimble and the whole device became inoperative. The "Ionvac" has since been removed from the market.

Possible Military Application:

Quite apart from its commercial application in TV and Hi Fi, the plasma-dynamic loudspeaker has, as a transducer, significant military applications. It appears to be the only transducer which can perform efficiently in the ultra-high frequency range. Electromagnetic transducers are non-operative at frequencies above 50,000 Hz. The piezo-electric transducers such as quartz, titanium oxide or barium titanate reach higher operating frequencies but are fragile, expensive, and in most cases resonant at a fixed frequency. Many military uses, some highly classified, require high-energy propagation. Only by carefully grinding, sorting and matching piezo-electric units can such high-energy arrays be built, and this is a costly procedure. No inexpensive solution has been found.

The plasma-dynamic transducer appears to have none of these limitations. Its upper frequency response, not yet determined, appears to be well above 250,000 Hz. Best of all, the transducer is non-resonant. Its construction is inexpensive and its dimensional tolerances are relatively unimportant in its performance. Efficiencies are not yet determined, but preliminary tests are highly favorable. A research and development contract with the Navy is a distinct possibility.

Summary:

The plasma-dynamic loudspeaker (transducer) is believed to posses the following desirable features:

1. No moving parts (no vibrating cone or diaphragm).
2. Long life - virtually no maintenance.
3. No inertial or Doppler distortion.
4. Life-like realism.
5. Flat frequency response, without resonant peaks
6. Estimated frequency range of 50 to 250,000 Hz., with possible extension into both lower and higher frequencies.
7. Efficiencies probably better than 5 times that of the average electromagnetic speaker.
8. Using modular construction, can be built to any size.

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