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Preliminary Patent Application
December 7, 1969
Menlo Park, Calif.
Copyrighted © by The Townsend Brown family. All rights reserved.
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.
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
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.
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.
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.
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.
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
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.
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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