The Plasma Tweeter
Massachusetts Institute of Technology
is a “Plasma Tweeter?”
sound is generated by a high voltage Corona
Discharge. An audio signal
causes the size of this electric flame to change rapidly, creating elastic
The flame has no practical mass, which implies a
perfect transient response and acoustical impedance matching.
It also has no directional preference, and thus acts like an
omnidirectional acoustic point source!
The lower frequency response limit is set by the
flame size and is around 2 kHz here as shown.
The upper limit is in the MHz!
feedback-driven oscillator passes a current though a Tesla-like coil at its
natural resonant frequency to generate high voltage.
audio signal controls the amount of this current changing in the coil,
altering the flame size.
The coil is basically a parallel resonant circuit; high Q, low
capacitance and low losses are desirable.
flame self-sustains as long as the voltage peaks above a critical electric
high frequency oscillation (greater than 3 to 4 MHz), the ions cannot move
very far in a period and form a region of space charge. This perpetuates the
corona flame and keeps it quiet.
ions bombard the electrode, electrons are freed by secondary emission
(heating). This sustains the
corona and causes sputtering, limiting electrode life.
Thoriated tungsten was used in the Ionovac.
the flame size! How low can the
frequency response go? Increase
the power efficiency to make larger flames practical.
the system as a whole. The flame
itself is surprisingly linear. I
didn’t do a very good job on the audio stages since I wanted to get the
with Helium gas, which should allow for high acoustic output.
Klein’s patents: #2,768,246
magazine, April 2003, Vol. 34, Issue 4, “Build a Plasma Tweeter”
Peek, Dielectric Phenomena in High
Voltage Engineering, 3rd Ed.,
Book Company, 1929.
Engle, Ionized Gases, Clarendon
download: plasma poster > Plasma