The Plasma Tweeter

1
The Plasma Tweeter

• A high frequency audio transducer that utilizes
  an ionized flame.
• Colin Joye,
• Grad student, PSFC, Waves and Beams,
• MIT

2
The Problem off-axis response
3
The Dynamic Cone Speaker
4
Solution shrink the speaker??

• Diffraction properties cause sound beaming at
  high frequencies.
• W D/?
• Wavelength ?/4 0.4mm at 20kHz ? Too small!!

5
Optimal Listening Area
6
Possible Solutions

• Siamese Tractrix Horn
• Only horizontal uniformity
• Diameter must be ?max
• Difficulty in guiding sound
• Line Source (ribbon)
• Only vertical uniformity

7
Possible Solutions (cont)

• Hypersonic Source
• Uses ultrasonic beat frequencies to accurately
  image sound.
• Not perfectly uniform
• Requires hard flat surface
• Sensitive to surface type
• Still only one optimal zone.

8
Another Option

• The plasma tweeter (a.k.a. ion tweeter) uses
  corona discharge to create an electric flame.
• Modulation of the flame size in time produces
  sound waves.
• Flame gives essentially perfect spherical
  uniformity.
• No moving mass yields theoretically perfect
  transient response and natural impedance matching!

9
How does it work?

• Siegrfried Klein, US Patent 2,768,246 (1952)
• Corona discharge heats the tip of the electrode,
  causing thermionic emission.
• The large potential difference imparts kinetic
  energy into the ions, which collide with air
  molecules, creating pressure.
• Modulating the potential causes changes in
  pressure, and thus audible elastic waves.

10
How to make Corona Discharge

• Corona discharge studies in early 1900s for
  power transmission F.W. Peek.
• Favors sharp points.
• Starts at around 20kV.
• Klein High voltage oscillation frequency must
  be greater than 3MHz.

11
How to get high voltage

• Ohms Law
• V IZ Large Z and moderate I gives us
  Large V!
• Use of a parallel resonant circuit with low loss
  gives very high Z (low Y) at resonant
  frequency.

12
The Resonant Coil
13
The System
14
The first version

• Coil f0 35MHz, Clean flame.
• Vacuum Tube switching device for high Gain-BW
  product and very low input capacitance.
• Good sound quality, demonstrates the concept
  well.
• Inherent tube loss generates waste heat.
• Develop high efficiency FET-based design.