Audio reproduction with atmospheric pressure plasmas:

Surface discharge device

Hugh Potts, Declan Diver

University of Glasgow

Why plasma?• No moving mass

– No resonances, smooth response– No upper frequency limit (CD 20kHz, SACD 100kHz)

• Looks nice

• Inefficient for low frequencies• Complex, dangerous• Ozone!*Although inaudible, has a significant effect on the perception of sound – shown in EEG brain measures (26-60kHz)

Why not?

History

Demonstrated 1800’s

First patent 1924

First practical device 1946: Siegfried Klein

The IONOPHONE

History

Demonstrated 1800’s

First patent 1924

First practical device 1946: Siegfried Klein

The IONOPHONE

History

Demonstrated 1800’s

First patent 1924

First practical device

“…after prolonged music-listening we felt some slight irritation of the skin in the auditory canal affected by the partially ionized air. “

Why does it work?

• Two main methods of sound production– Ion-wind coupling to neutral gas

• Poor efficiency, quantify

– Heating• Thermal expansion drives pressure waves• Tiny amplitued required: Normal speech 0.002-

0.02 Pa pressure amplitude - only 4nm amplitude!• Heat a 0.1mm slab of air: 0.01K, 0.4 W/m2

Physics

• How much heating is required

• What amplitude is required?

• Imagine heating 0.1mm slab, scale by 1/r^2

• results

Modern developments

• Atmospheric pressure plasma arc • 317 kHz resonant frequency (drawing ~1.2A)• 2D isotropic acoustic radiator measured acoustically over the frequency range 1-10 kHz

N.S. Braithwaite, Y.Sutton, Open University

Planar discharge

• Surface DBD discharge

• Electrode configuration optimised using 3D finite difference iterative solver to produce the most even discharge possible.

• Praps a picture of the discharge in Argon

• OZONE! Oh dear oh dear…

100mmdiam

Planar discharge at 30kHz

Air 90% Argon

Performance

• Early prototype• 100mm diam• Excellent frequency

response <100Hz - >>20kHz

• Potentially efficient• Low temperature,

hence ozone production a problem