2146 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 11, NOVEMBER 2011

Looking Into a Plasma LoudspeakerYvonne Sutton, Jon Moore, David Sharp, and Nicholas St. J. Braithwaite

Abstract—A 325-kHz atmospheric discharge can be modulatedat audio frequencies so that it acts as a loudspeaker by direct elec-troacoustic coupling, without any electromechanical components.In exploring the details of the mechanism, it has been useful tovisualize the heated gas within and around the discharge plumeusing Schlieren techniques. This has enabled a 2-D reconstructionof the translational temperature of the neutral gas (up to 2500 K)that complements spectroscopic measurements of rotational andvibrational temperatures (up to 2700 K) in the luminous region.

Index Terms—Acoustic transducers, atmospheric-pressureplasma, loudspeakers, plasma applications.

AN ATMOSPHERIC discharge is being studied in termsof its effectiveness as a source of sound. Modulated

discharges have previously been developed to provide directcoupling of audio-frequency (electrical) signals into sound, i.e.,a loudspeaker [1]–[3]. There are two distinct mechanisms thatcan be exploited. One is based on direct momentum transferthrough an ion wind drawn from a corona discharge. Theother involves a thermal mechanism similar to that in naturallightning, wherein sound is generated by the rapid expansion ofan ionized channel through the air. Although considerably lesssevere, the discharge involved in this study is closer to the latterthan the former.

The discharge is generated using a low-voltage chopperdriving a Tesla coil (resonant transformer) that produces asinusoidal waveform of 20–30 kV, which is high enough tobreak down a point-to-point gap of 15–20 mm. The circuit hasa natural frequency in the region of 325 kHz determined bythe inductances and capacitance of the coil. A control stageprovides the switching signal to a power stage containing fourpower transistors (MOSFET) in a full wave bridge configu-ration. Alternate switching of the transistors produces a high-voltage square-wave output to the primary coil. The Tesla coilthen picks out only the fundamental frequency of the squarewave to transfer energy into the secondary where, after high-voltage breakdown, a steady conducting path is sustained by∼2 kV rms. The power stage is driven by a dc supply; anincrease in the dc voltage translates into an increase in thepower transferred into the plasma. The discharge that is struckin the secondary circuit is rooted between sharpened points on

Manuscript received November 30, 2010; revised July 13, 2011; acceptedJuly 16, 2011. Date of publication September 22, 2011; date of current versionNovember 9, 2011.

Y. Sutton, D. Sharp, and N. St. J. Braithwaite are with the Department ofPhysics and Astronomy, The Open University, MK7 6AA Milton Keynes, U.K.

J. Moore is with Bowers & Wilkins, Steyning, BN44 3SA, U.K.Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPS.2011.2165560

copper electrodes, leading to visible hot spots [see Fig. 1(a)].The discharge column has a flamelike luminous axially sym-metric shape some 2–5 mm in diameter. Fig. 1(a) shows twoimages of visible light emission from discharges carrying rmsconduction currents of (upper) 11 and (lower) 30 mA, recordedusing a conventional camera. The column is stable and silent inambient air.

The 325-kHz (RF) discharge has some of the characteristicappearance of a dc arc, although it is not expected to be inthermal equilibrium. There is a clear top-to-bottom asymmetrythat can be attributed primarily to gas heating rather thanelectrical effects. When operated horizontally, the dischargeforms along a classic upwardly curved path (“arc”). Duringnormal operation, there is virtually no ozone in the surroundingair, which indicates that the gas temperature and gas mixing aresufficient to maintain high rates of ozone destruction. However,it is likely that, under these conditions, oxides of nitrogen aregenerated.

Electroacoustic transduction begins with the audio-frequency-modulated transfer of electrical energy into thedischarge plasma. This is readily visualized through theintensity of total optical emission, which shows a synchronousfluctuation, reflecting a modulation of the electron population.Spectroscopic analysis of emission from nitrogen moleculesin the 300–400-nm range reveals a very weak coupling ofthe acoustic signal through these channels, with barely anysynchronous structure in the temporal variation of rotationaland vibrational temperatures [4]. Such spectroscopy islimited to the locality of energetic electrons and cannot giveinformation outside the luminous plume. Instead, Schlierenimaging reveals refractive index gradients in transparent media,and it offers a means of “seeing” where there is gas heatingoutside (and within) the plume.

The contrast in a Schlieren image is obtained by illuminatingthe test volume with parallel rays (collimated from a pointsource) that are then brought through a focus at which a knifeedge cuts off half the field and most of the focal spot [5].An image formed from this plane with a third converginglens contains information about rays that were refracted bythe test volume, thereby missing the focus. On one side ofthe image, density gradients perpendicular to the knife edgemap onto enhanced illumination; the illumination of the otherside is diminished. Fig. 1(b) shows a Schlieren image recordedwith a 1024 × 1024 intensified CCD array (Andor TechnologyDH534-18F), using light from a tungsten–halogen lamp. A10-nm bandpass filter centered on 632 nm was used to providea monochromatic Schlieren field. By placing the filter betweenthe plasma and the imaging optics, most of the light emittedby the discharge was removed; background subtraction wasthen used to remove residual emission, particularly from the

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SUTTON et al.: LOOKING INTO A PLASMA LOUDSPEAKER 2147

Fig. 1. Two sets of images of the unmodulated RF discharge at the extremes of the operating range of conduction current: (Upper) 11 and (lower) 30 mA.(a) Total visible emission. (b) Schlieren image revealing gradients of refractive index perpendicular to the axis. (c) Translational gas temperature map (linearscale—axial peak 2700 K and ambient at large radius 295 K)—the dashed lines trace the luminous boundary from (a).

electrode spots. The light and dark bands demarcate a region ofrapidly changing refractive index that shrouds the plume.

The final component of the image [Fig. 1(c)] shows a con-tour plot of the radial variation of gas temperature, deducedfrom the Schlieren images. The temperature can be calculatedby using an Abel transformation [5] of the intensity map inFig. 1(b) together with an ideal-gas model of refractive index(i.e., the refractivity is inversely proportional to the absolute gastemperature). This map shows the hot gas core. Interestingly,there is not complete equilibrium between the peak translationaltemperature (2500 ± 200 K) of the whole gas and that derivedfrom the rotational and vibrational spectra of the nitrogenmolecules (2700 ± 500 K) [4].

REFERENCES

[1] F. Bastien, “Acoustics and gas-discharges—Applications to loudspeakers,”J. Phys. D, Appl. Phys., vol. 20, no. 12, pp. 1547–1557, Dec. 1987.

[2] P. Bequin, “Model of acoustic sources related to negative point-to-plane discharges in ambient air,” Acustica, vol. 83, no. 2, pp. 359–366,Mar./Apr. 1997.

[3] M. S. Mazzola and G. M. Molen, “Modelling of a DC glow plasmaloudspeaker,” J. Acoust. Soc. Amer., vol. 81, no. 6, pp. 1972–1978,Jun. 1987.

[4] Y. Sutton, G. V. Naidis, D. Sharp, J. Moore, and N. St. J. Braithwaite,“An experimental and numerical investigation of an axially-symmetric RFplasma,” J. Phys. D, Appl. Phys., submitted for publication.

[5] T. E. Carlsson, R. Mattsson, P. Gren, M. Elfsberg, and J. Tegner, “Com-bination of Schlieren and pulsed TV holography in the study of ahigh-speed flame jet,” Opt. Lasers Eng., vol. 44, no. 6, pp. 535–554,Jun. 2006.