A plasma loudspeaker using a DC carbon arc in a modulated magnetic field

Summary form only given. Plasma loudspeakers potentially have a more accurate frequency response than conventional loudspeakers, since they have no solid moving parts that possess inertia. The problem with plasma loudspeakers is that most require complicated apparatus and helium or argon gas to generate atmospheric pressure plasma. In this proof-of-principle experiment, we use a conventional carbon-welding arc to generate the plasma. We used a DC arc, and modulated a transverse magnetic field to cause the arc to vibrate in space to generate sound waves. Previous carbon arc experiments modulated the arc current to generate thermally induced pressure fluctuations that produce sound. Our experiments were successful in that an audio oscillator coupled to the magnetic field generated a wide spectrum of sounds. The apparatus was simple, and proved to be reliable. The DC arc voltage was about 100 V, and the arc current was about 5 A. The magnetic field was generated by two filter chokes that each had one leg of the magnetic circuit removed. The open sides of the magnetic circuits of the two chokes faced each other, with the carbon arc in between. However, the amplitude of the sound produced was not sufficiently high for a commercial product. Our impression is that the area of the oscillating arc was too small - a few square centimeters although the amplitude of the arc displacement was observed to be large - up to a centimeter. A possible future way to enhance sound production would be to surround the arc with a close-fitting tube that is coupled to an exponential horn for better acoustic coupling to the air in the room.