Castor K. and Béquin Ph.,
Laboratoire d'Acoustique de l'Université du Maine UMR CNRS 6613 Av. Olivier Messiaen B.P. 535 F 72 085 Le Mans Cedex 9 France

Most of the ionic loudspeakers use an ionised gas obtained by application of high voltage between electrodes having different curvature radii (e.g. a needle and a plane). The complex phenomena which take place in the electrode gap are called discharges in the gas. These discharges present different physical behaviours depending on the polarity of the small-radius electrode, the gap length and the gas. By choosing the appropriate geometric and electric configurations of the electrodes, the interactions between charged and neutral particles in the ionised gas can lead to either a predominant heat transfer between particles (Joule heat source) or a predominant momentum transfer which creates a gas flow, the so-called "electric wind" (force source, having a specific axis). If the current flowing through the gap is modulated by an external electronic circuit, the two energy transfer mechanisms act as two coupled acoustic sources.

The interaction mechanisms between charged and neutral are introduced in the acoustic equations by two source terms (heat and force sources). Assuming the neutral particle gas as an ideal gas, and considering adiabatic conditions and first-order of the acoustic quantities, an Helmholtz equation can be obtained using the set of linearised acoustic equations (continuity, Euler and Fourier equations). An analytical solution of the acoustic pressure is obtained for the case of a free-field boundary condition for both source terms. Furthermore, an analytical expression of the electroacoustic efficiency of each acoustic source is proposed.

An experimental set-up for acoustic pressure measurements has been developed and has allowed to confirm predictions. Based on experimental results, the evolution of the acoustic pressure and the electroacoustic efficiencies for different geometric and electric configurations have been traced and discussed.