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 smallradius
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 socalled "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 firstorder 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 freefield boundary condition for both source terms. Furthermore,
an analytical expression of the electroacoustic efficiency of each
acoustic source is proposed.
An
experimental setup 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.
