The speed of sound in a plasma for the common case that the electrons are hotter than the ions (but not too much hotter) is given by the formula (see here)
where
* m is the ion mass;
* μ is the ratio of ion mass to proton mass ;
* T is the electron temperature;
* Z is the charge state;
* k is Boltzmann constant;
* γ is the adiabatic index.
In contrast to a gas, the pressure and the density are provided by separate species: the pressure by the electrons and the density by the ions. The two are coupled through a fluctuating electric field.