In some combustion reactions the reaction front is propagated by a strong pressure wave,
which compresses the unreacted mixture in front of the reaction front above its autoignition
temperature. This compression occurs rapidly, resulting in an abrupt pressure change or shock
in front of the reaction front. This is classified as a detonation, resulting in a reaction front and
leading shock wave that propagates into the unreacted mixture at or above the sonic velocity.
For a deflagration the energy from the reaction is transferred to the unreacted mixture by
heat conduction and molecular diffusion. These processes are relatively slow, causing the reaction
front to propagate at a speed less than the sonic velocity