English: The shock is compressive because the characteristic speed (in air, the speed of sound) behind the shock is greater than that of the shock, which is greater than the characteristic speed in front of the shock. The characteristic speed is the speed of small, travelling perturbations.
to share – to copy, distribute and transmit the work
to remix – to adapt the work
Under the following conditions:
attribution – You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
share alike – If you remix, transform, or build upon the material, you must distribute your contributions under the same or compatible license as the original.