Shock wave cannot travel any further upstream and the pressure progressively builds in that region; a high pressure shock wave rapidly forms. Shock waves are not conventional sound waves; a shock wave takes the form of a very sharp change in the gas properties. Shock waves in air are heard as a loud "crack" or "snap" noise. Over longer distances, a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a sonic boom,
Undercompressive shock wave or undercompressive. Undercompressive shock wave An undercompressive shock wave is a shock wave that does not fulfill the Peter Lax conditions. Ordinary shock waves are stable, that is the speeds of characteristic behind the shock are greater than that of the shock itself, which is greater than the characteristics in front of the shock. The characteristic speed is the speed of small, travelling perturbations. These conditions are necessary for a shock wave to remain and not decay. If the peak of a wave moves faster than at its base, then the wave front becomes self-sharpening and eventually becomes a nearly