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 a sharp wave front which remains so when it travels. A shock wave is undercompressive if these conditions are not fulfilled. A solution exists where a sharp wave front may remain sharp whilst travelling even when perturbations behind the front travel slower than it. An experiment can be made to show this with travelling liquid steps : a thick film is spread on a thin one. The liquid steps remain sharp when they travel because the spreading is enhanced by the Marangoni effect. Making little perturbations with the tip of a hair, one can see whether shock waves are compressive