In low pressure gases, speed of sound is independent on pressure and depends only on temperature. Therefore increase of pressure will propagate as shockwave - front of the wave compresses and heats the gas, allowing the rear of the wave to travel faster and pile up into a shock. By the same cause, a decrease of pressure CANNOT propagate as shockwave in ideal gas - initial expansion cools the gas and slows down the rear of the wave, spreading out the unloading. How about fluids? The isothermal compressibility diverges to infinity at and below critical point. It does not and cannot get negative. Adiabatic compressibility therefore stays finite everywhere. How does the speed of sound around critical poind depend on pressure along adiabats? Is there any region where speed of sound increases on expansion, enabling decrease of pressure to propagate as shockwave?