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,
Aircraft noise Whitney PW1000G helped reduce the noise levels of the Bombardier CSeries, Mitsubishi MRJ and Embraer E-Jet E2 crossover narrowbody aircraft: the gearbox allows the fan to spin at an optimal speed, which is one third the speed of the LP turbine, for slower fan tip speeds. It has a 75% smaller noise footprint than current equivalents. The PowerJet SaM146 in the Sukhoi Superjet 100 features 3D aerodynamic fan blades and a nacelle with a long mixed duct flow nozzle to reduce noise. Aerodynamic noise arises from the airflow around the aircraft fuselage and control surfaces. This type of noise increases