Sources and Propagation of Aircraft Noise
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This chapter discusses sources and propagation of aircraft noise. The dominant noise from most aircraft is power-plant noise that generally increases with the output of the power plant. However, aerodynamic noise dominates in the case of the helicopter (blade noise) as well as the SST (sonic boom). Aerodynamic noise is also dominant in portions of the interior cabin of certain jet aircrafts. In jet aircraft there are two distinct kinds of engine noise. The jet engines currently in operation are basically of two types: turbojets and turbofans. The turbofans differ from the turbojets in that some of the air entering the inlet bypasses the engine combustion system and rejoins the burned gases at the exhaust tailpipe. Turbojets constituted the first generations of jet engines introduced in commercial service. The sonic boom is an aerodynamic phenomenon that occurs when a plane flies faster than the speed of sound. The intensity of the shock wave decreases with the three-quarter power of the distance from the aircraft.