Figure 3-51.  A pilot can direct thrust via the vectoring nozzle 20°  up or down to increase flight maneuverability.  Figure 3-50.  An example of a variable area exhaust nozzle used to  increase or decrease exhaust flow during afterburn.  3-31 Farther downstream, as the velocity of the jet stream  slows down, the jet stream mixes with the atmosphere and  turbulence of a coarser type begins. Compared with noise  from other portions of the jet stream, noise from this portion  has a much lower frequency. As the energy of the jet stream  finally is dissipated in large turbulent swirls, a greater portion  of the energy is converted into noise. The noise generated  as the exhaust gases dissipate is at a frequency near the low  end of the audible range. The lower the frequency of the  noise, the greater the distance the noise travels. This means  that the low-frequency noises reach an individual on the  ground in greater volume than the high-frequency noises,  and hence are more objectionable. High-frequency noise  is weakened more rapidly than low-frequency noise, both  by distance and the interference of buildings, terrain, and  atmospheric disturbances. A deep-voiced, low-frequency  foghorn, for example, may be heard much farther than a  shrill, high-frequency whistle, even though both may have  the same overall volume (decibels) at their source. Aircraft powered by gas turbine engines sometimes require  noise suppression for the engine exhaust gases when  operating from airports located in or near highly populated  areas. Several types of noise suppressor are used. A common  type of noise suppressor is an integral, airborne part of the  aircraft engine installation or engine exhaust nozzle. Engine  noise comes from several sources on the engine, the fan, or  compressor and the air discharge from the core of the engine.  There are three sources of noise involved in the operation  of a gas turbine engine. The engine air intake and vibration  from engine housing are sources of some noise, but the noise  generated does not compare in magnitude with that produced  Thrust vectoring is the ability of an aircraft’s main engines to  direct thrust other than parallel to the vehicle’s longitudinal  axis, allowing the exhaust nozzle to move or change position  to direct the thrust in varied directions. Vertical takeoff  aircraft use thrust vectoring as takeoff thrust and then change  direction to propel the aircraft in horizontal flight. Military  aircraft use thrust vectoring for maneuvering in flight to  change direction. Thrust vectoring is generally accomplished  by relocating the direction of the exhaust nozzle to direct the  thrust to move the aircraft in the desired path. At the rear of  a gas turbine engine, a nozzle directs the flow of hot exhaust  gases out of the engine and afterburner. Usually, the nozzle  points straight out of the engine. The pilot can move, or  vector, the vectoring nozzle up and down by 20°. This makes  the aircraft much more maneuverable in flight.  [Figure 3-51] by the engine exhaust.  [Figure 3-52]  The noise produced by  the engine exhaust is caused by the high degree of turbulence  of a high-velocity jet stream moving through a relatively  quiet atmosphere. For a distance of a few nozzle diameters  downstream behind the engine, the velocity of the jet stream  is high, and there is little mixing of the atmosphere with the jet  stream. In this region, the turbulence within the high speed jet  stream is very fine grain turbulence and produces relatively  high-frequency noise. This noise is caused by violent,  turbulent mixing of the exhaust gases with the atmosphere  and is influenced by the shearing action caused by the relative  speeds between the velocity and the atmosphere.  somewhat like a single burner can combustion chamber.  Operation in the afterburner mode is somewhat limited by  high fuel consumption, which can be almost double normal  consumption.  Thrust Vectoring  Engine Noise Suppression