In the engine nacelle, the area between the engine and the  cowl has acoustic linings surrounding the engine. This  noise-absorbing lining material converts acoustic energy  Fan engines require a larger turbine to provide additional  power to drive the fan. The large turbine, which usually has  an additional turbine stage, reduces the velocity of the gas  and, therefore, reduces the noise produced because exhaust  gas noise is proportional to exhaust gas velocity. The exhaust  from the fan is at a relatively low velocity and, therefore, does  not create a noise problem. Because of the characteristic of  low-frequency noise to linger at a relatively high volume,  effective noise reduction for a turbojet aircraft must be  achieved by revising the noise pattern or by changing the  frequency of the noise emitted by the jet nozzle. The noise suppressors in current use are either of the corrugated- perimeter type, or the multi-tube type.  [Figure 3-53]  Both  types of suppressors break up the single, main jet exhaust  3-32 Figure 3-53.  Noise suppressors currently in use are corrugated- perimeter type, or multi-tube type.  Noise levels vary with engine thrust and are proportional  to the amount of work done by the engine on the air that  passes through it. An engine having relatively low airflow  but high thrust due to high turbine discharge (exhaust gas)  temperature, pressure, and/or afterburning produces a gas  stream of high velocity and, therefore, high noise levels.  A larger engine, handling more air, is quieter at the same  thrust. Thus, the noise level can be reduced considerably  by operating the engine at lower power settings, and large  engines operating at partial thrust are less noisy than smaller  engines operating at full thrust. Compared with a turbojet,  a turbofan version of the same engine is quieter during  takeoff. The noise level produced by a fan-type engine is  less, principally because the exhaust gas velocities ejected  at the engine tailpipe are slower than those for a turbojet of  comparative size. stream into a number of smaller jet streams. This increases the total perimeter of the nozzle area and reduces the size  of the air stream eddies created as the gases are discharged  into the open air. Although the total noise-energy remains  unchanged, the frequency is raised considerably. The size  of the air stream eddies scales down at a linear rate with  the size of the exhaust stream. This has two effects: 1) the  change in frequency may put some of the noise above the  audibility range of the human ear, and 2) high frequencies  within the audible range, while perhaps more annoying, are  more highly attenuated by atmospheric absorption than are  low frequencies. Thus, the falloff in intensity is greater and  the noise level is less at any given distance from the aircraft. Figure 3-52.  Engine noise from engine exhaust is created by the turbulence of a high velocity jet stream moving through the relatively  quiet atmosphere.  30 D 10 D D Most of the noise radiates from this low frequency  turbulence region D = Nozzle diameter 10° Tail cone