**1. Introduction**

In modern commercial high bypass ratio aero engines, low specific fuel consumption together with low life cycle costs are mandatory to allow air transportation economically. Further‐ more, an environmentally friendly aero engine should have reduced CO2 emission by even lower fuel burn and generate low noise levels during takeoff (side line and cut back) and landing.

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At the beginning of the jet era noise emissions were dominated by jet mixing noise. That has beenreducedwhenintroducingmodernbypassenginesbyloweringthejetspeed.Noiseemitted by fan, turbine, and compressor has then become important with the reduction in jet speed. Now,thatnoisehas tobe reducedsignificantly.Since thepublicationoftheACAREgoals,which are targets defined by the Advisory Council for Aeronautics Research in Europe to reduce the environmental impact of air transportation, the commercial and political pressure to reduce CO2, NOx and noise (up to 20 dB reduction of noise level until 2020 compared to technologies of the year 2000) has been increased considerably. A lot of research over the past decades was done reducing fan and compressor noise in orderto achieve the noise reduction targets. During the last years, noise emission from the fan was much reduced that suddenly noise from the interactionofthe last stage lowpressure turbine andthe turbine exitguidevaneofthe exit casing became perceivable. Nowadays also manufacturers of low pressure turbine components have toconsideracousticaspects intheirdesigntobeabletoreachtheACAREgoals in2020.Therefore a lot of research is currently done in that field of expertise. An additional benefit of that low noise levels is that passengers as well as residents living in the vicinity of airports feel more comfortable. Basically an increasing acceptance to live close to airports and rise of life quality can be achieved if the noise level of aero engines is decreased significantly.

In this chapter measures to reduce noise generated and propagating from modern aero engine turbines are presented. The main issue is to find methods and/or new engine designs that reduce noise without causing considerable losses or a reduction of thrust. However, a lot of novel engine architectures are investigated in several national and international funded projects with the goal to reduce the emission of pollutants, e.g. by lowering the engine weight. This can be achieved by reducing the length of the entire engine by reducing the axial spacing between blade rows or integrating additional functions in one part, e.g. a non‐lifting strut in an intermediate turbine duct which also has to turn the flow and provide the next rotor with the correct inflow conditions. This leads to a so‐called turning mid turbine frame. However, all modifications on these parts of the engine will influence the noise generation and propa‐ gation. Also a considerable change of excitation of blades and vanes can be observed.
