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I know that the nacelles around the aircraft engines are actually a housing and are generally lightweight and have may components like inlet cowl, fan cowl, etc., but do they help in reducing the engine sound as well or are they just for housing?

Even if they help in reducing the sound, then how much % of sound has been reduced by them?

Is there any quantitative analysis available for that which can show that without housing engines were creating sound at X dB, and after nacelle and proper housing of all other components, the sound has been decreased to Y dB?

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  • $\begingroup$ How much is what? Can you please clarify? $\endgroup$ – Kamran Jan 22 '15 at 4:06
  • $\begingroup$ edited with more clarification $\endgroup$ – NitinG Jan 22 '15 at 4:25
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Take a modern turbofan: The tips of the fan run at approximately Mach 1.5. We know from unducted props that supersonic tips make them very noisy. The same turbofan without the shroud would be extremely noisy, too. Acoustic liners on the inside of the fan shroud have major contribution in suppressing this noise source.

But there is more: The nacelle also makes it possible to put an intake in front of the fan which helps to equalize flow conditions over the cross section of the intake and to accelerate or slow the flow of air to just the right amount for the engine. Without the intake, the engine would suffer from frequent compressor stalls and surges, so the nacelle in effect widens the operating conditions and avoids those noisy incidents, too.

Another important source of noise is the nozzle: Here the hot, fast gas from the engine is ejected into the surrounding flow, and the mixing of both creates most of the engine noise. Note that noise increases with the sixth to eighth power of jet velocity, so the most dramatic noise reduction is possible with higher bypass ratio engines.

Plot of exhaust noise level over jet velocity Diagram from the Stanford web page linked above. The jet velocity is in ft/s.

By serrating the trailing edge of the nozzle, this mixing process can be slowed down somewhat and the noise level can be reduced.

Without the nacelle the engine would create much less thrust and more drag, and bigger, more powerful engines would be needed. Bigger engines would create more noise, too, so this is also an indirect way how nacelles help to keep noise down.

A last noise reduction contribution to mention here is the capacity of the nacelle to contain the shrapnel from a failed fan blade. Fan blade failure is all but silent, but if the blade fragments would escape at high speed, the noise of what follows next would also be considerable.

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    $\begingroup$ Regarding your last paragraph, this one was a turbine disk rather than a fan blade, but it did generate quite a lot of noise in the form of discussion in the cockpit, radio chatter, media reports, and a few years of subsequent discussions about why it happened. $\endgroup$ – reirab Jan 22 '15 at 16:29
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The nacelles themselves reduce sound just by being another barrier before the sound gets to you for the most part. It's there to provide airflow around engine components, better aerodynamics, protection from elements, and depending on what kind of engine it is, it "may" help with noise reduction some (Turbofan).

On turbine engines, there are exhaust airflow options that do suppress the noise. This is not a nacelle, it's a noise suppression nozzle.

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  • $\begingroup$ You are mostly right, but there are multiple design features in a modern nacelle that play a large role in noise reduction. $\endgroup$ – fooot Jan 22 '15 at 18:28
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As you have correctly mentioned, the term Nacelle generally refers to a number of sections around an aircraft engine (In the context of turbofan aircraft engines). This generally the structures formed by the Inlet, Fan Cowl, Thrust Reverser and the Exhaust.

The nacelle does perform the basic function of a housing for all these subassemblies which means it contributes to protection of the engine from the elements and the environment from the engine (Say a blade failure). But its major function is to provide an aerodynamic body which can house the mechanicals of the engine, the pneumatic ducts, fuel lines and electrical wiring.

The nacelle can therefore provide noise reduction by reducing the drag-noise which would be otherwise generated. This is through the aerodynamic design and the various features built into the subassemblies such as Acoustic Liners used extensively througout the nacelle which convert acoustic energy into small amounts of heat. This is a major source of noise-reduction in modern aircraft engines.

Exhausts are the biggest culprits when it comes to aircraft engine noise and as such, the exhausts are designed to create a 'mixing' area where hot air from the exhaust is mixed with ambient air to reduce the exhaust noise.

I guess I'm just trying to say that the nacelle is formed of these various subassemblies and therefore their individual contribution towards noise reduction is very important. As far as a quantitative study on the reduction of noise, I could not locate any as therefore can only manage to answer the first part of your question.

Sidenote: Modern Inlets also use Laminar-flow design (777X for example) which contribute to immense fuel savings and very minor drag-noise reduction (Probably negligible).

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