# Quiet Jet Engines

A number of deceptively simple, but hopefully interesting questions.

1. Why are jet engines so noisy?
2. Is there a way to make them quieter, or otherwise register less on the human ear?

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Janitor
I've seen Mach diamonds in rocket engine exhaust, and I think also in military jet engine exhaust. Regardless of how much noise is coming from the engine proper, that sort of high-speed exhaust is going to interact with the atmosphere in a very noisy way, I would think.

enigma
Staff Emeritus
Gold Member
All a fanjet engine is, is a giant turbine. You're also pushing out air at really high velocity. You can definately hear wind at 30mph... you'll really hear it at 500.

Integral
Staff Emeritus
Gold Member
While I remember well, the screech of a F4 Phantoms afterburners, a question perhaps related to this one, and much more important to my current life...

How about a quite vacuum cleaner?

Cliff_J
There are a few quiet vacum cleaners. Go down to Sears and look at their best models, for $300 its got great suction and the canister is easier then the uprights IMO. The upright featured on the TV show Monk is also quiet, but a friend paid like close to$500 for his verison of that company's product. Low 70 dbs would be my guess, I could meter it on my Audiocontrol and tell you if you really want to know. :)

How about the jet engine diffusors on the stealth planes? Or the other jets that hide the sound behind the 'cone' of air as the plane approaches mach? Still not going to be as quiet as a luxury car.

Cliff

Noise reduction for jet engines is a major design problem, especially since federal noise regulations for airports have become tougher. You have to fight the fact that you are dealing with pressurized air (and sound is a pressure disturbance) the mechanical noise generated by the engine (friction, spinning shafts, bearings, etc.) and the noise of high speed flow.

You can see some of the design elements that help reduce noise next time you are at an airport. Look for a Boeing 727, a two engine domestic aircraft. Check out the shape of the cowl of the engines, that is to say, the front part of the engine. You will notice that it looks like a flexible hoop being smooshed against the ground, with a flatter part at the bottom. This design reduces the noise produced by the engine while having a minimal impact on engine performance.

LURCH
The sounds so simple that I'm sure someone has already thought it, but;

Am sure you all seemed noise-reducing headphones, right? Perhaps a similar trick could be applied to get exhaust. What about a secondary cowling inside the primary? This inner cowling could be perforated to allow one-half of the sound waves that reach it to pass through, while the other half are deflected out the rear of the engine. The one-half that pass through this barrier strike the outer cowling and are deflected out the rear also. By properly setting the distance between the primary and secondary cowlings the apparatus could be engineered in such a way that the two sets of sound waves was less emitted are of equal amplitude and frequency, with a one-half phase variance. These two sets of sound waves might then partially cancel one another out.

russ_watters
Mentor
Noise reduction in airflow is actually quite simple: lower velocity. I deal with the issue on a daily basis designing air conditioning systems.

This is the reason turbofan engines are quieter than turbojets: massive fans and high bypass ratios mean lower airflow velocities at low speed.

Unfortunately, this also means the only way to reduce noise further is make the engines so big you could park a semi inside.

enigma
Staff Emeritus
Gold Member
russ_watters said:
Unfortunately, this also means the only way to reduce noise further is make the engines so big you could park a semi inside.
Speaking of which:

A-380

that is one huge plane, I think right before the depression hit there were designs for a gigantic 16 prop plane or something like that.

Cliff_J