Can the noise level of jet engines be reduced?

In summary: 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. Jet engines are very noisy because of how they produce airflow. There are a few ways to reduce the noise made by an engine, but the most effective solution is to make the engine so big it can't produce much noise in the first place.
  • #1
FZ+
1,604
3
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?
 
Physics news on Phys.org
  • #2
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.
 
  • #3
All a fanjet engine is, is a giant turbine. You're also pushing out air at really high velocity. You can definitely hear wind at 30mph... you'll really hear it at 500.
 
  • #4
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?
 
  • #5
There are a few quiet vacuum 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
 
  • #6
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.
 
  • #7
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.
 
  • #8
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.
 
  • #9
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:

http://users.adelphia.net/~luv2hang/380.jpg
 
  • #10
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.
 
  • #11
Well I think the Howard Hughes "Spruce Goose" would be the biggest ever built, and its construction was during WWII. At 320ft for the wingspan, its got 58ft on the A380! Only 8 engines, plenty for its one and only flight.

http://www.sprucegoose.org/Specification.htm [Broken]

Cliff
 
Last edited by a moderator:

Question 1: What causes jet engines to be so loud?

Jet engines are loud due to the high-speed exhaust gases that are expelled from the engine during operation. The high temperatures and pressures inside the engine cause these gases to be expelled at a high velocity, creating a loud noise.

Question 2: Are all jet engines equally loud?

No, not all jet engines are equally loud. The loudness of a jet engine depends on its size, type, and design. Smaller engines tend to be quieter than larger engines, and newer designs with improved noise-reducing technologies are typically quieter than older designs.

Question 3: How do engineers reduce the noise of jet engines?

Engineers use various techniques to reduce the noise of jet engines, such as incorporating sound-absorbing materials in the engine, using aerodynamic design elements to reduce turbulence, and implementing exhaust nozzle designs that help to reduce noise. They also conduct extensive testing and analysis to identify and address noise sources within the engine.

Question 4: Why are jet engines louder during takeoff than during cruising?

Jet engines are louder during takeoff because they are operating at maximum power to lift the aircraft off the ground. This requires a higher amount of thrust, which results in more noise. During cruising, the engine is typically operating at a lower power setting, resulting in less noise.

Question 5: Can jet engine noise be harmful to human health?

Exposure to jet engine noise at high levels and for extended periods can potentially have negative effects on human health, such as hearing loss and increased stress levels. However, strict regulations and noise reduction technologies have been implemented to minimize the impact of jet engine noise on both passengers and the surrounding environment.

Similar threads

  • Aerospace Engineering
Replies
2
Views
1K
  • Aerospace Engineering
Replies
2
Views
1K
Replies
2
Views
1K
  • Aerospace Engineering
2
Replies
35
Views
3K
Replies
11
Views
2K
Replies
2
Views
1K
Replies
8
Views
2K
Replies
3
Views
576
  • Thermodynamics
Replies
3
Views
2K
  • Introductory Physics Homework Help
Replies
5
Views
2K
Back
Top