Aviation Sound Attenuation

[Superman]

Perhaps someone else can answer my latest aviation question, while Mr. Higgins is designing special duct tape to be used on 747 fuselages.

My cell phone conversations are usually complexified by jet airplane noise. The MD-80's are the worst by far, but they are all terrifically loud. I have to wait for them to clear the area to resume my conversations. But then I noticed they are a quarter mile away when they drown out my phone conversation. The passengers are ten feet away. How do engineers design these planes so that the engines do not deafen the passengers? The solution must have been lightweight. What sound mat do they use and where can I get a sheet?

[Rot 911]

I am reporting you for posting an OT post in the tech forum.

[RWebb]

no it links ot the sound attenuation thread for light wt. 911 mods...

[Superman]

Originally Posted by Kurt V

I am reporting you for posting an OT post in the tech forum.

I throw myself, postrate, to the mercy of the Moderators.

[wastintime]

I'm going to disagree on the MD-88s being the worst, but... the turbofans used in all commercial aviation craft are nowhere near as bad as afterburning turbofans used in fighters. This actually has to do with the speeds the turbines are spinning at... there really isn't a whole lot of speed differential that you can change when adjusting the throttles end engine nozzles, especially in terms of exhaust speed. Even if the plane isn't supersonic yet, the exhaust might be... engines for higher speed craft tend to spin faster and be noisier. That was a gross oversimplification, btw...

You also hear a lot of aerodynamic noise totally unrelated to the engines when aircraft go by.

As for the passenger compartments... if you think about it, other than at takeoff, you're rarely anywhere near the peak sound levels a jet engine will make, and some jets even at takeoff are only around the db levels of some race cars... take a race motor and put it next to double paned windows, thermal, and sound insulation, plus the loss of frequency from a pressurized cabin, etc... all of a sudden it's really not hard to imagine that it's at a tolerable level. I'm not 100% sure of the materials used... my dad might have known he flew in the Air Force and for Delta for like 30 years.

The two neatest jets I've gotten to fly in were a T-38 and an F-15D, and honestly from inside the planes the sound isn't nearly as bad as on the ground as they fly over. In fact, did you ever see the movie "Behind Enemy Lines" with owen wilson... terrible and horribly inaccurate movie, but the cockpit noise was shockingly accurate in a jet fighter.

[BlueSideUp]

Most of the noise you hear from outside the airplane is from the windshear as the high speed exhaust air encounters the relatively slow moving air around it. This is the ripping sound you can hear. You might notice that aircraft like a 757 are much more quiet, the high velocity core exhaust is surrounded by the much lower velocity fan exhaust. The Mad Dogs or MD-80s are especially loud because their engines are not high bypass like the more modern 757 so you have a lot more high-velocity air.

Inside the airplane you hear a lot more fan noise because all of that windshear business is behind you in the case of an MD80. In the case of a 737 if you've ever ridden in the front and the back you probably noticed a big difference in ambient noise. That is the difference between fan and exhaust noises.

On some newer aircraft, the CRJ900 for example they have installed a chevron exhaust nozzle to reduce the noise. The new GENx powerplant for the 787 also uses this design.





The only aircraft I know of that uses active noise canceling is the Bombardier Dash 8 Q400. Apparently it works well but is usually broken.

I can't find a picture but if you look inside the inlet of the engine cowling on a CRJ you can see a bunch of little holes. The holes are designed to reduce noise made by the fan.

[tcar]

Originally Posted by Kurt V

I am reporting you for posting an OT post in the tech forum.

I think he's asking, pretty round-a-bout, how to make his P-car quieter.

Or to build a 'cone of silence' for use with his cell phone.

Maybe.

Possibly.

[Jim727]

But.but.but.but.but....

I was chewed out for posting prop noise charts in that thread ...

Supe, you ask a complex question, but here goes. (Some gaps in info guaranteed). There are several sources for jet engine noise, some obvious - like combustion - others not. The high-pitched whine you hear comes from the inlet compressor stages. In some engine intakes you can see perforations around the inside of the ductwork; this is to help attenuate the noise and cannot be completely efficient.

Another big source of noise, much lower in pitch, is at the business-end. Jets derive their thrust from the acceleration of mass; that, in turn, results in a high-velocity exhaust. Much of the noise comes from the shear layer between the high-velocity exhaust and the surrounding air. The earliest jets (turbojets) were extremely noisy in that regard. Trying to quiet them down many things were tried, including exhaust diffusers which you see on many old engines.

The latest engines (turbofans) use a large portion of the exhaust energy to drive much bigger front fan sections. These are like multi-bladed propellers, and model airplane people have been using something very similar for years with "ducted fan" models. The advantage of the turbofan engine is better performance at lower altitude and, because much of the thrust is derived from the lower-velocity air mass accelerated by the fan section, there is less shear to create noise. The colder, slower fan air also creates a secondary noise reduction by reducing the speed differential between itself and the core flow.

For real noise (and real fun!) you get to play with ramjets and afterburners, but they don't see much application in commercial aviation except, of course, for SSTs like the Concorde. If you ever watched a Concorde takeoff it was fantastic.

OK - probably more info than you wanted, but you did ask. The MD-80 series normally uses older style engines and you won't find them flying into the most noise restrictive airports. Inside the MD-80, they are quite a bit noisier in the forward pax section than most other commercial aircraft due to the air turbulence generated by the two small strakes you see on the lower portion of the fuselage just aft of the cockpit.

Jim

Ok - I see you there were plenty of good answers while I was typing. Note to self: take remedial typing course.

[Jim727]

Andrew -

T-38. What a love affair - I absolutely loved every minute I spent in that thing. Never drove an F-15, so I'm very, very, jealous.

Uh-oh. Must. Regain. Control....

[Jagshund]

ey lik airplane. it ez fun. Seriously, now, let's keep this to a third-grade level so I can keep up.

[Superman]

Originally Posted by tcar

I think he's asking, pretty round-a-bout, how to make his P-car quieter.

Maybe.

Possibly.

That's my story, and I'm sticking to it.

[wastintime]

Jim,
I was lucky enough to have a cool dad. For the most part I stuck to Bonanzas, but I actually got to fly the F-15 while I was in college, one of my dad's friends had made General and got command of a reserve fighter wing and until I went blind and got into physics had been really interested in trying to get into one of their pilot training slots. So my dad and I flew out and met everyone in the squadron and they took me for a ride and it was a blast.
The T-38 is definitely it's own world though, lol, I was unfortunately the victim of a small practical joke where I found out exactly how fast the roll rate was in it though.


As for absurd noise from Aircraft... my top two votes were the F-4 and the C-5... the C5 was just annoying with that hum and drove me nuts, but the F-4 was a thing of beauty as the earth shook as it went by.

[Jim727]

Roll rate: 720 degrees per second. Yep, that would get your attention if you didn't expect it.

.
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We now return control of the thread to Supe.

[safe]

Originally Posted by BlueSideUp

The only aircraft I know of that uses active noise canceling is the Bombardier Dash 8 Q400. Apparently it works well but is usually broken.

It's also used in the SAAB 2000, discontinued since 1999.

It works amazingly well! I wonder why no manufacturer has used the technology in cars?

[wastintime]

Honda, Lexus, Audi, Mercedes... those are the only ones I know off the top of my head, but many auto-manufacturers do use it. More so than aircraft manufacturers!

[Formerly Steve Wilkinson]

Honda, Lexus, Audi, Mercedes etc. use active noise-canceling technology? Is that true?

[wastintime]

Yes, lol, the first two jobs I got after moving to DC post divorce were selling Hondas and then Porsche/Audi so I am saying this with 100% certainty. Honda offers it on their new accord V6s excluding the 6-spd coupe (they assumed people wanted to hear the motor) Same with Audi, they didn't offer it on cars like the RS4, S4, etc... but it is getting very common in cars.

If you think about it, it makes sense. People expect aircraft to be noisy, but apparently the goal of every rich Mercedes driver is to be so unable to hear the world around them thus distancing themselves from reality... and the 18-wheeler honking at them

[tcar]

Originally Posted by wastintime

Honda, Lexus, Audi, Mercedes... those are the only ones I know off the top of my head, but many auto-manufacturers do use it. More so than aircraft manufacturers!

So, how do they do it? Through radio speakers????

Or do I not understand something?

Is it this system???

http://www.autoblog.com/2008/07/18/toyota-introduces-noise-canceling-tech-on-crown-hybrid/print/

[safe]

Cool! Do you know how it works in a car, if it uses the audio system or what?

I think it would be cool to use in a Porsche, light weight noise control! No more earplugs on long trips.

[tcar]

Look at my link, safe...