bsergonomics
Topic Author
Posts: 458
Joined: Thu Jan 31, 2002 5:07 am

### Question Re: Engine Exhaust Noise

I was reading through the crew chief notes for one of the aircraft that I work on and it had the noise (dB(A)) profiles at various distances from the aircraft at various engine settings.

The curious (to me) thing was that the highest noise spike came out at about 35-40 degrees (horizontally - these are ground measurements) from the aircraft centreline. I usually do the pointy end of the aircraft (cockpits, mission data etc) so the nearest I usually get to the engines is the throttle box. In my ignorance, I would have expected the loudest noise to be directly behind the engines.

I've asked around the office, but the ones who would know have been in management for too long to remember anything useful, and the intern (who would remember) is out at the moment.

It has been suggested that the loudest noise comes from the largest area of turbulence, which would be where there is the largest speed differece (and, therefore, pressure difference) between air pockets. This would mean that the highest pressure difference would be between the stationary air at the side of the exhaust nozzles and the exhaust gas itself.

So, is the sound all generated at this point and continues outwards (I would have expected more of a hemisphere effect); or does it travel with the moving air (which is being blown backwards); or does the noise continue to be generated along the line of the dissipating exhaust flow; or have I got it badly wrong and there is something else at work here?

The aircraft is a twin-engined 'multi-role' fighter built by a European consortium. Can you guess what it is?
The definition of a 'Pessimist': an Optimist with experience...

dairbus
Posts: 518
Joined: Mon Nov 17, 2003 10:45 am

### RE: Question Re: Engine Exhaust Noise

I do remember when I was in college learning that the highest noise level from a jet engine being at 35-45 degrees of the exhaust. Sorry but I do not remember the exact scientific explanation for this but your explanation seems to be accurate. Maybe someone else knows.
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DC-10Tech
Posts: 291
Joined: Fri Jun 15, 2001 6:40 pm

### RE: Question Re: Engine Exhaust Noise

You are correct. The difference in speed between the engine exhaust and the still air has a direct impact on the sound level. This is one reason that turbofan engines are generally quiter than turbo jets - their exhaust gas is traveling slower than the turbojets.

As for the angle from the tailpipes, if you look at jet blast danger areas, you'll see that the 35-45 degree angle coincides with the boundry of jet blast and still air.
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Fly2HMO
Posts: 7184
Joined: Sat Jan 24, 2004 12:14 pm

### RE: Question Re: Engine Exhaust Noise

An Eurofighter for sure

bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

### RE: Question Re: Engine Exhaust Noise

As for the angle from the tailpipes, if you look at jet blast danger areas, you'll see that the 35-45 degree angle coincides with the boundry of jet blast and still air.

Hi Dan, why does the exhaust air exit in such a cone shape rather than a straight gradually-dissipating tube of jetblast? I would have thought that it is a tubular jetblast and that the boundary is inexistent (very wide) because it would be a gradual speeding of the surrounding air, and not on a specific line. Could you please enlighten me on this? Thanks in advance

-Alfredo

A/c train
Posts: 674
Joined: Sun Jun 10, 2001 7:57 am

### RE: Question Re: Engine Exhaust Noise

Went out on engine runs the other day, idle outside hangar then full power runs up the bay, ill never stop being amazed at the aweseome power and noise those RB211's produce ! wings bouncing up and down and horizontal stab shaking very violently !! yea man.

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