Modern jet engine t/o noise

[Trislander]

Hi there,

I was wondering what it is that makes that 'Owwwwwwww' kind of noise when jet engines on the 757, A320 series and 146 are run at high RPM for take-off? It can only be heard in front of the engine and at high engine RPM. It's very noticable on t/o on the a/c I work on (146) when I'm sat up front and the 757 when sat near to the front of the engine.

It can be heard in and outside the a/c.

Not all a/c jet engines make this noise such as the 767 or older 737's.

Any ideas?

Tri

[Aerofoil]

I am going to have a complete guess at this and it is probably completely incorrect...
Maybe it is to do with the engine rating, and that rating causes the engine to produce a sound at a certain wavelength which gives the engine that chopping whirring sound?

Maybe one of the engineers could confirm what it exactly is but thats my guess.

Regards

Dave

[747FOCAL]

In the Noise industry that is known as "Buzzsaw" noise and airlines do not like it cause you can hear it really bad in first class. :E

[Tinstaafl]

It's a low(er) frequency sound from the fan.

[Trislander]

Thanks guys

Tri :ok:

[PaperTiger]

The fan noise, often referred to as a “buzz saw” or whining noise, is caused by small shocks produced near the tips of the fan blades and the interaction of the wake of the fan blades with neighboring sets of guide vanes, called stators or stator blades. The relatively still air that enters the engine is suddenly coming into contact with a very fast moving fan, and at the tips of the fan blades the air becomes unstable and creates disturbances in the flow. These disturbances create a wake in the air that travels down the engine. The air then hits the stators, which straighten the flow out and allow it to hit any subsequent compressor stages at an optimal angle. When the wakes from the rotating blades hit the stators, they make a noise.
Accoustic lining in the intake is one way of alleviating the soundprint. For more discussion, see: http://www.its.caltech.edu/~sciwrite/journal03/williams.html (from where the above quote was taken)

[411A]

The first transport jet to make this particular noise was the B707-320B (advanced cowl), with JT3D engines.
As a brand new Captain on same, it sure sounded good. :)

[Mark 1]

PaperTiger,

'not sure of the validity of your quote, but 'buzz-saw' noise doesn't rely on blade-vane interaction (although wake interaction is an important source of fan noise).

Fan tips typically reach M1.4 locally at take-off and the shocks propagate non-linearly giving rise to a set of harmonics of the engine RPM (other fan tones only occur at harmonics of the blade passing frequency). Typically the spectrum peaks at about half the blade frequency - eg for 25 blades at 4000 rpm it would be around 800Hz.

It tends to propagate dominantly near to the plane of the fan.

Nacelle acoustic treatment does alleviate it considerably, though that is normally designed to treat the sub-sonic fan noise that typically dominates at approach power settings. Some older engines experimented with 'shock equalization' liners, that needed to be quite deep to be effective at the lower frequencies.

[PaperTiger]

Mark 1

I agree, I think the quote was badly worded so as to read (a misplaced and) that the 'buzz-saw' noise was a combination of factors. I think he meant that overall noise was the shock waves (buzz-saw only) plus the vane interaction (other noise).