Buzzing engines
 

I'm not a jet engine designer or mechanic so forgive this question if it is naive.

What causes the loud buzzing noise that one hears from some jet engines, particularly at take-off and high throttle settings? It seems to occur especially in recent-model, large, high-bypass turbofans, like, say, those of a 767.

I'm guessing that it has something to do with the front fan and blade speeds getting close to the speed of sound, but I don't know if that's on the right track. In any case, I'm curious about the specifics.

You got that right, it's the fan blade tips going transsonic.

Due to the high RPM the sound merges and creates that buzz

Yup it's the fan without Inlet guide vanes to support a bearing in front of the rotor. The vanes create a siren effect, while the open face blades with vastly different twist angles create a buzz.

The story, as told by McDouglas: Boeing, Daco and Lockheed were all faced with this buzz saw noise in the cabin at takeoff for the first time with their new widebodies. Lockheed L-1011 engineers were super smart, so they drilled, trimmed, etc., to quiet the noise at the source.

Boeing and McDouglas weren't so smart; they just added a ton of sheet lead noise dampening to the sidewalls of the fuselages of the 747s and DC-10s.

Result: They were all comfortable noise-wise on takeoff. When they got to cruise, the 747 and DC-10 were quiet inside, while the L-1011 was noisy.

GB

You shoulda heard the buzzing noise we got when that ibis went throught the right engine........:ok:

I don't understand. It's the presence of inlet vanes that has an effect, or their absence, or what?

How come the L-1011 was noisier in cruise, despite the engineering changes?

Come to think of it, I don't really know what causes the classic high-pitched shriek of a jet engine. Is it just from the high-speed flow of air through the engine, or is it a siren effect from blades on compressors or turbines, or what? The shrieking sound always seems to be quite smooth compared to the buzz, although it has a certain edge to it that seems more audible at low engine speeds.

As for the white-noise roar that one hears especially on turbojets, I've always assumed that it simply comes from the high-speed exhaust (?), particularly since it seems to be really obvious on pure turbojets and aircraft in afterburner.

Hi AnthonyGA,

If you have a look at :
Arrangement for minimizing buzz saw noise in bladed rotors - Patent 4732532
it mentions several of the reasons for the buzz saw noise.

"This noise arises from the production of unequal shockwaves during the rotation at sonic or supersonic velocities of a bladed rotor, the dimensional characteristics of whose blades are subject to manufacturing tolerances."

The L1011 air conditioning was the noisiest I've come across. I bet most of the cabin noise came from there - it was just as loud with idle thrust.

(Thanks for the warning about 411A - I meant to explain that the engine noise on the L1011 is so quiet, that the only perceptible noise in the cabin was from the conditioning - louder in descent with idle thrust when the HP air was used as the bleed source.)

Someone being critical of the Tristar?




Standby for 411A!

Where are you 411A?

Noise at cruise, especially ahead of the wing, is primarily air noise. Before the first big fuel price jump in the 1970s, the 727 also had sheet lead in the sidewalls, so I heard.

GB

Cool! It does indeed go into considerable detail that explains quite well whence comes the buzzing noise. Thanks!

Come to think of it, I don't really know what causes the classic high-pitched shriek of a jet engine. Is it just from the high-speed flow of air through the engine, or is it a siren effect from blades on compressors or turbines, or what? The shrieking sound always seems to be quite smooth compared to the buzz, although it has a certain edge to it that seems more audible at low engine speeds.

As for the white-noise roar that one hears especially on turbojets, I've always assumed that it simply comes from the high-speed exhaust (?), particularly since it seems to be really obvious on pure turbojets and aircraft in afterburner.

Bet you won't find an easy answer to that one: it was begining to become a topic of research back in the 70's. Would imagine that there are now models of the 'source' generators but can't imagine it's simple. Interesting about the sheet lead on the 747 etc: implies it was the resonating fuselage which amplified the effect rather than the driving source being the direct problem itself.

like Mr Optimist has stated, the excitation of natural frequencies in the cabin and wing can create such buzzing sounds. I recall some issues with an airline, I think Air France, who had complaints over buzzing sounds on the A330 ( I think), it turned out that the frequency of the EDP at high RPM (e.g. during take-off) excited resonant frequencies which lead to 'buzzing' sounds in the cabin. I think this was rectified by increased number clamps and potentially an attenuator.

No, it was probably the Bee model.

What is it about lead that makes it good for soundproofing, anyway? Is it inertia, or the fact that lead is relatively soft, or what?

At a science museum once I went into a soundproof chamber that appeared to use large pyramids of some soft material to deaden sound. It took up a lot of space, though, so I suppose that wouldn't be any more practical for an aircraft than the weight of lead sheets would be.

AnthonyGA: the pyramid-shaped form/foam was to eliminate sound reflections. On it's own, that material doesn't deaden sound much at all.

Behind it would have been a heavier and pliable layer that would absorb the sound energy rather re-transmit it (which is what a rigid layer - something like 'rigidized' aluminum sheeting - would).

What is it about lead that makes it good for soundproofing

Stretching the memory cells to a previous life so forgive me if I err in the detail.

Lead usually is in the form of lead wool. Weighs a bit but does a fine job in dampening a reasonable frequency spectrum of noise.

Higher frequency noise attentuates well in light material such as fibreglass batting, lower frequency needs some mass to vibrate.

soundproof chamber

anechoic chambers use multifaceted foam wall sections to disperse and attenuate incident sound energy. Try not to get caught in one over the weekend, though ... the sensory underload is quite stressful as the odd victim has observed.

Supersonic blades?
 

So, pardon my ignorance but, fan, compressor and turbine blades reach supersonic speeds commonly?

I thought that was undesirable.

It generally is, but since the speed is greatest at the tip of the blade therefore any part of a blade on the same rotor that is smaller than the fan tip diameter will not be supersonic. A little bit of trading going on for a short period of time just to keep the turbine happy, then along comes geared fans and a win win :)

What is it about lead that makes it good for soundproofing, anyway? Is it inertia, or
 

The Last Word - 25 May 1996 - New Scientist - might help.

Think it is partly the high density which means that a lot of energy is needed to set up vibrations (the accelerated mass is high and from what I recall the energy associated with a wave goes up as frequency squared or something like that) - so an external noise source doesn't get the stuff vibrating in sympathy, plus lead sheets plastic response (if you hit a steel bar with a hammer it rings and hurts, hit lead sheet and you get a dimple as the metal flows out of the way) which I guess causes high energy loss for vibrations so sound is turned into heat.