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DocLightning
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The Buzzsaw Noise

Sat Apr 18, 2015 6:45 pm

I'm looking for a better explanation than the one I usually hear.

http://www.youtube.com/watch?v=QdP-VMcREyg

This is a video of a DL L-1011 with, of course, RB211 power, making the characteristic "Buzzsaw" noise that we all know and love.

The common explanation that I am given for the buzzsaw noise is that the fan blades move faster than Mach 1 at takeoff power and the shockwaves make the noise. But there are two problems with this explanation.

1) I did a little frequency matching and found out that the buzzsaw noise has a frequency of ~120Hz. But the RB-211 has 33 fan blades. I don't know the RB-211 N1 max RPM but the CF6 is around 4,000, so I'll assume that the RB-211 is well within an order of magnitude of same. So at 4,000 RPM N1 rotates at 66.6666...RPS. With 33 blades, the frequency of the shockwaves from those blades detected at a given point near the engine will be 2,200Hz. So where is the predominant ~120Hz noise coming from?

2) The blade tips would continue to move at supersonic speed at cruise since air approaches the inlet at ~M=0.8 and the blades are spinning quickly in cruise. Certainly the tips are moving faster than M=0.2. But the buzzsaw noise drops off in cruise.

So...what causes the buzzsaw?
-Doc Lightning-

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SPREE34
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 3:20 am

At cruise altitude the power reduction to "Cruise Power" may reduce the speed at the fan tips to below Mach. I don't know for sure. Also, the air density is lower at altitude, perhaps resulting in a noise reduction.

I don't know that there is a direct relation to the cruise mach number of the aircraft to the air speed at the fan tips. I hope a knowledgable person chimes in on that. I do know you can't feed a jet engine air at a velocity above mach one. You may recall, many fighter jets have doors and nacel devices to reduce the speed of intake air to below mach. The air going into the RB211s is certainly below mach, so agian, I wonder what the relation of the aircraft mach number to the fan ti speed is.
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Okie
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 3:41 am

http://www.southampton.ac.uk/enginee...oise_and_non_linear_acoustics.page

See if this helps Doc.

Okie

[Edited 2015-04-18 21:01:11]
 
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DocLightning
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 3:45 am

Quoting SPREE34 (Reply 1):
I don't know that there is a direct relation to the cruise mach number of the aircraft to the air speed at the fan tips.

No, but the air approaches the engine inlet at M=0.78-0.85 depending on aircraft type. A Fan the size of the RB-211 is spinning, say, at 50% N!, so ~2000RPM. That is 33.3333 RPS.

The RB-211 is 84.4 inches across. Multiplying that by pi, I get a circumference of about 266 inches. I then multiplied that by the 33.3 times a fan blade travels around it in a second and then converted that to MPH. The answer is that at 50%N1, a fan blade tip moves at ~500 MPH. Now, I made a lot of assumptions and did a lot of rounding, but I'm probably within 20-30% of the actual number. But if the incoming air is at 500 MPH and the fan blade is moving orthoganally at 500 MPH then the total speed is 500mph times the square root of 2 or ~707 MPH, which is faster than Mach 1 at 35,000 feet.

And given the fact that fan blades are airfoils, even if the actual absolute speed is under Mach 1, they accelerate airflow passing over their suction sides and so shockwaves will form there.
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DocLightning
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 4:01 am

Quoting Okie (Reply 2):
http://www.southampton.ac.uk/enginee...oise_and_non_linear_acoustics.page

See if this helps Doc.

A bit. All I gather is that somehow in the propagation of soundwaves down the inlet they somehow interact and energy is transferred from higher frequencies to lower ones. It did not escape my attention that by my calculation, the dominant frequency I heard in that video is about twice the rotation frequency.
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"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Okie
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 4:18 am

Quoting DocLightning (Reply 4):
It did not escape my attention that by my calculation, the dominant frequency I heard in that video is about twice the rotation frequency

Recheck that article and look at the shock wave diagram.
There appears to be 2 shock waves per blade which will fit into your analogy.
One sound wave being the primary. The second is the combination of the low frequency of the preceding and the high frequency of the following.


Okie

[Edited 2015-04-18 21:34:13]
 
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DocLightning
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 7:31 am

Quoting Okie (Reply 5):
Recheck that article and look at the shock wave diagram.
There appears to be 2 shock waves per blade which will fit into your analogy.
One sound wave being the primary. The second is the combination of the low frequency of the preceding and the high frequency of the following.

Yes, but the 66.6 is rotations per second. If the shockwaves themselves are the source of the noise, then it's very high-pitched...and indeed this is the sound we first hear when a fan spins up until it goes supersonic at the tips. Then the buzz starts. That buzz is twice the frequency I would get if it was just accounted for by regular variations in the overlying harmonic. 66Hz is barely audible unless it's really loud, in which case you feel it more than you hear it. 120Hz is a low hum that is easily heard.
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"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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N243NW
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 5:07 pm

Quoting DocLightning (Reply 6):
That buzz is twice the frequency I would get if it was just accounted for by regular variations in the overlying harmonic. 66Hz is barely audible unless it's really loud, in which case you feel it more than you hear it. 120Hz is a low hum that is easily heard.

Are you certain? 66Hz is very audible, especially when it's produced as more of a square wave instead of a sine wave, as is common with most buzzsaw noise. I've found that most engine buzzsaw noise is the same speed as the N1 engine RPM. Multiply this by the number of fan blades, and you'll get the higher-pitched scream or whine that's more audible at all engine speeds.
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Horstroad
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 5:29 pm

Quoting DocLightning (Reply 3):
No, but the air approaches the engine inlet at M=0.78-0.85 depending on aircraft type.

The engine inlet is designed as a diffuser. This decreases the velocity and increases the pressure of the approaching air.
 
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DocLightning
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 7:05 pm

Quoting N243NW (Reply 7):
Are you certain? 66Hz is very audible

No, really, it's not unless its very loud.

http://www.youtube.com/watch?v=E_Sf7rSOU78
-Doc Lightning-

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Horstroad
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 7:19 pm

Quoting DocLightning (Reply 9):
No, really, it's not unless its very loud.

pretty audible to me. And jet engines are very loud...
 
N243NW
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 7:25 pm

Quoting DocLightning (Reply 9):
No, really, it's not unless its very loud.

That's because you're playing a sine wave. The buzzsaw noise is nowhere near a sine wave - it's closer to a square wave, as you can hear if you input 60Hz here and choose the "Square wave" option.

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N243NW
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 7:28 pm

My last post got all messed up.

I meant to include a link to onlinetonegenerator.com, but the buggy forum HTML code won't allow it for some reason.
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DocLightning
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RE: The Buzzsaw Noise

Sun Apr 19, 2015 7:43 pm

Quoting N243NW (Reply 11):
That's because you're playing a sine wave. The buzzsaw noise is nowhere near a sine wave - it's closer to a square wave, as you can hear if you input 60Hz here and choose the "Square wave" option.

Well, that's interesting. I wonder why that is.

Anyway, when I play those test tones, I get concordance at both 60 and 120 Hz. It's best with a sawtooth pattern, which is what you would expect if these noises were made by the differences between fan blades repeating at 60Hz.
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SPREE34
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RE: The Buzzsaw Noise

Mon Apr 20, 2015 12:57 am

Quoting Okie (Reply 2):
See if this helps Doc.

Okie

Thanks, Okie. Good info.

Good info and comments by all so far.
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DocLightning
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RE: The Buzzsaw Noise

Mon Apr 20, 2015 3:48 am

Quoting horstroad (Reply 8):
This decreases the velocity and increases the pressure of the approaching air.

Air entering the inlet at takeoff power undergoes a pressure drop because the fan is lowering the pressure in front of it. That accelerates air into the inlet, which is why you see condensation in there on humid days. The acceleration into the inlet involves a pressure decrease as the air accelerates down the pressure gradient.

At cruise speeds, I don't know if the opposite is true, but it would strike me as odd if the overall pressure on the front of the fan were above ambient. You want the pressure on the front of the fan to be low and the pressure on the back of the fan to be high since that's...yanno...the definition of thrust and all that.   So my guess is that the air entering the inlet is accelerating down a pressure gradient into the fan.

But this may explain why the buzzsaw is not heart at cruise and gets quieter as speed increases. On the ground there is a big pressure drop from, say, ten meters in front of the inlet to just in front of the fan. With the incoming air accelerating so much (and thus dropping pressure so much), the sound waves somehow have to transfer their energy into lower frequencies as they travel up the pressure gradient. At cruise, the air doesn't accelerate as much into the inlet (the true efficiency of a low-specific impulse, high-bypass ratio turbofan) and so there is less distortion of the sounds.
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Jetlagged
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RE: The Buzzsaw Noise

Mon Apr 20, 2015 12:49 pm

Quoting DocLightning (Reply 15):
You want the pressure on the front of the fan to be low and the pressure on the back of the fan to be high since that's...yanno...the definition of thrust and all that.

Whatever the inlet pressure is the fan will cause it to increase, and the core will increase it even more. So you will get thrust.
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akiss20
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RE: The Buzzsaw Noise

Mon Apr 20, 2015 3:25 pm

Quoting DocLightning (Reply 15):
At cruise speeds, I don't know if the opposite is true, but it would strike me as odd if the overall pressure on the front of the fan were above ambient. You want the pressure on the front of the fan to be low and the pressure on the back of the fan to be high since that's...yanno...the definition of thrust and all that. So my guess is that the air entering the inlet is accelerating down a pressure gradient into the fan.

That is incorrect. At cruise the fan face axial mach number is below that of free-stream (typically on the order of 0.5, whereas free-stream Mach is of the order of 0.8-0.85). To first order, the inlet is isentropic (inlet recovery is typically on the order of 0.985-0.99), so the static pressure at the fan face at cruise is above the ambient static pressure.

At takeoff the inlet does the exact opposite, increasing the fan face mach number from the ambient (essentially 0) to approximately 0.5-0.6. The fan wants a constant inlet mach number, so the inlets job is to provide that in a variety of conditions from takeoff, cruise, max Cl, and crosswind.
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jambrain
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RE: The Buzzsaw Noise

Mon Apr 20, 2015 5:47 pm

The buzzsaw (my favourite sound in the world) is down to the variation of intensity of the fan shock waves leading to sub harmonics of the rotor passing tone, see page 64 of

Aircraft Noise (Cambridge Aerospace Series) by Michael J. T. Smith (Author)

you can read online at :-

http://books.google.co.uk/books?id=J...tcover#v=onepage&q&f=false
Jambrain
 
PhilBy
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RE: The Buzzsaw Noise

Mon Apr 20, 2015 6:44 pm

Beats?

In acoustics, a beat is an interference between two sounds of slightly different frequencies, perceived as periodic variations in volume whose rate is the difference between the two frequencies.
 
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DocLightning
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RE: The Buzzsaw Noise

Tue Apr 21, 2015 3:39 am

Quoting PhilBy (Reply 19):

I thought about beats, but what two higher frequencies could be so close as to account for that?
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Aaron747
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RE: The Buzzsaw Noise

Tue Apr 21, 2015 6:11 am

Fascinating questions Doc.

This prompts me to re-ask another age old question:

https://www.youtube.com/watch?v=MDwn6mJg2yU

What in the world causes the 'woo woo woo' in the PW4070-4090 series???
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musang
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RE: The Buzzsaw Noise

Tue Apr 21, 2015 11:55 am

I'm still trying to get my head around 66.6 RPS for an RB-211 fan……….

Regards - musang
 
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akiss20
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RE: The Buzzsaw Noise

Tue Apr 21, 2015 12:42 pm

Quoting musang (Reply 22):
I'm still trying to get my head around 66.6 RPS for an RB-211 fan……….

Why? 4000RPM (thus 66.6 Hz shaft frequency) for an older, higher FPR fan is reasonable.

Doc you are over-simplifying the problem a bit. While yes in the ideal world with every blade being identical and the 3D shock structure for each blade being identical, you wouldn't expect much spectral energy below BPF (and this is more true for subsonic turbomachinery), that is not how it is in reality. The shock structure on the blades is incredibly complex and varies somewhat from blade to blade due to minute differences in manufacturing as well as non-uniformities in the inlet condition. As a result you have shocks of varying different strengths around the annulus. These shocks can merge and and create 1/rev, 2/rev, 3/rev etc non-uniformities (see the second image, which is obviously exaggerated for clarity). You get a superposition of non-uniformities at frequences from 1/rev up to BPF. This superposition is largely responsible for the buzz-saw noise. Also the buzzsaw noise cannot be said to be at a singular frequency as it is not a pure sinusoid. The spectral content of the buzzsaw noise can be found in the image below.

http://i.imgur.com/Ou249BD.jpg
http://i.imgur.com/nH92YK6.png

[Edited 2015-04-21 06:21:10]

[Edited 2015-04-21 06:22:37]
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DocLightning
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RE: The Buzzsaw Noise

Tue Apr 21, 2015 4:57 pm

Quoting akiss20 (Reply 23):

Thank you. That's really fascinating. I think I'd have to read another textbook to really understand it, but this makes more sense.
-Doc Lightning-

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akiss20
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RE: The Buzzsaw Noise

Tue Apr 21, 2015 5:18 pm

Quoting DocLightning (Reply 24):
Thank you. That's really fascinating. I think I'd have to read another textbook to really understand it, but this makes more sense.

No problem.

For a general intro to gas turbines I can recommend "Aircraft Engines and Gas Turbines" by Jack Kerrebrock. A more advanced text on compressors can be found in "Compressor Aerodynamics" by N.A. Cumpsty.
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kurtverbose
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RE: The Buzzsaw Noise

Tue Apr 21, 2015 6:21 pm

Thank you all for a really informative thread. I've very much enjoyed following it.
 
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RE: The Buzzsaw Noise

Wed Apr 22, 2015 5:02 am

The buzzsaw goes away as the airplane goes through the air faster and faster, too. N1 increases with altitude (cruise RPM is typically higher than takeoff RPM, particularly with a reduced thrust takeoff).

I always figured it had to do with the AOA of the fan blade.
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