Charlie757
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Jet buzzsaw noise

Tue Jul 02, 2019 2:53 am

When a jet aircraft takes off you get the famous ‘buzzsaw’ sound that carries on for most of the climb and just gradually fades away. But during cruise you can’t hear that sound even though the engines are still operating at let’s say 70-80%

So does this buzzsaw sound disappear in-flight because of external wind noise or because thrust on the aircraft is gradually reduced?

I fly a fair bit so any info would be greatly appreciated as I’ve been wondering for a while! :)
 
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AirKevin
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Re: Jet buzzsaw noise

Tue Jul 02, 2019 3:09 am

Captain Kevin
 
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Starlionblue
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Re: Jet buzzsaw noise

Tue Jul 02, 2019 7:11 am

The buzzsaw comes from the tips of the fan blades going supersonic. A large number of tiny sonic booms.

As you say it goes away because thrust is reduced and the blades slow down. Also with thinner air sound carries less well, I suppose.
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acjbbj
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Re: Jet buzzsaw noise

Tue Jul 02, 2019 2:30 pm

The low-pitched buzzsaw sound is actually from the rotation of the fan (frequency of the buzz is the rev/s angular velocity of the fan); the higher-pitched whine is from the fan blades.
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masi1157
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Re: Jet buzzsaw noise

Tue Jul 02, 2019 2:39 pm

Charlie757 wrote:
So does this buzzsaw sound disappear in-flight because of external wind noise or because thrust on the aircraft is gradually reduced?

There are a number of reasons why buzzsaw noise changes during the climb. The reason why it disappears at cruise is primarily the helical tip speed dropping to subsonic.


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akiss20
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Re: Jet buzzsaw noise

Wed Jul 03, 2019 2:12 pm

You are both right, the buzzsaw noise is due to shocks with a tip Mach number greater than unity but is also at shaft frequency and it’s harmonics up to blade passing frequency. See my post from a long time ago about this

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masi1157
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Re: Jet buzzsaw noise

Wed Jul 03, 2019 2:40 pm

It is not "also" at shaft frequency, it is only at shaft frequency and many many of its harmonics. You can also have a pronounced peak at the blade passing frequency, but that one doesn't disappear at cruise level. It might just get less pronounced.


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508 different segments on 100 airlines to 210 airports in 55 countries
 
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akiss20
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Re: Jet buzzsaw noise

Wed Jul 03, 2019 4:07 pm

masi1157 wrote:
It is not "also" at shaft frequency, it is only at shaft frequency and many many of its harmonics. You can also have a pronounced peak at the blade passing frequency, but that one doesn't disappear at cruise level. It might just get less pronounced.


Gruß, masi1157


That’s exactly what I said. “And its harmonics up to blade passing frequency”
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masi1157
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Re: Jet buzzsaw noise

Wed Jul 03, 2019 6:14 pm

akiss20 wrote:
That’s exactly what I said. “And its harmonics up to blade passing frequency”

You said "also". Only the shaft frequency and its harmonics are created by supersonic blade tip speed and are perceived as buzzsaw noise. And the harmonics of the shaft frequency reach far beyond the BPF.


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akiss20
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Re: Jet buzzsaw noise

Wed Jul 03, 2019 6:39 pm

masi1157 wrote:
akiss20 wrote:
That’s exactly what I said. “And its harmonics up to blade passing frequency”

You said "also". Only the shaft frequency and its harmonics are created by supersonic blade tip speed and are perceived as buzzsaw noise. And the harmonics of the shaft frequency reach far beyond the BPF.


Gruß, masi1157


That “also” was in reference to the two posters both being correct, not that the buzzsaw noise was something else and also shaft frequency and it’s harmonics. Perhaps I should have included the word “both” for clarity.

Obviously there are an infinite number of harmonics, but what is the mechanism of producing energy at higher than BPF from shocks? There probably is one, but either way the acoustic energy of harmonics above BPF is much lower than that at BPF or 1/rev or 2/rev. The 1000th engine order is not of much interest generally.
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masi1157
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Re: Jet buzzsaw noise

Wed Jul 03, 2019 8:15 pm

akiss20 wrote:
[but what is the mechanism of producing energy at higher than BPF from shocks? There probably is one, but either way the acoustic energy of harmonics above BPF is much lower than that at BPF or 1/rev or 2/rev. The 1000th engine order is not of much interest generally.

The mechanism is simply from the fact, that the signal is periodical with the shaft frequency, but far away from a pure sine. That gives you lots of higher harmonics. And "shock" implies very steep slopes in the time signal, which is equivalent to a large high frequency content in the spectrum.

The acoustic energy? I don't really know. But the annoying part of the spectrum is (as always) in the mid frequency range from a few hundred to a few thousand Hz.


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Farsight
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Re: Jet buzzsaw noise

Thu Jul 04, 2019 4:02 pm

As for the posters original question, why does the buzzsaw sound gradually reduce....I think its mostly due to masking by the wind noise, which becomes the predominate sound you hear at cruise due to increased speed.
 
Farsight
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Re: Jet buzzsaw noise

Thu Jul 04, 2019 4:08 pm

Its interesting to me that at cruise engine settings...you are in the correct RPM range to create the buzzsaw sound, but you can no longer hear it. Throughout the cruise the engines will still be going 'grrrrrrrr'.

You can notice it especially on a 757 during cruise, you can't really hear it anymore, but if you walk from one end of the aircraft to the other...you can perceive a low droning bass noise when going over the wing spars...sounds like a sine wave at about 60hz. I can only imagine this is the vibration from the engine buzzsaw sound being transferred up through the pylon to the wing/fuselage.
 
masi1157
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Re: Jet buzzsaw noise

Thu Jul 04, 2019 4:13 pm

Engine fan and N1 shaft imbalance, it has nothing to do with buzzsaw noise.


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Farsight
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Re: Jet buzzsaw noise

Thu Jul 04, 2019 7:27 pm

masi1157 wrote:
Engine fan and N1 shaft imbalance, it has nothing to do with buzzsaw noise.


Gruß, masi1157



?? What question was this the answer too?
 
masi1157
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Re: Jet buzzsaw noise

Thu Jul 04, 2019 7:40 pm

It was an answer to your post immediately before it. The imbalance of the fan and shaft also have (of course) the same fundamental frequency as buzzsaw noise would have, but the generating mechanism is completely different. Other than the frequency it has nothing to do with it.


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508 different segments on 100 airlines to 210 airports in 55 countries
 
Farsight
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Re: Jet buzzsaw noise

Thu Jul 04, 2019 8:56 pm

What is fan & N1 shaft imbalance?
 
masi1157
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Re: Jet buzzsaw noise

Fri Jul 05, 2019 3:46 am

Farsight wrote:
What is fan & N1 shaft imbalance?

What do you not understand here? The fan is sitting on the N1 shaft, so they both rotate at the same frequency. An imbalance of the fan or the shaft and the inhomogenuous flow condition across the fan inlet introduce a vibration at that same frequency, that is transmitted into the engine, nacelle, pylon, wing, fuselage, cabin lining, seats. Cabin floor and lining radiate noise at that frequency into the cabin. That is what you hear, and you feel the vibration of cabin floor ans seats. The generating mechanism is the imbalance and the induced vibration from it, but not the supersonic tip speed. It (obviously) also occurs at subsonic tip speed.


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508 different segments on 100 airlines to 210 airports in 55 countries

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