hitower3
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777W in cruise - noisy on ground?

Tue Aug 27, 2019 8:37 am

Dear all,

I have the privilege to live in a quiet place in the suburbs of Vienna, Austria.
It happens to me that I am using my telescope in the nearby vineyards at night and I can see - and hear - aircraft flying above my head at cruise altitude. Sometimes, one aircraft is significantly louder than any other plane overflying me and when I look it up on FR24, it is very often a 777-300ER.

While it is pretty understandable to me that airplanes with smaller engines (like the usual bunch of A320 and B737) make less noise, I notice that also aircraft with a similar amount of installed power (B747, A340-600, A350...) appear to be noticeably less noisy than the 777W.
Interestingly, the GE90 engine is renowned to be particularly NIMBY-friendly, a fact that I can confirm when I listen to the Austrian 777-200ER compared to their 767-300ER (PW4000 powered) when they take off from Vienna: the latter being a lot louder in spite of being less powerful.

So this brings me to ask two questions:
- Does anyone notice the same phenomenon when a 777W flies over at cruise altitude?
- What could be an explanation for this "quiet at takeoff, but noisy at altitude" behaviour?

Many thanks & best regards,
Hendric
 
flipdewaf
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Re: 777W in cruise - noisy on ground?

Tue Aug 27, 2019 8:59 am

The 77W has a high wing loading and as a result is often not able to achieve high altitudes at similar flight stages as other aircraft. It may well be louder because its lower and therefore closer to you.

Fred
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FlyHossD
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Re: 777W in cruise - noisy on ground?

Wed Aug 28, 2019 9:46 pm

flipdewaf wrote:
The 77W has a high wing loading and as a result is often not able to achieve high altitudes at similar flight stages as other aircraft. It may well be louder because its lower and therefore closer to you.

Fred


That's my thought, too. At heavy loads, the 77W will be at a lower - closer - cruise altitude.

Fred gets the points for the first correct answer.
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masi1157
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Re: 777W in cruise - noisy on ground?

Fri Aug 30, 2019 9:12 am

flipdewaf wrote:
It may well be louder because its lower and therefore closer to you.

Flying at FL330 instead of say FL370 would make a 1dB difference. That is the smallest noise difference to distinguish in a direct comparison. But hearing them several minutes/hours/days apart you would not be able to notice any difference at all. The flight altitude surely is not the major reason.


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508 different segments on 100 airlines to 211 airports in 55 countries
 
flipdewaf
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Re: 777W in cruise - noisy on ground?

Fri Aug 30, 2019 9:31 am

masi1157 wrote:
flipdewaf wrote:
It may well be louder because its lower and therefore closer to you.

Flying at FL330 instead of say FL370 would make a 1dB difference. That is the smallest noise difference to distinguish in a direct comparison. But hearing them several minutes/hours/days apart you would not be able to notice any difference at all. The flight altitude surely is not the major reason.


Gruß, masi1157

I guess its an inverse square law thing (been 11 years since I did the acoustics at uni) so (37/33)^(1/2) = 1.058. Not sure how that equates to dB but I'm sure you are right.

Fred
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zeke
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Re: 777W in cruise - noisy on ground?

Fri Aug 30, 2019 12:27 pm

masi1157 wrote:
flipdewaf wrote:
It may well be louder because its lower and therefore closer to you.

Flying at FL330 instead of say FL370 would make a 1dB difference. That is the smallest noise difference to distinguish in a direct comparison. But hearing them several minutes/hours/days apart you would not be able to notice any difference at all. The flight altitude surely is not the major reason.


Gruß, masi1157


The 77W is normally starting off lower than that in a long haul. I find the cruise engine noise much louder in the cabin against other wide bodies I have been in.

Also much louder during engine start than other aircraft.
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estorilm
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Re: 777W in cruise - noisy on ground?

Fri Aug 30, 2019 12:49 pm

Interesting thread - most has been covered but I will say, the strange "howl" they make as the pilots advance the throttles to stabilize before TO power is set is VERY distinctive on the 777 (not sure if it's the W or 2, but very distinctive!)

I've worked on the north end of the 3 parallel runways at IAD for 15 years, there's no mistaking when a 777 has taxi'd into position and is about to roll haha (even if it's over a mile away!)
 
hitower3
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Re: 777W in cruise - noisy on ground?

Fri Aug 30, 2019 2:36 pm

Thank you for all the answers so far, greatly appreciated!
Wor what it's worth, the distinctively loud noise that I hear on the 777s passing over my head is a frequency-less, deep white noise, so unrelated to the spool-up howl described above. It's really just a deep rumble.
 
masi1157
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Re: 777W in cruise - noisy on ground?

Fri Aug 30, 2019 3:14 pm

flipdewaf wrote:
I guess its an inverse square law thing (been 11 years since I did the acoustics at uni) so (37/33)^(1/2) = 1.058. Not sure how that equates to dB but I'm sure you are right.

It is not "^(1/2)" but simply "^2". And the sound pressure level difference is 10*log((37/33)^2) or 20*log(37/33), which is the same.

zeke wrote:
The 77W is normally starting off lower than that in a long haul.


May be. Let it be FL310 on the B777, while others would fly FL330 or FL350 initial cruise altitude. It wouldn't explain more than 1-2dB difference, which is inaudible if not in direct A-B comparison.

zeke wrote:
I find the cruise engine noise much louder in the cabin against other wide bodies I have been in.


There you would 2 possible explanations, or a combination of both: The engines (incl. the jet noise generated behind the engine) might be noisier and the noise transmission into the cabin might be higher.

zeke wrote:
Also much louder during engine start than other aircraft.


That can't be "extrapolated". During engine start the noise sources and noise generation will be very different than in cruise.

estorilm wrote:
Interesting thread - most has been covered but I will say, the strange "howl" they make as the pilots advance the throttles to stabilize before TO power is set is VERY distinctive on the 777 (not sure if it's the W or 2, but very distinctive!)


We are talking about noise in cruise heard from the ground. As I said, that has other dominant noise sources and generating mechanisms.

hitower3 wrote:
Thank you for all the answers so far, greatly appreciated!
Wor what it's worth, the distinctively loud noise that I hear on the 777s passing over my head is a frequency-less, deep white noise, so unrelated to the spool-up howl described above. It's really just a deep rumble.


The domimant noise contribution heard on the ground will be the broadband low frequency stochastic jet noise.


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zeke
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Re: 777W in cruise - noisy on ground?

Fri Aug 30, 2019 10:21 pm

masi1157 wrote:
May be. Let it be FL310 on the B777, while others would fly FL330 or FL350 initial cruise altitude. It wouldn't explain more than 1-2dB difference, which is inaudible if not in direct A-B comparison.


If you are talking about a 77W at FL310 the A330/A350/A380 would be at FL370 at the same corresponding fraction of actual weight/MTOW.

At MTOW a 77W will only get to FL290 normally, the A350 straight to FL350.

masi1157 wrote:
That can't be "extrapolated". During engine start the noise sources and noise generation will be very different than in cruise.


I can tell the sound of a 77W starting if I’m behind glass inside a terminal or even in the cockpit of an aircraft facing the other direction.
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masi1157
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Re: 777W in cruise - noisy on ground?

Sat Aug 31, 2019 8:05 am

zeke wrote:
If you are talking about a 77W at FL310 the A330/A350/A380 would be at FL370 at the same corresponding fraction of actual weight/MTOW.

At MTOW a 77W will only get to FL290 normally, the A350 straight to FL350.


That would give 1.2dB and 1.6dB difference. As I said, still below 2dB and inaudible if not in direct A-B comparison.

zeke wrote:
I can tell the sound of a 77W starting if I’m behind glass inside a terminal or even in the cockpit of an aircraft facing the other direction.


Maybe, but that doesn't matter in this context. We are talking about the noise emitted in cruise heard from the ground below, not about the noise emitted by starting engines. After travelling 10km down to the ground all the higher frequency content will be gone (by damping in air) and only the low frequency part can be heard, mostly mixing noise of the jet.


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zeke
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Re: 777W in cruise - noisy on ground?

Sat Aug 31, 2019 8:20 am

masi1157 wrote:
zeke wrote:
If you are talking about a 77W at FL310 the A330/A350/A380 would be at FL370 at the same corresponding fraction of actual weight/MTOW.

At MTOW a 77W will only get to FL290 normally, the A350 straight to FL350.


That would give 1.2dB and 1.6dB difference. As I said, still below 2dB and inaudible if not in direct A-B comparison.

zeke wrote:
I can tell the sound of a 77W starting if I’m behind glass inside a terminal or even in the cockpit of an aircraft facing the other direction.


Maybe, but that doesn't matter in this context. We are talking about the noise emitted in cruise heard from the ground below, not about the noise emitted by starting engines. After travelling 10km down to the ground all the higher frequency content will be gone (by damping in air) and only the low frequency part can be heard, mostly mixing noise of the jet.


Gruß, masi1157


I can tell the difference between lots of different aircraft engines as they fly overhead. It is not just the absolute noise levels as they pass overhead, it also depends on the sound approaching and departing.
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SQ22
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Re: 777W in cruise - noisy on ground?

Sat Aug 31, 2019 10:22 am

zeke wrote:
The 77W is normally starting off lower than that in a long haul. I find the cruise engine noise much louder in the cabin against other wide bodies I have been in.

Also much louder during engine start than other aircraft.


I agree with you, compared to 772 (PW4077), 77E (RR Trent 892) and 773 (RR Trent 892) I have flown with the 77W is more noisy in the cabin.
 
Farsight
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Re: 777W in cruise - noisy on ground?

Sat Aug 31, 2019 5:00 pm

Perhaps due to the 77W higher wing loading there is disproportionally more aerodynamic noise generated in cruise that one would hear from the ground....possible?
 
masi1157
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Re: 777W in cruise - noisy on ground?

Sat Aug 31, 2019 6:12 pm

SQ22 wrote:
I agree with you, compared to 772 (PW4077), 77E (RR Trent 892) and 773 (RR Trent 892) I have flown with the 77W is more noisy in the cabin.


Being noisier in the cabin does not necessarily mean it is also noisier when heard from the ground. It may be a hint, but it surely is not related 1 to 1.

Farsight wrote:
Perhaps due to the 77W higher wing loading there is disproportionally more aerodynamic noise generated in cruise that one would hear from the ground....possible?


That is surely possible. But I have no idea if it would dominate the noise from the jet mixing.


Gruß, masi1157
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BravoOne
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Re: 777W in cruise - noisy on ground?

Sat Aug 31, 2019 6:43 pm

I have always thought the RR powered -200ER was really noisy in the cabin, even forward of the L2 door. Sounds like a buzz saw and quite unpleasent at times.
 
masi1157
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Re: 777W in cruise - noisy on ground?

Sat Aug 31, 2019 6:49 pm

BravoOne wrote:
I have always thought the RR powered -200ER was really noisy in the cabin, even forward of the L2 door. Sounds like a buzz saw and quite unpleasent at times.


That is called buzzsaw noise (really!) and I am perfectly sure it occurs during climb (on all aircraft, some noisier, some less) but not during cruise. And it will absolutely not be heard from the ground when the aircraft is at cruise level above you (which is the topic here, it is not cabin noise!).


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Strato2
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Re: 777W in cruise - noisy on ground?

Sun Sep 01, 2019 7:16 am

zeke wrote:
Also much louder during engine start than other aircraft.


Indeed. I still have occasional tinnitus from the startup of AF 777-300ER on a flight back in 2011 sitting near the wing.
 
tomcat
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Re: 777W in cruise - noisy on ground?

Sun Sep 01, 2019 9:18 am

masi1157 wrote:

Farsight wrote:
Perhaps due to the 77W higher wing loading there is disproportionally more aerodynamic noise generated in cruise that one would hear from the ground....possible?


That is surely possible. But I have no idea if it would dominate the noise from the jet mixing.


Gruß, masi1157


The GE90-115 being the most powerfull engine by a significant margin, could it be that due to its intrinsically high air flow, the noise resulting from the jet mixing is simply higher than for any other engine? Also, has the exhaust speed any effect on the low frequency noises?
 
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zeke
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Re: 777W in cruise - noisy on ground?

Sun Sep 01, 2019 11:36 am

Strato2 wrote:
Indeed. I still have occasional tinnitus from the startup of AF 777-300ER on a flight back in 2011 sitting near the wing.


My tinnitus got significantly worse after I got married, hardly her anything she says apparently.
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masi1157
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Re: 777W in cruise - noisy on ground?

Sun Sep 01, 2019 7:52 pm

tomcat wrote:
The GE90-115 being the most powerfull engine by a significant margin, could it be that due to its intrinsically high air flow, the noise resulting from the jet mixing is simply higher than for any other engine? Also, has the exhaust speed any effect on the low frequency noises?


Of course a more powerful engine will -ceteris paribus- generate more noise. I am just not sure if "ceteris paribus" is a useful starting point here. I am also not sure if a B777 really is noisier than other A/C flying overhead at cruise altitude. I could a number of arguments for and another number against it.


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DarkSnowyNight
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Re: 777W in cruise - noisy on ground?

Wed Sep 04, 2019 8:49 am

zeke wrote:

I can tell the sound of a 77W starting if I’m behind glass inside a terminal or even in the cockpit of an aircraft facing the other direction.



I live more than four miles south of LAX. In the quiet hours of the late night/early morning hours, I can clearly hear sound of our nightly sortie of 77Ws and Fs starting up.

It's not intrusive or anything, but if you know what it is, it's unmistakable.
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mxaxai
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Re: 777W in cruise - noisy on ground?

Sun Sep 08, 2019 8:24 am

I know what you mean. I sometimes hear faint jet noise, look up, and find the aircraft at cruise altitude. It's almost always a 77W or A343. The volume seems to be weather-dependent; it's most noticeable on clear winter days.

In Vienna, I would suspect that the 77W is from nearby Munich airport.

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