Aircraft noise attenuation calculations

Discussions about serious topics, for serious people
User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Aircraft noise attenuation calculations

Post by Bird on a Fire » Mon Jun 22, 2020 9:37 pm

Hello hivemind!

I'm currently trying to check/modify some calculations of aircraft noise from an Environmental Impact Assessment. The document presents a figure showing the area where the sound is ≥65 dB, which they claim is the threshold above which disturbance of birds should be considered - however, the paper they cite to support this actually shows disturbance occurring from 55 dB. As their 65 dB area only very narrowly avoids a key nature reserve, I would like to work out how much greater the area would be with lower levels of disturbance.

The EIA used a bit of software called AEDT from the US FAA, but (a) their website seems to be down, (b) I'm assuming it's paid for and (c) it looks very complicated. Can I get reasonable estimates just by doing some maths? If somebody could point me in the right direction and/or help me sense-check, that would be awesome. It's for a good cause*.

Thanks very much for the help.

*Here's a bit more about the case if you're interested.


Stuff I tried already:

There's an R package called seewave that uses the equation dB = lref - (20 * log10(d/dref)) where dref is a 'reference distance' at which the source noise was measured - and I cannot find what a plausible value of that would be.

Similarly I've tried the opeNoise plugin for QGIS but it seems to have a bug and won't actually accept input layers (including the example datasets it comes with).
Born at 356.32 ppm CO2 #ShowYourStripes

bmforre
Fuzzable
Posts: 303
Joined: Mon Nov 11, 2019 1:15 pm
Location: Trondheim

Re: Aircraft noise attenuation calculations

Post by bmforre » Mon Jun 22, 2020 9:52 pm

Is it clear which decibel weighting is used for this and whether the weighting is correct for birds involved?

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Mon Jun 22, 2020 10:04 pm

bmforre wrote:
Mon Jun 22, 2020 9:52 pm
Is it clear which decibel weighting is used for this and whether the weighting is correct for birds involved?
I'm afraid this is so out of my area that I'm not 100% sure I even understand the question ;)

The environmental impact assessment itself doesn't give those details, it just cites the AEDT software as a "black box". Tomorrow I can dive into the technical detail of that software if it helps - my ideal scenario would be to recreate their analysis exactly but explore a wider range of noise scenarios
Born at 356.32 ppm CO2 #ShowYourStripes

bmforre
Fuzzable
Posts: 303
Joined: Mon Nov 11, 2019 1:15 pm
Location: Trondheim

Re: Aircraft noise attenuation calculations

Post by bmforre » Mon Jun 22, 2020 10:17 pm

Noise effects on wildlife
Fortunately, according to this source
The threshold for hearing in birds is higher than for humans at all frequencies
In other words: Humans are more sensitive to sound than birds are.

That publication is from the Federal Highway Administration and has a good deal of information.

User avatar
Martin Y
Dorkwood
Posts: 1246
Joined: Mon Nov 11, 2019 1:08 pm

Re: Aircraft noise attenuation calculations

Post by Martin Y » Mon Jun 22, 2020 10:21 pm

Not my area at all but environmental noise assessment is exactly the field my student son is working in on placement at the moment. I'll put your question to him and see if he has any insights that might help.

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Mon Jun 22, 2020 10:32 pm

Thank you!

The figure for bird responses comes from a study of blasting an air-horn for 3 seconds at various noise levels and seeing what birds did in response at various distances. (It's not a very good model for aircraft noise, and as studies directly of aircraft noise also exist I'm slightly surprised - perhaps even skeptical - that they depend on it)

That study says
A-weighting of decibels was used
when calculating overall broadband
sound levels. This weighting scheme, which
is based on the inverse of a curve of sound
intensities perceived as being equally loud
to humans, has also been found to
approximate to hearing in birds (Dooling &
Popper 2007).
I'll see if I can find the weighting used in the EIA - it's a bit tangential, but I hadn't considered this at all and it's interesting.
Born at 356.32 ppm CO2 #ShowYourStripes

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Mon Jun 22, 2020 10:33 pm

Martin Y wrote:
Mon Jun 22, 2020 10:21 pm
Not my area at all but environmental noise assessment is exactly the field my student son is working in on placement at the moment. I'll put your question to him and see if he has any insights that might help.
Thanks very much, Martin!
Born at 356.32 ppm CO2 #ShowYourStripes

User avatar
dyqik
After Pie
Posts: 2204
Joined: Wed Sep 25, 2019 4:19 pm
Location: Masshole
Contact:

Re: Aircraft noise attenuation calculations

Post by dyqik » Tue Jun 23, 2020 12:00 am

The really naive option is to just use the inverse square law: i.e. sound pressure drops by 6 dB every time you double the distance from the source.

So the level will drop 10 dB at a distance 3.16 times further away from the source.

If they've done all the weightings in calculating the original distance, and there aren't big new propagation effects from terrain or atmospherics, that'll be pretty fairly close.

If the source is a narrow flight path, then just expanding the area by a factor of 3 will do the job. If it's a broader path as the source, then you need to increase the distance from the edge of the path to the edge of the 65 dB threshold zone by a factor of three.

User avatar
dyqik
After Pie
Posts: 2204
Joined: Wed Sep 25, 2019 4:19 pm
Location: Masshole
Contact:

Re: Aircraft noise attenuation calculations

Post by dyqik » Tue Jun 23, 2020 2:18 am

dyqik wrote:
Tue Jun 23, 2020 12:00 am
The really naive option is to just use the inverse square law: i.e. sound pressure drops by 6 dB every time you double the distance from the source.

So the level will drop 10 dB at a distance 3.16 times further away from the source.

If they've done all the weightings in calculating the original distance, and there aren't big new propagation effects from terrain or atmospherics, that'll be pretty fairly close.

If the source is a narrow flight path, then just expanding the area by a factor of 3 will do the job. If it's a broader path as the source, then you need to increase the distance from the edge of the path to the edge of the 65 dB threshold zone by a factor of three.
You also need to remember that the source is probably substantially above the ground, so it's a further spread on the ground to get to 3 times the distance from the source. Use Pythagoras.

User avatar
Matatouille
Clardic Fug
Posts: 212
Joined: Mon Nov 11, 2019 5:26 pm

Re: Aircraft noise attenuation calculations

Post by Matatouille » Tue Jun 23, 2020 7:15 am

dyqik wrote:
Tue Jun 23, 2020 2:18 am
dyqik wrote:
Tue Jun 23, 2020 12:00 am
The really naive option is to just use the inverse square law: i.e. sound pressure drops by 6 dB every time you double the distance from the source.

So the level will drop 10 dB at a distance 3.16 times further away from the source.

If they've done all the weightings in calculating the original distance, and there aren't big new propagation effects from terrain or atmospherics, that'll be pretty fairly close.

If the source is a narrow flight path, then just expanding the area by a factor of 3 will do the job. If it's a broader path as the source, then you need to increase the distance from the edge of the path to the edge of the 65 dB threshold zone by a factor of three.
You also need to remember that the source is probably substantially above the ground, so it's a further spread on the ground to get to 3 times the distance from the source. Use Pythagoras.
Further to dyqik's point on noise source elevation, are the wetland areas you're looking at under/adjacent the landing approach glide-path, directly adjacent the airport etc?

If you're looking at landing noise:
The standard airport landing glideslope is 3 degrees, and extends for about 5-7 statute miles from the intersection point (usually a couple of hundred meters down the runway from the threshold. To start with you could just measure from the runway threshold, I'll see if I can find plans for their runway operations (unless you have access to them and could share?), but due to the shallow angle the height difference at a point along the glideslope won't be that big and measuring from the threshold would probably just give you a decent figure for an aircraft that is a little lower than ideal on the slope (not unusual).
Heathrow I recall a few years ago was trialling a 3.2 degree glideslope specifically for noise abatement, and if successful were going to then extend to trialling 3.5 degrees. I'll see if I can find out what the outcomes were for that as it might give you some useful references. Some airports in mountains or within cities with rich neighbours use 5 degrees (London City for example), but it starts to limit the types of aircraft that can achieve that, so I'd bet the plan out of the box for a capital city's new primary airport won't be anticipating that.
Unless the runway operations plans state otherwise, you could probably assume 3 degrees is what was simulated for this new airport, which will give you a solid height over ground.

If you're looking at takeoff noise:
Commercial aircraft take off at full thrust as steeply as they can for noise abatement. Despite the engines being in max noise mode at the time, getting distance from the ground as quickly as possible actually makes this quieter around the airport than a lower thrust gentler ascent. Runways are over-length for normal takeoffs to provide safety margins for engine failures and aborted takeoff braking distances so aircraft will be pretty high by the time they get to the boundary fence. The highest noise will be concentrated around the boundary area of perhaps the last 1/3 of the runway. If takeoff noise is something you're looking at I can see if I can find some sources to get heights of noise source, but given this depends more on the performance differences of various aircraft types, there is going to be a good bit of variation.

User avatar
Sciolus
Fuzzable
Posts: 296
Joined: Mon Nov 11, 2019 6:42 pm

Re: Aircraft noise attenuation calculations

Post by Sciolus » Tue Jun 23, 2020 8:19 am

AEDT is the best (only?) tool for this kind of assessment. It's free (the US knows how to do FoI) but complicated - not something you can just install and mess around with.

See Matt for geometry and Dyqik for sound propagation. You should also be aware that there are shedloads of noise metrics, mostly based on human disturbance, and you will need to be consistent (I doubt there is good evidence about which is most appropriate for bird disturbance).

The Manston Airport application also involved planes flying low over bird sites, so may be worth comparing their assessment to see what criteria they use and whether Natural England agreed. I'll try to have a rummage later.

User avatar
Martin Y
Dorkwood
Posts: 1246
Joined: Mon Nov 11, 2019 1:08 pm

Re: Aircraft noise attenuation calculations

Post by Martin Y » Tue Jun 23, 2020 10:38 am

I got a reply from son Robb and most of what he mentions is already covered above, except that he pointed out an extra complication that the contribution from ground-based noise around the airport (or flying aircraft low down from your point of hearing) drops off at only 3dB per doubling of distance rather than the 6dB for aircraft above you (due to the hemispherical rather than free space propagation).

The UK NATS has representative data for noise from aircraft types landing or taking off but I'm not sure if that's useful or just nerdily curious:
https://www.nats.aero/environment/aircr ... lmax-data/

He has access to AEDT-generated noise contour maps of UK airports but not in Portugal.
Luton_contour.JPG
Luton_contour.JPG (167.96 KiB) Viewed 442 times

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Tue Jun 23, 2020 11:36 am

Thanks everybody! Will go through and reply to questions.
Matatouille wrote:
Tue Jun 23, 2020 7:15 am
Further to dyqik's point on noise source elevation, are the wetland areas you're looking at under/adjacent the landing approach glide-path, directly adjacent the airport etc?
The figures in the EIA are based on landing noise - they have one runway with landing happening north-south and one south-north, so if I'm following correctly this is based on landing noise in both directions.

I've digitised the figure from the EIA (it's for a proposed new airport, rather than the current one):
airport.png
airport.png (429.46 KiB) Viewed 427 times
The yellow polygon is their 65 dB area. The red boundary is an SPA, so EU-level protected area. The blue X is the proposed airport location, and the blue polygon is a wetland complex that was created as mitigation for the last infrastructure project that impacted these wetlands (Vasco da Gama bridge).
Matatouille wrote:
Tue Jun 23, 2020 7:15 am
If you're looking at landing noise:
The standard airport landing glideslope is 3 degrees, and extends for about 5-7 statute miles from the intersection point (usually a couple of hundred meters down the runway from the threshold. To start with you could just measure from the runway threshold, I'll see if I can find plans for their runway operations (unless you have access to them and could share?), but due to the shallow angle the height difference at a point along the glideslope won't be that big and measuring from the threshold would probably just give you a decent figure for an aircraft that is a little lower than ideal on the slope (not unusual).
Heathrow I recall a few years ago was trialling a 3.2 degree glideslope specifically for noise abatement, and if successful were going to then extend to trialling 3.5 degrees. I'll see if I can find out what the outcomes were for that as it might give you some useful references. Some airports in mountains or within cities with rich neighbours use 5 degrees (London City for example), but it starts to limit the types of aircraft that can achieve that, so I'd bet the plan out of the box for a capital city's new primary airport won't be anticipating that.
Unless the runway operations plans state otherwise, you could probably assume 3 degrees is what was simulated for this new airport, which will give you a solid height over ground.
Some info is given about their assumptions in the EIA (in Portuguese - my translation):
1. Landing: From 1800m altitude and about 44 km from the runway. Less sound emission because the aircraft is reducing speed.
2. Landing [maintain altitude and landing gear): From about 800m of altitude and about 15km from the runway, and typically before about 9km distance from the runway, until landing. Greater sound emission, due to the use of flaps and power to maintain altitude and release landing gear.
3. Landing (reverse thrust): About 400m after touchdown. Even greater sound emission, due to the use of reverse thrust which helps decelerate the plane after touching down.
However, these are just generic figures taken from ANP (Aircraft Noise and Performance) for a Boeing 737-800 and the document “Civil Aviation Authority – Managing Aviation Noise. 2014”. I don't know if specific plans for runway operations were taken into account for their analysis.
[/quote]
Born at 356.32 ppm CO2 #ShowYourStripes

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Tue Jun 23, 2020 11:39 am

dyqik wrote:
Tue Jun 23, 2020 2:18 am
dyqik wrote:
Tue Jun 23, 2020 12:00 am
The really naive option is to just use the inverse square law: i.e. sound pressure drops by 6 dB every time you double the distance from the source.

So the level will drop 10 dB at a distance 3.16 times further away from the source.

If they've done all the weightings in calculating the original distance, and there aren't big new propagation effects from terrain or atmospherics, that'll be pretty fairly close.

If the source is a narrow flight path, then just expanding the area by a factor of 3 will do the job. If it's a broader path as the source, then you need to increase the distance from the edge of the path to the edge of the 65 dB threshold zone by a factor of three.
You also need to remember that the source is probably substantially above the ground, so it's a further spread on the ground to get to 3 times the distance from the source. Use Pythagoras.
Thanks for this - it's sounding like this kind of naive approach (probably a couple of conservative scenarios) might be the way to go. As you can see from the map, even doubling the width includes a lot of extra area.
Born at 356.32 ppm CO2 #ShowYourStripes

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Tue Jun 23, 2020 12:00 pm

Sciolus wrote:
Tue Jun 23, 2020 8:19 am
AEDT is the best (only?) tool for this kind of assessment. It's free (the US knows how to do FoI) but complicated - not something you can just install and mess around with.
Ok thanks! Annoyingly the AEDT website still seems to be down, but I probably don't need to go into that much detail - for these purposes a rougher indication using the inverse square law is probably enough to show that the particular choice of 65 dB makes a reasonably large difference, both to the total area and which areas are potentially affected.
Sciolus wrote:
Tue Jun 23, 2020 8:19 am
See Matt for geometry and Dyqik for sound propagation. You should also be aware that there are shedloads of noise metrics, mostly based on human disturbance, and you will need to be consistent (I doubt there is good evidence about which is most appropriate for bird disturbance).
As much as possible, I'm trying to take the EIA on its own terms. They've used a study looking at bird responses to an airhorn, so I'm going to stick with that even though it's pretty silly.
Sciolus wrote:
Tue Jun 23, 2020 8:19 am
The Manston Airport application also involved planes flying low over bird sites, so may be worth comparing their assessment to see what criteria they use and whether Natural England agreed. I'll try to have a rummage later.
Awesome, thanks - there does seem to have been a bit of back-and-forth about noise levels and particular bird populations which will be good to read.

As an aside, submissions seem to refer to previous submissions by codes like REP4-018, and while that indicates it was a response by deadline 4 there doesn't seem to be any way of searching by code specifically, which is a bit of a PITA (unless I'm being daft).
Born at 356.32 ppm CO2 #ShowYourStripes

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Tue Jun 23, 2020 12:06 pm

Martin Y wrote:
Tue Jun 23, 2020 10:38 am
I got a reply from son Robb and most of what he mentions is already covered above, except that he pointed out an extra complication that the contribution from ground-based noise around the airport (or flying aircraft low down from your point of hearing) drops off at only 3dB per doubling of distance rather than the 6dB for aircraft above you (due to the hemispherical rather than free space propagation).

The UK NATS has representative data for noise from aircraft types landing or taking off but I'm not sure if that's useful or just nerdily curious:
https://www.nats.aero/environment/aircr ... lmax-data/

He has access to AEDT-generated noise contour maps of UK airports but not in Portugal.
Luton_contour.JPG
Thanks (and thanks Robb!) - it's very useful to have some other sources for reliable figures to sense-check what I come out with, and I have no idea where to look for any of this.
Born at 356.32 ppm CO2 #ShowYourStripes

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Tue Jun 23, 2020 12:46 pm

As it happens, I have found a whole range of noise contours plotted in the appendix of a different section of the report, including the lower noise levels of interest. They weren't used for the birds assessment, but I should be able to digitise them into a GIS program and make use of them anyway. ETA - not what I wanted (they average sound levels over 24 hour periods, and only show plots for urban areas)
Born at 356.32 ppm CO2 #ShowYourStripes

User avatar
Sciolus
Fuzzable
Posts: 296
Joined: Mon Nov 11, 2019 6:42 pm

Re: Aircraft noise attenuation calculations

Post by Sciolus » Tue Jun 23, 2020 12:54 pm

Bird on a Fire wrote:
Tue Jun 23, 2020 12:00 pm
As an aside, submissions seem to refer to previous submissions by codes like REP4-018, and while that indicates it was a response by deadline 4 there doesn't seem to be any way of searching by code specifically, which is a bit of a PITA (unless I'm being daft).
Yeah, it's not easy to navigate through the various docs, but as well as filtering on the webpage there's this pdf index with hyperlinks which has REP numbers and is sometimes easier to use.

User avatar
Bird on a Fire
Stummy Beige
Posts: 3454
Joined: Fri Oct 11, 2019 5:05 pm
Location: nadir of brie

Re: Aircraft noise attenuation calculations

Post by Bird on a Fire » Tue Jun 23, 2020 12:59 pm

Sciolus wrote:
Tue Jun 23, 2020 12:54 pm
Bird on a Fire wrote:
Tue Jun 23, 2020 12:00 pm
As an aside, submissions seem to refer to previous submissions by codes like REP4-018, and while that indicates it was a response by deadline 4 there doesn't seem to be any way of searching by code specifically, which is a bit of a PITA (unless I'm being daft).
Yeah, it's not easy to navigate through the various docs, but as well as filtering on the webpage there's this pdf index with hyperlinks which has REP numbers and is sometimes easier to use.
Aha! That is easier, thanks.
Born at 356.32 ppm CO2 #ShowYourStripes

Holylol
Buzzberry
Posts: 35
Joined: Tue Nov 19, 2019 10:14 am

Re: Aircraft noise attenuation calculations

Post by Holylol » Tue Jun 23, 2020 1:39 pm

I deleted the post, I have something to check before.

Holylol
Buzzberry
Posts: 35
Joined: Tue Nov 19, 2019 10:14 am

Re: Aircraft noise attenuation calculations

Post by Holylol » Tue Jun 23, 2020 2:06 pm

Martin Y wrote:
Tue Jun 23, 2020 10:38 am
I got a reply from son Robb and most of what he mentions is already covered above, except that he pointed out an extra complication that the contribution from ground-based noise around the airport (or flying aircraft low down from your point of hearing) drops off at only 3dB per doubling of distance rather than the 6dB for aircraft above you (due to the hemispherical rather than free space propagation).
I am not sure that this is correct. If you consider a point source emitting over a broad frequency range above a (rigid) ground and a receiver above the ground (at a low altitude compared to the source), you can assume that the ground effect adds 3 dB when compared to the case where the source is radiating in a free-field. If the observer is exactly at ground level it is a 6 dB addition instead of 3.
But the rate of decay with the distance is still 20*log10(d), so 6 dB per doubling.

ETA: to complete this post. The sound pressure level Lp in dB for a broadband point source radiating above a rigid ground and an observer above the ground (at a low altitude compared to the source) and a distance d is

Lp = LW - 20*log10(d) - 8

where LW is the acoustic power of the source in dB

User avatar
Martin Y
Dorkwood
Posts: 1246
Joined: Mon Nov 11, 2019 1:08 pm

Re: Aircraft noise attenuation calculations

Post by Martin Y » Tue Jun 23, 2020 2:24 pm

Holylol wrote:
Tue Jun 23, 2020 2:06 pm
I am not sure that this is correct.
You're right. Nor was I, so I queried it and Robb pointed me to https://www.engineeringtoolbox.com/outd ... -d_64.html and now that you made me double-check properly instead of just skimming, you're correct as the 3dB figure applies not to hemispherical propagation but to cylindrical propagation from a linear source. I think he was too hasty. Students, eh?

It makes more sense that it's still 6dB as the area of the sound wave is still increasing as the square of the radius irrespective of whether it's hemispherical or spherical. It's twice as loud to begin with, but it drops off at the same rate.

User avatar
dyqik
After Pie
Posts: 2204
Joined: Wed Sep 25, 2019 4:19 pm
Location: Masshole
Contact:

Re: Aircraft noise attenuation calculations

Post by dyqik » Tue Jun 23, 2020 2:50 pm

Martin Y wrote:
Tue Jun 23, 2020 2:24 pm
Holylol wrote:
Tue Jun 23, 2020 2:06 pm
I am not sure that this is correct.
You're right. Nor was I, so I queried it and Robb pointed me to https://www.engineeringtoolbox.com/outd ... -d_64.html and now that you made me double-check properly instead of just skimming, you're correct as the 3dB figure applies not to hemispherical propagation but to cylindrical propagation from a linear source. I think he was too hasty. Students, eh?

It makes more sense that it's still 6dB as the area of the sound wave is still increasing as the square of the radius irrespective of whether it's hemispherical or spherical. It's twice as loud to begin with, but it drops off at the same rate.
Propagation in a cylinder might be appropriate under certain circumstances - an inversion layer above the source, for example, that traps a significant portion of the sound under it. The sound pressure level will then drop as 1/R, although there may also be significant attenuation from multiple bounces off the ground and the inversion layer, giving an additional e^-aR attenuation factor.

Holylol
Buzzberry
Posts: 35
Joined: Tue Nov 19, 2019 10:14 am

Re: Aircraft noise attenuation calculations

Post by Holylol » Tue Jun 23, 2020 3:07 pm

dyqik wrote:
Tue Jun 23, 2020 2:50 pm
Martin Y wrote:
Tue Jun 23, 2020 2:24 pm
Holylol wrote:
Tue Jun 23, 2020 2:06 pm
I am not sure that this is correct.
You're right. Nor was I, so I queried it and Robb pointed me to https://www.engineeringtoolbox.com/outd ... -d_64.html and now that you made me double-check properly instead of just skimming, you're correct as the 3dB figure applies not to hemispherical propagation but to cylindrical propagation from a linear source. I think he was too hasty. Students, eh?

It makes more sense that it's still 6dB as the area of the sound wave is still increasing as the square of the radius irrespective of whether it's hemispherical or spherical. It's twice as loud to begin with, but it drops off at the same rate.
Propagation in a cylinder might be appropriate under certain circumstances - an inversion layer above the source, for example, that traps a significant portion of the sound under it. The sound pressure level will then drop as 1/R, although there may also be significant attenuation from multiple bounces off the ground and the inversion layer, giving an additional e^-aR attenuation factor.
Cylindrical waves radiated by a line source are also used in road traffic noise exposure calculations, if the road can be assumed to have a relatively high vehicle density.

User avatar
Matatouille
Clardic Fug
Posts: 212
Joined: Mon Nov 11, 2019 5:26 pm

Re: Aircraft noise attenuation calculations

Post by Matatouille » Tue Jun 23, 2020 3:32 pm

Bird on a Fire wrote:
Tue Jun 23, 2020 11:36 am
I've digitised the figure from the EIA (it's for a proposed new airport, rather than the current one):
airport.png

The yellow polygon is their 65 dB area. The red boundary is an SPA, so EU-level protected area. The blue X is the proposed airport location, and the blue polygon is a wetland complex that was created as mitigation for the last infrastructure project that impacted these wetlands (Vasco da Gama bridge).
Yes that is definitely a landing sound profile, takeoff ones concentrate much closer to the airport, and tend to veer off in specific directions as they can direct jets onto egress headings that do most noise abating for the neighbours.

I think I've found a noise plot that they've done including your 65dB plot above, but also 55db, 60dB, and 70dB lines which might save you some work.
download link, I found it here. Open the pdf and go to page pdf page 106 (marked page number 102). It looks like the exact same plot for 65dB as in your map, so you might be able to translate the 50dB and 60dB lines onto a background more useful for your case of seeing how much of which wetlands are affected. It is then a matter of finding what assumptions they made for this - aircraft type, loading, glide angle etc and see if they hold up. I'm trying to get a copy of this pdf small enough that I can feed it into a translator because I'm lazy.
Bird on a Fire wrote:
Tue Jun 23, 2020 11:36 am
Some info is given about their assumptions in the EIA (in Portuguese - my translation):
1. Landing: From 1800m altitude and about 44 km from the runway. Less sound emission because the aircraft is reducing speed.
2. Landing [maintain altitude and landing gear): From about 800m of altitude and about 15km from the runway, and typically before about 9km distance from the runway, until landing. Greater sound emission, due to the use of flaps and power to maintain altitude and release landing gear.
3. Landing (reverse thrust): About 400m after touchdown. Even greater sound emission, due to the use of reverse thrust which helps decelerate the plane after touching down.
However, these are just generic figures taken from ANP (Aircraft Noise and Performance) for a Boeing 737-800 and the document “Civil Aviation Authority – Managing Aviation Noise. 2014”. I don't know if specific plans for runway operations were taken into account for their analysis.
Cheers, that is helpful. I'll do a bit more snooping about on this stuff.

Post Reply