Noise Barrier

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Noise Barrier

Postby Mohammad » Fri May 23, 2008 11:21 pm

Hello all.
does anyone know about how to do calculations for a noise barrier.
in specific the noise generated by a machinery.

thank you all
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Postby avare » Sat May 24, 2008 1:23 am

Yes.  Be more specific on what you want!

Andre
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Postby Bob » Sat May 24, 2008 3:25 am

Avare:

You have more guts than me. I debated writing "Yes" and nothing else, but chickened out. :)



Anyway these came to mind:
a) how loud is the device and at what frequencies
b) how far away is the listener that you have to protect, and what's between the device and the listener
c) what's the bylaw noise requirement in that area. Hours/dB(A)
d) vibration/isolation mounts (perhaps acousimat with concrete patio stones on top, or solymer)
e) walls and roof - see IR761.pdf and similar
f) absorbers on the inside of the enclosure to knock the resonant sound down

Is it an air conditioner outside that's near a studio or neighbours, is it in an alleyway or on a roof
Is it a free standing tool in the middle of a shop floor
If on a roof, are you trying to keep the noise from coming through the roof, or though air vents, etc
etc.

And then there's books like "Noise Control In Buildings" and "Noise and Vibration Control" and "Flakt Woods Practical Guide to Noise Control". and "Engineering Noise Control" and "Acoustics and Noise Control" and "Noise Control For Engineers" and "Fundimentals of Noise and Vibration Analysis for Engineers" and "Sound and Structural Vibration" etc.
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Postby avare » Sat May 24, 2008 9:10 am

Does this answer your question?  Or are you looking for the effects of a wall?

Andre
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Postby Mohammad » Sat May 24, 2008 10:32 am

it is simply:
7 chillers and 7 pumps on the top of a shopping mall.
the total dB of all items is 107dB(A). the barrier is requested to be on only one side [i mean one wall of 4 walls].
the required dB level after 10meter distance is 50dB to be measured at night.
the wall barrier that will be installed is fabricated by Forster [Austrian company]. and the anti-vibration pads are Sylomer by our company [Getzner].
the problem is that i made my calculations and according to the barrier height which is 4m i found that the screen attenuation is roughly 20dB. and the effect of the brrier itself is 30dB, [ACTUALLY, i dont know the total dB reduction amount].
maybe i need to improve my knowledge in this field.

anyway, if anyone has PDF in this regard i will be grateful.


thank you very much BOB & Andre.....
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Postby Terry Montlick » Sat May 24, 2008 12:00 pm

Mohammad wrote:it is simply:
7 chillers and 7 pumps on the top of a shopping mall.
the total dB of all items is 107dB(A). the barrier is requested to be on only one side [i mean one wall of 4 walls].
the required dB level after 10meter distance is 50dB to be measured at night.
the wall barrier that will be installed is fabricated by Forster [Austrian company]. and the anti-vibration pads are Sylomer by our company [Getzner].
the problem is that i made my calculations and according to the barrier height which is 4m i found that the screen attenuation is roughly 20dB. and the effect of the brrier itself is 30dB, [ACTUALLY, i dont know the total dB reduction amount].
maybe i need to improve my knowledge in this field.
.

Ah, that's a horse of another color! This involves diffraction effects at the top of an outdoor sound barrier. Fortunately, this has been well studied for building highway sound barriers. There are probably some good on-line references, but I have in my library the book:

Leo Beranek,editor, Noise and Vibration Control, Chapter 7, "Sound Propagation Outdoors," pp. 164-193, McGraw Hill, 1971.

I thought I also had some good papers on this, but my disorganization prevents me from finding these at the moments. I'll let you know If I find these.

Regards,
Terry
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Postby Terry Montlick » Sat May 24, 2008 12:19 pm

Here's a practical online source, which references the Noise and Vibration Control book I mentioned:

http://www.mne.psu.edu/lamancusa/me458/10_osp.pdf

See section 10.3.6.

Regards,
Terry
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Postby avare » Sat May 24, 2008 12:42 pm

Not meant as a silly question, but did you include the sound pressure build up by on the source side in your calculations or have absorbent material there?

Andre
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Postby bert stoltenborg » Sun May 25, 2008 11:31 am

Mohammad wrote:it is simply:
7 chillers and 7 pumps on the top of a shopping mall.
the total dB of all items is 107dB(A). the barrier is requested to be on only one side [i mean one wall of 4 walls].
the required dB level after 10meter distance is 50dB to be measured at night.
the wall barrier that will be installed is fabricated by Forster [Austrian company]. and the anti-vibration pads are Sylomer by our company [Getzner].
the problem is that i made my calculations and according to the barrier height which is 4m i found that the screen attenuation is roughly 20dB. and the effect of the brrier itself is 30dB, [ACTUALLY, i dont know the total dB reduction amount].
maybe i need to improve my knowledge in this field.

anyway, if anyone has PDF in this regard i will be grateful.


thank you very much BOB & Andre.....


First you need to explain if we're dealing with a sound level or a power (as is usual in these cases). This can make a 11 dB(A) differecne.
Then we need a sound spectrum in octaves or better.

These barrier calcs involve bended sound paths and other fancy stuff.

In practice you won't get more than 20 dB(A) attenuation, in the very best case.
Added with the geometrical attenuation, also 20 dB(A) in the best case (no refelctions and stuff) you get 40 dB(A) attenuation.
In the very best case.
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Postby Eric.Desart » Sun May 25, 2008 1:01 pm

Mohammad,

The isolation of barriers is defined by the path length difference between the direct sound (if no barrier should be there) and the path length over the barrier.

As Bert said this is very frequency dependent.
As Bob, Terry, and so on said you should read about it.
As Eric said (now), you have to do with environmental regulations here.  Most likely there are standards or published calculation methods available you have to apply for such situations.

Without frequency spectrum it has hardly sense to start calculating this stuff.
You give a level "AFTER 10 m".  This doesn't work as enclosures. At longer distance this can increase again (think about this path length difference).   The closer source or recipient (or combination) to the barrier the higher the isolation (this path length difference ratio becomes larger)

And it also depends on the environment.
In lot of countries penalties can be involved for tonal noise which is possibly applicable in your situation.

If you work for Getzner, it looks as you are trying to extend your advise to more general environmental acoustics.
If that's the case, either you or whoever in your company should follow a related course.
I don't think it's a responsible approach trying to do industrial acoustics via a DIY studio group.

You are dealing here with environmental laws and regulations, and mostly a lot of money and responsibility is involved.
When I hear 20 dB effect of a barrier for low frequent pumps, you MAYBE can get this at a short distance from the barrier but at a longer distance this seems very much like wishfull thinking to me.
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Postby bert stoltenborg » Sun May 25, 2008 3:17 pm

Woho,

I said: "in the very best case".
I meant: in practice it will almost never be that good, and almost always much worse.

and it's only easy to calculate these things when you're in regulation, paper acoustics.
Some research of TNO (the Dutch NRC, I geuss) on models for road noise barriers, showed recently the official calculation software sucks.
Although the guys who make this software say it ain't that bad.
:mrgreen:
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Postby Eric.Desart » Sun May 25, 2008 4:23 pm

:twisted:  Bert I wasn't correcting you, but wanted to warn Mohammad.

:) This type of industrial/environmental acoustics is what I mainly earned my living with.
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Postby avare » Sun May 25, 2008 4:56 pm

The more I think about this, the more I think we do not have enough information to give any useful comments.  

What is the height of the roof they are on?  
What is the relative height of the monitoring position.  
On a basic level, at what distance is the 107 dB SPL (A) taken?  
With it being a combination of sources, are they acting as a line source instead of a point source?  
What is the power level of the noise?
What is the frequency spectrum of the noise?  
Not being rude, but if the concern is A weighted sound levels, why are resilient mounts even being designed for?
One big barrier for seven sources seems like like IT MIGHT BE not as efficient as smaller barriers closer to each source.

All this is before getting into the specifics of the laws that have to be complied with.

Andre
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Postby Bob » Sun May 25, 2008 5:45 pm

Not being rude, but if the concern is A weighted sound levels, why are resilient mounts even being designed for?

With less mass to vibrate (due to decoupling), might make it louder at the neighbours.
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Postby bert stoltenborg » Sun May 25, 2008 6:08 pm

Andre,

in environmental issues they most of the time refer to sound power.

A measured sound level is normalized to power, so you have a fixed reference about the acoustical properties of the source.
(More or less, at least  :D)

SPL doesn't mean much if you don't know how and where it is measured, of course.
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Postby bert stoltenborg » Sun May 25, 2008 6:11 pm

Mohammad wrote:the screen attenuation is roughly 20dB. and the effect of the brrier itself is 30dB


8O  ??
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Postby Mohammad » Sun May 25, 2008 6:26 pm

thank you all,

my replies for Eric:
Mr. Eric, there is still no regulations in the middle east for noise levels.
acoustics in the middle east is still "premature science", it is really not of Getzner domain, but i try to do it personally for my friends beacause i am really happy with studying this science. by the way, if you have any source for courses "online" paid or not, i hope you can advice me.
the resilient mounts are being designed for vibration protection which is necessary for all equipments but it is not required to reduce the air-borne noise.

anyway, thank you all.
i am gathering the information you required and i will attach the sketch of chillers and every related info.

by the way, i did my own study and i found that the screen attenuation is at least 20dB and i know form the data sheets of the barrier that the dB reduction of this barrier is 30dB. i don't know how to correlate between these two amounts. I have read that the required reduction of any barrier is to be 5dB higher than the attenuation gotten by distance. it is really disturbing, but  challenging.

very soon i will attach all the data needed.
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Postby Bob » Mon May 26, 2008 1:57 am

I'm pretty sure I posted here more than a year ago about a television show about an air conditioner that was in an alley. One side of the alley was commercial, for which the air conditioner was for. The other side was residential, a half dozen houses. Whenever the air conditioner started, it was scarry loud.
The solution was:
a) put the air conditioner on the roof of the two storey commercial building, rather than behind it in the alley.
b) put up a two sided wall around the air conditioner on the roof
c) reduce the activation of the unit at night
d) since it was the starting of the unit that was loudest, during the day the new unit was better sized to the commercial requirements so that it would run longer when it started, rather than starting and stopping as often.
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Postby Mohammad » Mon May 26, 2008 6:56 am

attached is the chiller data and sketch

the red line represents the sound barrier.
and in the area between the chiller and red line there are 7 pumps located. the maximum pump noise level is 72dB.
the barrier height is 4meter.

behind the red line there is te rest of the shopping mall building but in a lower level, in this area the dB required to be not greater than 54dB.

the untreated wall is represented by bouble line in blue color, the area behind this wall is bare are [there is no buildings or neighborhood].
the untreated wall height is recommende to be 4.6m unless the acoustic analysis requires another height. the height of chillers is 2.80m for all.

the 54dB required, will be measured at night, and it is required 10m away from wall in the curved area and directly after next to wall in the pumps area.

the maximum noise level of eachi chiller is 98dB bu they are supposed working together. (107dB)
Attachments
chiller sketch &data.pdf
(1.04 MiB) Downloaded 136 times
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Postby Rod Gervais » Mon May 26, 2008 10:43 am

Mohammad,

Let me preface this with the understanding that I am trying to be neither funny nor disrespectfull when I say what I am about to say.

However - that being said, why don't you hire an acoustical engineer with experience in this regard and be done with it?  He could (perhaps) give you a better understandng of how it all works as he does his analysis.

You do not have the experience required to do this yourself - so hire someone who does.

If it's something you really are interested in - perhaps you should take a few college courses on the subject.

Sincerely,

Rod
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