design of transport noise barriers

8/6/2019 DESIGN OF TRANSPORT NOISE BARRIERS

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Ecology and Environmental Protection

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DESIGN OF TRANSPORT NOISE BARRIERS

Assoc. Prof. Nikolai Nikolov1, D.Sc.

Prof. Metodi Mazhdrakov2, D.Sc.

Dobriyan Benov3

1Institute of Building Physics, Technology and Logistics,Bulgaria

2University of Mining and Geology St. Ivan Rilski,Bulgaria3ACMO-2006 LTD,Bulgaria

ABSTRACT

Transport noise barriers areecologicalstructuresthataredesignedandbuiltalongtheroads

(motorways, expresswaysandrailwaysetc.), inorder to reducethenoiselevels tothelimitvalues forurbanareasandinrooms of buildings. Assessment of theeffectiveness of

thebarrieriscarriedouttheoretically, empiricallyorby a combination of bothmethods. Toautomatethedesign of thinnoisebarriersarecreatedfourmodulesfromtheseriesScreen *,

whicharepart of thesoftwarepackageforacousticcalculationsSoundBG.

Keywords:noise barriers, acoustical design, software

1. Definition, acousticefficiency and classification of transport noise barriers

Transportnoisebarriersareecologicalstructuresthataredesignedandbuiltalongtheroads

(motorways, expresswaysandrailwaysetc.), inorder to reducethenoiselevels to

thelimitvalues forurbanareasandinrooms of buildings.The barriers are made of different

sizes (length and height), of different construction materials, have various cross sections

and architectural appearance. In acousticspoint of view,the design of transport noise

barriers cant be standard (uniform), as opposed to elements from which are theydesigned. This complexity lies in their construction-acoustical design, which is a

function of many requirements, factors, phenomena and parameters. From an economic

perspective, this factor is leading to determine the value of the facility [1].

Transport noise barriers can be defined as solid, practically adequately soundproofed

enclosures,that creates a sound shadow zone behind them by breaking the direct

distribution of noise in the line of direct sight from the center of the source to the point

of impact (assessment point), where the noise level decreases due to diffraction of sound

waves (Fig. 1).

The effect of the noise barrier is based on acoustic processes taking place after itsconstruction. The main effect of noise reduction is achieved thanks to the creationof

acoustic shadow due to diffraction of sound on the free edge of the barrier.

Quantitativemeasure of noise protectionby thebarrierisitsacousticefficiency,

definedasthedifferencebetweennoiselevelsintheassessmentpointbeforeandafterconstruction of thebrrier, with allthe same other conditions.

According to thedesignandachievedefficiency, barriersaredividedintoseveralclasses [1].


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1. Thin barriers. Hei ht ratio is 2 to 6 m, and their acoustic efficiency is bet een 6 and

18 dB .

Fi . 1.Acoustic desi n of a thin barrier

2. Thick barriers.Hei ht ratio is 2 to 3 m, thickness is over 3 m, and their efficiency is

bet een 5 and 10 dB A).

3. Acoustic tunnels. Their efficiency is 25-30 dB (A).

4. Compound barriers - a combination oft o types of barriers. Their hei htis 3 to 5 m,and efficiency bet een 12 and 17 dB(A). The most common combination is

embankment with a thin barrier.

In the following statement will be considered acoustical design only ofthin barriers that

are applied most widely in Europe and the USA.

2. E i i calculati of thi barri r.

Efficiency evaluation of the barrier is carried out theoretically, empirically or by a

combination of both methods. The following formulas solves the so-called "right

problem", i.e. calculated is efficiency with given height ofthe barrier and its location in

relation to the noise source and the assessment point (Fig. 1).

Example oftheoretical and empirical determination of the efficiency of the barrier are

the works ofMaekawa, who has studied the diffraction in a thin barrierin experimentalconditions [2].

For an infinite noise source (car stream) from Maekawas studies is obtained, that the

efficiency ofthe barrieris:

( ) , :

)1(dB( ),,.1.

2

.

2.

!!(

P

HNL

NB

whereNis Fresnels number;


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length of the sound wave, m;

difference between the shortest distance from the noise source (1) to theassessment point (2) and the road that passes the diffracted sound, m.

From Fig. 1 is that:

)2(m.,2212

21

2

2

2

2

2

1

2

1 hhddhhdhhddBA !!H

Formula (1) hasphysicalmeaningwhen 0"5 , i.e.ifsoundis diffractedthethe upperedge

of thebarrier. When 0!5 itisassumedthattheefficiency of thebarrier is 5 dB(A).

Themainfactorsthatdeterminetheefficiency of thebarrier is determined by formulas (1)

and (2). These are:

- heightof the barrierh;- lengthofthesoundwave; for car streams is accepted m84.0!P , for railway

transport - m42.0!P ;

- geometric parameters of the location of the source (d1,h1) and of the assessmentpoint(

d2,h

2);- type of the source with infinite or finite length.Formula (1) refers to thenoisesourcewithinfinitelength; for a singlenoisesourceor a

sourcewith finitelength (railways) thecorresponding formula is:

)3(d ( ).,03.159.12

22.0

22.0

!!(

P

HNL

NB

Whentheassessmentpointislocatedin a room,

theoverallreductioninnoiselevelinthepresence of thebarrierwillbe:

)4(dB(A),,CARNBLLLLL ((((!(

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where RL( is the recuction in noise level, because of the increased distance from source,dB(A);

AL( -reductioninnoiseleveldue to absorption of soundenergyin the air, dB(A);

CL( - soundinsulationfromairbornenoise of thesurroundingstructure, dB(A).

For open space dB(A)0!(C

L .

For a continuoussource (carstream) the reductioninnoiseleveldue to increasing

distancefromsource is:

)5(dB(A),,lglg

lg

1100

rr

rLR

!( T

N

whereis the average distance between the unit sources in the stream, m;

r distance from noise source to assessment point, m;


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r0 basicdistancefordetermination of thenoisecharacteristics of thestrem(for car

streams m5.70 !r ).

With a sourcewith finite length, thereductioninthenoise levelbetweenpointsAandB,

whichlieson normal to themiddle of thesource,at a distancesrAandrB, is

)6(dB(A).,)(

)(lg20

B

A

Ad

rf

rfL !(

Ingeneral, thefunctionf(r)issignificantlymorecomplexthanformula (5) [3]. For a smallnumber of individual sources - 2, 3 or 4, the function is simplified [4]. Forexample, for

foursourcesat a distanceapart, we have:

(7),111

)(21

2

2

2

1rrrr

rfA

!

where .2,1,5.0 222 !! iirr Ai N

The formula forf(rB)is analogical.

The reduction in noise level due to absorption of sound waves fromtheairis:

)8(dB(A).,005.0 rLA

}(

Thelength of thesides of thebarriermustnotallowdirectsound, whichispassedthroughit, to

exceedthenoiselevel (Fig. 2). Taking into account the particularities of power

summation, we obtain:

)9(d ( ),,5'' (((u(( ARNBAR LLLLL

wherethereductions 'RL( and 'AL( dependsonthelength of theside of thebarrier.

Fig. 2.Determination of thelength of thebarrier

From Fig. 2 is that the total length of the screen:

)10(m,,1'

2

2

2N

!

r

rdW


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where 'r isthedistancefromtheassessmentpoint to thenearestnoise

sourcethatisoutsidetheactionofthebarrier, m;

- the length ofthe protected object, m.

3. Determi i the required efficiency.

Noiseregimeintheassessmentpointdependsonthenoisecharacteristics of thesourceL0, the

noise reductionfromthescreen, thedistanceandsurroundingstructures,

aswellaspermissible noise levelLLD, whichisregulatedbythelegal documents [5].

Inorderto meetthelegalrequirementsitis necessary

)11(.d (A),0 CARNBLD LLLLLL ((((e

Fromformula (10) we definetherequired (design) efficiency ofthebarrier:

)12(.dB( ),0 LDCARNB LLLLLL (((!(

Therelationshipbetweendesignheight of thebarrierandrequiredefficiencyisthe"inverseproblem" of theformula (1). Inthiscase, theunknownparameter istheheight ofthebarrierh, whichleads to a transcendentalequation.

4. Software.

To automatethedesign of thinnoisebarriersarecreatedfourmodulesintheseriesScreen *,

whicharepart ofthesoftware packageSoundBG [6].

The modulesScreen H VarandScreenHLVarintendedfor a feasibilitystudy of thesite.WithScreen H Varcan be finded thedependenciesbetweentheefficiency of thebarrier

anditsheightanddistance to theassessmentpoint (Fig. 3) and/orheight ofthat point (Fig.

4).


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Fig. 3.ModuleScrren H Var, executedwith a

variabledistancebetweenthebarrierandassessmentpoint;the height oftheassessmentpointis 12.5 m.

Fig. 4.ModuleScrren H Var, executedwith a variableheight ofthe assessment point; the

distancebetweenthebarrierandassessmentpointis30 m.

The moduleScreenHLVarexpandsthepossibilitiesforpreliminaryanalysis.

Inninegraphsare showthedependenciesbetweentheparameters of thebarrier - efficiency,

height, length, andnoisecharacteristics of thenoisesource, theindex of

airbornesoundinsulationfromthesurroundingstructureandregulatorynoiselevel.

The module Screen Project calculatestherequiredheight of thebarrier

underspecificconditions: geometry, noise levelsintheprotectedarea, soundproofing of

thepremises. The calculation is carried outin transverse acoustic profiles (Fig. 5).

Fig. 5.Computiveacousticprofile[7]

As a result, for eachprofile aregiventhenecessaryheightandlength ofthebarrier.


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Fromthecalculatedvalues areobtainedthe finaldesignheights,

takingintoaccounttwoadditionalrequirements.

1.1. Sizesof the barrier mustbemultiples of thedimensions oftheelementsthatareassembledorconstructed, eg. panels withsizes0.5x3 m.

2.Theheight of thebarrier mustberelativelyconstantforlongstretches of theroute.The efficiency of the barrier with the adopted design sizes is assessed with module

Screen Project B, which performs by the same computing profiles. Depending on theresults, follows different solutions - to increase the design height, increasing the sound

insulation of the surrounding structure of the upper floors of buildings, etc.

*

Thedescribedtechnologywasappliedforthedesign of transport noise

barriersinthereconstruction of theoverheadroad to BrusselsblvdinSofia [7].

REFERENCES

[1] .. . : . , 2006. . 236.

[2] . . .:

. : -. 1986. 423 .

[3] .. ,

.. ,5, , 2009, . 224-

228.

[4] .. . . , No. 1, , 2011.

[5] 6 , ,

,

, , 58/2006.7.

[6] Nikolov N.D., Mazhdrakov, M.G., Benov D.M., Trapov G.I. Software

suiteforacousticalcalculationsSoundBG. 11th International GeoconferenceSGEM 2011,

Albena, Bulgaria, 2011.

[7] .., .., .., .. . , . . .

, , 2011, . 1.

design of transport noise barriers

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