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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Ecology and Environmental Protection
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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.