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CPP 301 Core Project Part IDESIGNING OF A NOISE BARRIER Submitted by Lalit Aggarwal &Gayathri Lakshmi Kulukuru Supervisor Dr. Navin Kumar School of Mechanical Materials & Energy EngineeringINDIAN INSTITUTE OF TECHNOLOGY ROPAR Nov 2012 ~1~
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CERTIFICATEThis report is submitted by Mr.Lalit Aggarwal &Ms.Gayathri Lakshmi Kulukuru detailingthe work done during the 1st semester, 2011-2012. The report written and all material takenfrom other sources(books, manuals, journals,etc.) have been fully acknowledged.Lalit Aggarwal Gayathri Lakshmi KulukuruP2009ME1085 P2009ME1062SMMEE, IIT Ropar SMMEE, IIT RoparDate: 09-Nov-2012 Date: 09-Nov-2012Mr. Lalit Aggarwal &Ms.Gayathri Lakshmi Kulukuru have worked under my supervisionduring this semester. I have read this report; it meets the expectations and it accuratelyreflects the work done by the students.Dr. Navin Kumar(Supervisor)Date: 08-Nov-2012 ~2~
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ACKNOWLEDGMENTSThe authors acknowledge the support of the Director, Defense and Research Organization(DRDO) for giving the opportunity to work on the project. The authors acknowledge theguidance acquired from the project work of Mr. SahilBagat performed in the previous year.The authors acknowledge the support of their Project advisor Dr. Naveen Kumar for theircontinuous guidance and support throughout the semester.Lalit Aggarwal Gayathri Lakshmi KulukuruP2009ME1085 P2009ME1062SMMEE, IIT Ropar SMMEE, IIT Ropar ~3~
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ABSTRACTThe project which was given to us addressed the investigation of the technical, aesthetic, andeconomic feasibility of deploying special noise barrier application into baffle range ofDRDO.So we have to suggest a most suitable design of noise and maximum sound attenuation can beachieved under having some constraints like cost, weight and aesthetic requirements.Sound attenuation in noise barrier depends on various parameters like (height, type ofmaterial, top surface modification, receiver or source position and several other parameters,barrier shapes).There is a threefold process in which we are going to do this project.1. Literature Survey and Theory.2. Modeling and calculating Results.3. Make prototypes and Verifying Results.In Literature Survey in which there a cumulative study about the past design, all the researchdone in the field and all of the different possibility is there in designing. In order tounderstand the physics behind it thorough study about the theory of acoustics design is alsorequired.In Modeling, Using Sysnoise software the problem will be modeled and result will beobtained and compared with the theoretical results. Simple barrier can be solved analyticallyusing empirical relations but advanced barrier needs BEM or Some kind of simulationsoftware (SYSNOISE). The simulation will help in estimating the behavior of the noisebarrier under different parameters e.g.- changes in the barrier parameter, change in materialand other parameters.In making Prototype and verifying results, there is series of experiments have to beperformed to see the practical behavior of the parameters involved in the formulation of theproblem which is performed using microphone and other devices.Then all the results will be analyzed and final design will be made.In this report we continued our work after internship work from literature survey to modelingin matlab and taking physical readings from existing wall and on wooden barrier. Then wecompared this reading with problem formulated in Matlab and with software output. Softwarewe used was Oliver Lab Terrain. ~4~
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TABLE OF CONTENTSABSTRACT ........................................................................................................................... 4Problem statement and motivation ........................................................................................ 7INTRODUCTION .................................................................................................................. 7REVIEW OF PAST WORK.................................................................................................... 8 Conclusion from literature ............................................................................................ 12Work done in this semester ................................................................................................. 13 Experiment work .............................................................................................................. 13 MATLAB Programming .................................................................................................... 15 MODELLING IN OLIVE LAB TERRAIN ........................................................................... 18 Comparison of results with experimental results .............................................................. 22Future Work ........................................................................................................................ 23 References ...................................................................................................................... 23 ~5~
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LIST OF TABLES & FIGURESTable 1 Literature related to insertion losses in a barrier sorted based on the parametersbeing varied .......................................................................................................................... 9Figure 1 Noise barriers with various shape edges and surface conditions............................. 8Figure 2 Schematic of the experiment performed ................................................................ 13Figure 3 Experimental Setup for the soft ground ................................................................ 13Figure 4 Experiment setup for hard ground ......................................................................... 14Figure 5Definitions of symbols used to determine Fresnel Number ‘N’ ................................ 15Figure 6Paths considered in Lam Method ........................................................................... 16Figure 7 Flowchart of the matlab code ................................................................................ 16Figure 8 Matlab GUI ............................................................................................................ 17Figure 9 Comparison of insertion losses with change in height at different frequencies ...... 18Figure 10Comparison of insertion losses with frequency variation at different heights ........ 19Figure 11 Variation of thickness .......................................................................................... 20Figure 12 a) single panel barrier b) double panel barrier ......................................... 20Figure 13 IL variation in single and double panels (frequency=1000hz) .............................. 21Figure 14 IL variation in single and double panels (frequency=2000hz) .............................. 21Figure 15Comparision of matlab and software results with experimental results at 500 Hertz........................................................................................................................................... 22Figure 16Comparisonof MATLAB and software results with Experimental results at 1000Hertz ................................................................................................................................... 22 ~6~
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PROBLEM STATEMENT AND MOTIVATIONSpecific Aim: To design an effective and cost-efficient noise barrier for the baffle rangewhich can reduce the unwanted noise up to the required level.Usually firing ranges where soldiers are trained for the shooting at different conditions aremade out of the city so that there was no disturbance to the local residents. But some of the ranges arewithin the residential areas where common people feel a lot of noise. So, to get the control over thistype of noise a noise barrier need to be designed around the range to reduce the noise up to a desirablelevel. INTRODUCTIONIncreasing noise pollution will lead to an ever increasing need to control noise of all forms.Noise barriers are the most common solution for controlling noise from surroundings andseveral methods have been developed for improving their efficiency without increasing theirheight.Examples of already deployed noise barriers in India:1. Noise Barriers in BKC, Mumbai (2010)2. IIT Powai noise barrier (2012)3. Commonwealth Games noise barrier (2010)4. Sound barriers at Suman Nagar, Navghar flyovers (2012)The vast majority of these have been vertical, reflective wall made of concrete, wood or steel.The standard top for these walls is a “knife-edge”, providing a single diffraction edge with areflective diffraction zone.Clearly, there are many other options for noise barrier shapes than vertical reflective wallswith knife-edge diffraction zone. In addition to it, there are option to make barriersabsorptive, to displace the diffraction zone through the use of a slanted section on top, or toprovide for a double- diffraction zone through the use of a T-top section and other modifiedtops of the walls. ~7~
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REVIEW OF PAST WORKDuring summer internship most of the literature work was performed and all the work ispresented in the tabular form in table 1 and table 2. During this time also complete study wasperformed and the entire phenomenon was studied.Different types of barrier that were studied are: - Shaped Barrier - Conventional Barrier - T-shaped barrier - Multiple Edge Barrier - Arrow shape BarrierFigure 1 illustrates various noise barriers obtained by varying the shape edges and surfaceconditions. FIGURE 1: NOISE BARRIERS W ITH VARIOUS SHAPE EDGES AND SURFACE CONDITIONS ~8~
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TABLE 1LITERATURE RELATED TO INSERTION LOSSES IN A BARRIER SORTED BASED ON THE PARAMETERS BEING VARIEDDifferentbarrier Simple T-top y-top Cylindrical Wedge Multipl Angled edgeParameter top shaped e edgeBarrier height D. C. David J. HOTHERSA Oldham , LL, S. N. Christopher A. CHANDLER Egan (2011) -WILDE AND M. N. P.J. Thorsson HAJMIRZAE (2003) (1990) Takashi Ishizuka*, Kyoji Fujiwara (2003)Change in David J. D J Oldham and C A Egansource position Oldham , (2009) Christopher A. Egan (2011) D. C. D. C. D. C. HOTHERSA HOTHERSALL HOTHERReceiver height LL, S. N. , S. N. SALL, S. CHANDLER CHANDLER- N. -WILDE WILDE AND CHANDL AND M. N. M. N. ER- HAJMIRZAE HAJMIRZAE(1 WILDE (1990) 990) AND M. N. HAJMIR ZAE(1990 )Change in A. Muradali David J. D. C. C.A.receiver and K. R. Oldham , HOTHER Egan, Vposition Fyfe (1994) Christopher A. SALL, S. Chilekw Egan (2011) N. a and D. D. C. D. C. CHANDL J. HOTHERSA Hothersall, D. ER- Oldham LL, S. N. H. Crombie & WILDE (2006) CHANDLER S. N. Chandler- AND M. -WILDE Wilde N. AND M. N. HAJMIR HAJMIRZAE C.A. Egan, V ZAE(1990 (1990) Chilekwa and ) D. J. Oldham(2006) D. C. HOTHERSALL , S. N. CHANDLER- WILDE AND M. N. ~9~
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HAJMIRZAE(1 990)Width of top David J. D. H. Oldham , Crombi Christopher A. e, D. C. Egan (2011) Hothers all& S. D. C. N. Hothersall, D. Chandle H. Crombie & r-Wilde S. N. Chandler- (1994) Wilde C.A. Egan, V C.A. Egan, V Chilekw Chilekwa and a and D. D. J. J. Oldham(2006) Oldham (2006)Cap depth D. C. TomonaoO D. H. Hothersall, D. kuboa) and Crombi H. Crombie & Kyoji e, D. C. S. N. Chandler- Fujiwara Hothers Wilde all& S. P.J. Thorsson N. (2003) Chandle r-Wilde (1994) D. C. D J Oldham and C A EganAngle variation HOTHER (2009) SALL, S. N. CHANDL ER- WILDE AND M. N. HAJMIR ZAE(1990 ) D. C. Takashi Hothersall, D. Takashi D. C. Takashi Takashi Ishizuka*, KyojiComparison Ishizuka*, H. Crombie & Ishizuka*, HOTHER Ishizuka Fujiwara (2003)with T-top Kyoji S. N. Chandler- Kyoji SALL, S. *, Kyoji Fujiwara Wilde(1990) Fujiwara N. Fujiwar (2003) (2003) CHANDL a (2003) Takashi ER- D. C. Ishizuka*, WILDE HOTHERSA Kyoji Fujiwara AND M. LL, S. N. (2003) N. CHANDLER HAJMIR -WILDE ZAE AND M. N. (1990) HAJMIRZA E (1990) ~ 10 ~
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barrier surface David J. David J. D. C. D. H. D J Oldham and C A Eganvariation Oldham , Oldham , HOTHER Crombi (2009) Christopher Christopher A. SALL, S. e, D. C. A. Egan Egan (2011) N. Hothers (2011) CHANDL all& S. D. C. P.J. Thorsson ER- N. HOTHERSA (2003) WILDE Chandle LL, S. N. AND M. r- CHANDLER N. Wilde(1 -WILDE HAJMIR 994) AND M. N. ZAE(1990 HAJMIRZA ) E(1990) top surface David J. D. C. D. H. D J Oldham and C A Eganabsorption Oldham , Hothersall, D. Crombi (2009)variation Christopher A. H. Crombie & Takashi e, D. C. Egan (2011) S. N. Chandler- Ishizuka*, Hothers Wilde (1990) Kyoji all& S. D. C. Fujiwara N. Hothersall, D. (2003) Chandle H. Crombie & r- S. N. Chandler- Wilde(1 Wilde 994) C.A. Egan, V C.A. Chilekwa and Egan, V D. J. Chilekw Oldham(2006) a and D. J. D. C. Oldham HOTHERSALL (2006) , S. N. CHANDLER- WILDE AND M. N. HAJMIRZAE(1 990) M.R. Monazzam, Y.W. Lam(2006)Ground surface Muradali and D. C.variation K. R. Fyfe Hothersall, D. (1994) H. Crombie & S. N. Chandler- D. C. Wilde HOTHERSA LL, S. N. P.J. Thorsson CHANDLER (2003) -WILDE AND M. N. HAJMIRZA E(1990) ~ 11 ~
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MahdiyehNaderPerforated zadeh a,⇑,sheet on Mohammaddiffuser Reza Monazzam b, ParvinNassiri b, SamanehMome nBellahFard (2011)QRD on top M.R. Monazzam, Y.W. Lam(2006) MahdiyehNader zadeh a,⇑, Mohammad Reza Monazzam b, ParvinNassiri b, SamanehMome nBellahFard(20 11)Soft TomonaoO kuboa) and Kyoji FujiwaraReflective top D. H. D J Oldham and C A Crombi Egan(2009) e, D. C. Hothers all& S. N. Chandle r- Wilde(1 994) CONCLUSION FROM LITERATURE Going through all the research papers and by seeing their comparative study, it was vaguely suggested that T-type barrier having absorptive coating suits best to get maximum sound abatement. Since for a single barrier height/cost ration peaks at 3meter so height of T-top barrier should be taken 3meter and width of the top can be taken as 1 meter so that aesthetically is looks good and having maximum sound abatement. Till internship the project was not having any practical touch and in this research paper experiments were conducted and results was compared to get the real and practical touch ~ 12 ~
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WORK DONE IN THIS SEMESTER EXPERIMENT WORK SCHEMATIC FIGURE 2: SCHEMATIC OF THE EXPERIMENT PERFORMEDFigure 2 shows the basic schematics of the experiments that were performed and it showsvarious parameter that is involved during calculations. EXPERIMENT ON SOFT GROUND FOR AN EXISTING WALL FIGURE 3 : EXPERIMENTAL SETUP FOR THE SOFT GROUND ~ 13 ~
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Figure 3 shows the experiment that was performed on an existing wall at the fuel-zap in IITRopar to get the basic insight of the insertion loss values obtained. The material of the wall isconcrete with a height of 170 cm and thickness of 22 cm. The parameters that are varied inthis experiment are frequency (varied between 100-1000Hz at an interval of 100 Hz) andreceiver distance (varied between 3-15m from the wall) while keeping all other parametersconstant. The source height and receiver height are kept in the shadow region with values of50 cm and 100 cm respectively. EXPERIMENT ON HARD GROUND FIGURE 4EXPERIMENT SETUP FOR HARD GROUNDThe experiment was performed on a finite barrier on hard ground to enable comparisons asmost of the analytical solutions are for hard ground. Figure 4 shows the experimental setup ofthe experiment. The material of the barrier is wood with a height of 90 cm, width of 108 cmand thickness of 2 cm. The parameters that are varied in this experiment are frequency(varied between 100-1000Hz at an interval of 100 Hz) and receiver distance (varied between0-16m from the barrier) while keeping all other parameters constant. The source height andreceiver height are kept in the shadow region with values of 50 cm and 83 cm respectively. ~ 14 ~
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MATLAB PROGRAMMING THEORY: LAM’S METHOD USING MAEKAWA’S CURVEMaekawa introduced an empirically based diffraction model that provides the insertion lossdue to a thin-walled barrier in terms of the Fresnel number.Maekawa then suggested that theinsertion loss for a finite-length barrier could be determined by multiple application of thiscurve to the diffraction paths around the barrier and then summing the energy contributionsof these paths. Maekawa‟s curve can be represented by the following two equations: where N is Fresnel Number given byWhere (A+B-d) is the path difference and λ is the wavelength. The symbols are defined asshown in the Figure 5. FIGURE 5 DEFINITIONS OF SYMBOLS USED TO DETERMINE FRESNEL NUMBER ‘N’Lam improved on Maekawa‟s method by summing complex pressures, instead of energies, ofeach diffraction path around the barrier. This was done to incorporate the phase interactionand interference between the paths, the absence of which, Lam suggested, was the cause ofthe poor agreement between Maekawa‟s method and experimental results.A semi-infinite barrier is equivalent to a 2D geometry. Diffraction Paths considered for asemi-infinite thin barrier are as shown in Figure 6. ~ 15 ~
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FIGURE 6 PATHS CONSIDERED IN LAM METHOD The barrier insertion loss is given by:Mi represents the insertion loss value from Maekawa‟s curve for the ithpath. The subscript „o‟refers to the direct path (from the source to receiver) and the subscript „r‟ refers to the groundreflected path (from the source image to receiver).The Lam method fell short when the receivers were in the proximity of the line-of-sight, andwhen parallel geometries in 2D were considered. This is due to the fact that this method doesnot predict a unique phase shift at the diffraction edge for each path. GUI IMPLEMENTATION FIGURE 7: FLOWCHART OF THE MATLAB CODE ~ 16 ~
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A MATLAB GUI has been made for calculating the insertion loss. It includes analysis for aninput data.A GUI (graphical user interface) allows users to perform tasks interactively through controlssuch as buttons and sliders. Within MATLAB®, GUI tools enable you to perform tasks suchas creating and customizing plots, fitting curves and surfaces, and analyzing and filteringsignals.Figure 8 shows the typical GUI that was modeled in the Matlab using Lam‟s equation.Parameters that were involved in GUI are barrier height, Source and receiver height, sourceand receiver distance, and frequency of the sound.It also shows the graphical variation of the variation in one parameter by taking 5 otherparameter constant and vary 6th one. FIGURE 8 MATLAB GUI ~ 17 ~
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MODELLING IN OLIVE LAB TERRAIN ABOUT THE SOFTWARETHEORYThe acoustic calculations are made by the software based on Hadden& Pierce Diffraction 3Dmodel implemented with finite impedances faces using Salomons semi-analytical methodincluding ground effects. Multiple barrier diffraction is calculated in a recursive way at anydiffraction order.Ground effect is included using the One Parameter Theory of Chessell basedon Delany and Bazley.LIMITATIONSThe thickness of the barrier cannot be reduced to a value less than 3cm. The numerical valuesof the readings are available only at octave and 1/3rd octave frequencies. Any other parameterexcept frequency cannot be varied in the same model. ANALYSIS MADE VARIATION OF HEIGHT PARAMETERS Barrier Material: mineral wool Barrier type: Thin Barrier Receiver Height= 50 cm Source Height= 50cm Source distance=5m 35 30 Insertion Loss(in dB) 25 20 1000 Hz 15 1995 Hz 10 3981 Hz 5 0 0 1 2 3 4 5 Barrier height(in m) FIGURE 9: COMPARISON OF INSERTION LOSSES WITH CHANGE IN HEIGHT AT DIFFERENT FREQUENCIES ~ 18 ~
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35 30 25 Insertion Loss(in dB) 20 h=1m h=2m 15 h=3m 10 h=4m 5 0 2 2.5 3 3.5 4 -5 log(frequency(in Hz)) FIGURE 10: COMPARISON OF INSERTION LOSSES WITH FREQUENCY VARIATION AT DIFFERENT HEIGHTSIt can be seen that the insertion loss increases with increase of barrier height. From the figure7, it can be seen that the variation of the insertion loss values is not much when it is changedfrom a height of 3m to 4m. VARIATION OF THICKNESSMASS LAWWhen sound is incident upon a wall or partition, some of it will be reflected and some will betransmitted through the wall. The transmission loss obtained can be determined using masslaw at a particular frequency.Where m=mass density and f=frequency PARAMETERS Barrier Material: mineral wool Flow resistivity=20000 Pas/m2 Barrier Height = 2m Receiver Height= 1 m Source Height = 1 m Source distance=2m ~ 19 ~
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Frequency=1500 Hz 30 25 20 Insertion Loss(in dB) thickness_3cm 15 thickness_3.5cm thickness_4cm 10 thickness_4.5cm 5 0 0 10 20 30 40 50 60 -5 Receiver distance (in m) FIGURE 11 VARIATION OF THICKNESSFrom figure 8 it can be clearly seen that the value of insertion loss doesn‟t vary much withthe receiver distance for thickness values greater than the optimum value calculated from themass law (≈2.5cm). SINGLE VS DOUBLE PANELSFIGURE 12 A) SINGLE PANEL BARRIER B) DOUBLE PANEL BARRIERPARAMETERSBarrier Material: mineral woolFlow resistivity 20000 Pas/m2Barrier Height 2mBarrier Thickness 20cm single 10 cm doubleReceiver Height 1mSource Height 1mSource distance 2m ~ 20 ~
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45 Frequency = 1000 Hz 40 35 single panel 30 25 double Insertion Loss(in dB) 20 panel_gap20cm 15 double panel_gap 30cm 10 5 double panel_gap50cm 0 0 2 4 6 8 10 12 Receiver distance(in m) FIGURE 13 IL VARIATION IN SINGLE AND DOUBLE PANELS (FREQUENCY=1000HZ) 45 40 35 Frequency = 2000 Hz Insertion Loss(in dB) 30 single panel 25 double panel gap 20 20 cm double panel_gap 15 30cm 10 double panel_gap50cm 5 0 0 2 4 6 8 10 12 Receiver distance (in m) FIGURE 14 IL VARIATION IN SINGLE AND DOUBLE PANELS (FREQUENCY=2000HZ)From the figures 13 and 14 it can be clearly seen that double panel barriers are more effectivein sound reduction compared to single panel barrier and the insertion loss increases withincrease in gap between the two panels. ~ 21 ~
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COMPARISON OF RESULTS WITH EXPERIMENTAL RESULTSPARAMETERSBarrier Material: woodBarrier Height=90 cmBarrier Width=105 cmBarrier Thickness= 2cmReceiver Height= 80 cmSource Height= 50 cm FIGURE 15 COMPARISION OF MATLAB AND SOFTWARE RESULTS WITH EXPERIMENTAL RESULTS AT 500 HERTZ FIGURE 16COMPARISONOF MATLAB AND SOFTWARE RESULTSWITH EXPERIMENTAL RESULTSAT 1000 HERTZFrom the figures 15 and 16 it can be seen that the results from the software are in coherencewith those of the experimental results within acceptable error limits. It can also be seen thereis a considerable variation in the results obtained from MATLAB program, the reasons ofwhich can be attributed to the assumptions made in the theoretical model, where it considersthe barrier is semi-infinite with negligible thickness and which doesn‟t include the effect ofthe material of the barrier. ~ 22 ~
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FUTURE WORKSome part of the project is completed in B.Tech-1 project which was in this semester and thisproject will be continued in coming semester also as B.Tech-2 project.The things which are planned for coming semester are: 1. In current work, Material selection for the barrier was not suggested so work will be done in this context in the upcoming semester. 2. In current work, output from the software of single and double panels was compared but there physical modelling was not done. It will be includedin the further studies. 3. The software which is presently used has certain limitations due to which one cannot vary the shape of the barrier. The new software SYSNOISE, which is BEM/FEM software, was purchased for getting more accurate results and help in physical modelling of the system. 4. In current study the MATLAB formulations are donefor a semi-infinite, thin barrier. In 2nd part the more focus will be made in including parameters like thickness, finiteness, etc. 5. In our next work formation of the optimization problem will be included and will be solved using different optimization techniques. REFERENCES Web site reference 1. http://sciencedirect.com/ 2. http://www.acoustax.com/noise-barrier-specs.php 3. http://www.acousticalsurfaces.com/wall_barrier/wall_barrier.htm 4. http://www.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/85-sound-tranmission.html 5. http://articles.timesofindia.indiatimes.com/2012-05-17/mumbai/31747986_1_noise- barrier-noise-levels-sumaira-abdulali 6. http://www.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/85-sound-tranmission.html 7. http://www.otlterrain.com/Research Papers [1] R.O.Feher, proc.Ann.Nat. Noise Abatement Symp.,1951,p-98 [2] A.Muradali and K.R Fyfe,A study of 2d and 3d barrier insertion loss using improved diffraction based methods, applied acoustics,vol.53,no-1-3,pp 49-75,1998 [3] D. C. HOTHERSALL, S. N. CHANDLER-WILDE AND M. N. HAJMIRZAE, Efficiency of Single Noise barrier, Journal of Sound and Vibration (1991) 146(2), 303-322. ~ 23 ~
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[4] David J. Oldham , Christopher A. Egan.. A parametric investigation of the performance of T-profiled highway noise barriers.. Applied Acoustics 72 (2011) 803–813[5] C.A. Egan, V Chilekwa and D. J. Oldham, Top edge treatment to enhance the performance of a noise,Acoustics Research Unit, University of LiverpoolLiverpool, L69 3BX, United Kingdom[6] Watts, G.R., Barrier design to reduce road traffic noise. Proceedings of the Institution of Civil Engineers, 2002. 53(2): p. 79- 86.[7] Takashi Ishizuka, KyojiFujiwara,Performance of noise barriers with various edge shapes and acoustical conditions.. Applied Acoustics 65 (2004) 125–141.[8] MahdiyehNaderzadeh, Mohammad Reza Monazzam, ParvinNassiri, SamanehMomenBellahFard, Application of perforated sheets to improve the efficiency of reactive profilednoise barriers, Applied Acoustics 72 (2011) 393–398.[9] A. Muradali and K. R. Fyfe, A Study of 2D and 3D Barrier Insertion Loss using Improved Diffraction-based Methods, Applied Acoustics, Vol. 53, No. I-3, pp. 49- 15, 1998[10] Maekawa, Z., Noise reduction by screens. Applied Acoustics, 1968, 1, 157- 173.[11] Pontus J. Thorsson,Combined effects of admittance optimisationon both barrier and ground, Applied Acoustics 64 (2003) 693–711.[12] D J Oldham and C A Egan, The development of a practical top edge device for a noise barrier, 16th international congress on noise and vibration July 2009. ~ 24 ~
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