Evaluation of pavement noise emission model : A case study in Tehran

Noise pollution and its influence on environment and life quality of human beings may be considered as a hot topic in scientific research. Since there are no sufficient studies on noise pollution in Tehran, this study was carried out to determine noise levels of pavement in Tehran City. SMM1 (The first standard of noise measurement method) model is one of the most common models used in calculating noise emission in Tehran City. Because factors which this model uses are close to Tehran traffic situation, in this study, SMM1 noise emission model has been modified by including pavement statement factor. In other words, the pavement age will affect noise emission in environment. Thus, a case-study is chosen and its pavement statement factor is added to SMM1 noise emission model. Equivalent noise level changes in different distances with sound noise. This effect is calculated and indicated in this paper. Another factor that could affect the sound level is traffic volume, which is also shown in this paper. In this study, it is concluded that when asphalt pavement is fresh and new, equivalent noise level is 2 (dB) less than when the pavement becomes old due to congestion of pavement and its decreasing void. The results showed that pavement noise should be mentioned among the major environmental problems, and studies aim at preventing it should have great priority.


INTRODUCTION
Sound is a physical phenomenon that is caused by air pressure waves, which emanate from a vibrating source, and alerts the hearing sense of human beings.While sound is an energy type that emits waves, noise is simply an unwanted sound.The magnitude of the highway traffic noise is analyzed in this study.Discrete sources of noise from industrial units like factories, construction, aircrafts or vehicles in a flowing traffic, turn into a linear source, the most stable of all known random noise sources in the city (Turgut and Zübeyde, 2012).Like other engineering problems, there are two major ways of analyzing the problem of noise emissions in residential areas.One way is by using sound level meters (SLMs) in the study area by which the level of the generated sound in the area is directly measured and the necessary considerations will be thought.Using SLMs is a very precise technique for studying sound level in the area.On the other hand, sound measuring in each certain area would take a considerable time and be costly (Golmohammadi et al., 2007).This is because the level of noise emitted by vehicles in streets varies depending on traffic condition and such parameters should be considered in measurements.Noise pollution is recognized as a major problem for the quality of life in urban areas all over the world ((ElifEbru and Emin, 2011;Olayinka, 2012).Because of the increase in the number of cars and industrialization, noise pollution has also increased.Noise in cities, especially along main arteries, has reached up disturbing levels.Residences far from noise sources and near silent secondary roads are currently very popular.People prefer to live in places far from noisy urban areas (Serkan et al., 2009).
Another way used to measure noise pollution in a certain area is employing noise pollution model, which indeed is a mathematical technique.Mostly traffic-based parameters, which are very diverse, are used as the model's inputs.There are many factors that may affect sound emission in the space; hence, numerous models have been developed in this area so far.Some of these models are described in this reference (Garg and Maji, 2014).Most of them are physical factors which cover sound properties.Measuring and examining such parameters is very difficult and complicated.However, other parameters including traffic-related ones such as velocity of vehicles and traffic flow are measured very easily (Ranjbar et al., 2012).
First attempts of making a traffic noise prediction can be collocated into 1950/1960 decades; they mainly evaluate the percentile L 50 , defined as the sound level exceeded by the signal in 50% of the measurement period.These models refer principally to a fluid continuous flux, considering a common constant velocity with no distinction between vehicle typologies.One of the first models, developed in 1952, is the one reported in Handbook of Acoustic Noise Control (Johnson and Saunders, 1968).This model states that the 50 percentile of traffic noise for speed of 35-45 mph (about 55-75 Km/h) and distances greater than 20 feet (about 6 m) are given by:  (Department of Transport, 2008).It estimates the basic noise level L 10 both at 1h and 18 h reference time.This level is obtained at a reference distance of 10 m from the nearest carriage way edge of the highway.The parameters involved in this model are: traffic flow and composition, mean speed, gradientof the road and type of road surface.The basic hypotheses of the model are a moderate wind velocity and a dried road surface.Operatively, the basic hourly noise level is predicted at a distance of 10 m from the nearest carriage way, according to the following equation: (2) and the basic noise level in terms of total 18-hour flow is: Where q and Q are the hourly traffic flow (vehicles/hour) and 18 h flow (vehicles/hour), respectively.Here it is assumed that the basic velocity is v = 75 km/h; the percentage of heavy vehicles is P = 0 and road's gradient is G = 0%.It is also assumed that the source line is 3.5 m from the edge of the road for carriage ways separated by less than 5.0 m.Tire/pavement interaction is not the only source of traffic noise, but it is the dominant component for passenger cars traveling at highway speeds (McNerney et al., 2000).Reducing traffic noise at the source by placing a noise reducing pavement surface is becoming an attractive option for many highway agencies.A noise reducing surface is defined as "a road surface which, when interacting with a rolling tire, influences vehicle noise by causing at least 3 dB (A) lower vehicle noise than what is obtained on conventional and most common road surfaces" (Sandberg and Ejsmont, 2002).Literature shows that open graded asphalt mixes in Europe can reduce traffic noise (Sandberg, 2005;Colwill et al., 1993;Camomilla et al., 1990).
Tire/pavement noise is evidently affected by the characteristics of the tires and the pavement.Only the latter factor is addressed in this paper.The physical properties of the materials that constitute the upper structural layer of the pavement play a major role in the generation of noise.Pavement mixes with higher air void contents porosity are known to reduce noise levels.There are two noise reduction mechanisms for open graded (porous) surfaces: noise absorption and noise propagation.The presence of air voids in the surface layer helps dissipate trapped air in the tire's tread grooves.This results in reduced air pumping, and therefore decreased noise emissions (Nelson, 1994).Porosity also gives the pavement acoustical absorption properties where sound waves are dissipated into heat within the voids of the surface layer (Sandberg and Ejsmont, 2002).This study tried to analyze a traffic model developed in Germany.The effect of asphalt life span on noise emission through asphalt was considered as the pavement correction factor.Its proper adaptability with the studied area, which is a street in Tehran City, is why this model and its correction factor are used.
A mathematical model was used to design noise emissions level around the studied traffic axis.This model is useful for measuring level of equivalent noise or environmental noise pollution level through road traffic standards and various types of vehicles that cross the mentioned axis.For this model, various parameters were studied and applied which will be pointed out as follows.First of all, there is need to show you the general equation of equivalent noise level (Ranjbar, 2012): In this equation, the sole unknown factor, which is important for this study, is equivalent sound level in decibel, which is represented by symbol L eq .Other parameters defined in other sides of the equation are the ones that can be measured regarding the traffic factors.All parameters of the equations are summarized in Table 1.For road surface, Table 3, which concerns the pavement characteristic and as a factor used in the SMM1 model, was used.For the other factors in the model, the parameter that is described in Table 2 was used.As the table indicates, there is a main parameter, represented by symbol E, for measuring noise level.Other parameters are applied to reform this parameter.In the equation E, there are three noise related parameters for different sorts of vehicles including light, medium and heavy.The equation designed for these three parameters is (Ranjbar et al., 2012): Where, l v , m v and h v indexes are used for light, medium and heavy vehicles, respectively.V represents velocity, while Q is used to represent traffic flow in a certain time period.E is the equivalent noise level for each class of vehicle.
At the end of these equations, there is a factor which is the noise correction regarding the type of pavement.For the standard model, the factor has been defined separately for each class of pavements.However, what is complicated is the pavement lifespan, which can be an effective factor in pavement porosity and hence the sound emission level through pavement (Hampshire, 2009).In this study, we tried to achieve a certain level of correction through analyzing the pavement lifespan effect in noise emission using as a case study one of Tehran City streets.It helps the designed model to function more precisely.

METHODOLOGY
As already mentioned, this study focuses on the noise on pavements with a model that has a parameter that is related to asphalt pavements on highways.When possible, noise emission from light and heavy vehicles is analyzed separately.The areas for pavement and noise investigation are shown in Figures 1 and 2. Azadegan Expressway has been aligned from Northwest to Southeast of Tehran City.The expressway reaches Tehran-Karaj Freeway from Northwest and ends at Afsariyeh (Basij-e Mostaza'fin) three-way from Southeast.After crossing Tehran-Karaj Freeway, Azadegan Expressway in its path towards southeast meets the following highways: ShahidLashgari Expressway, Fat'h Highway, Tehran-Saveh Freeway, Ayatollah Saeedi Highway (Saveh Road), Khalije Fars Freeway (Tehran-Qom) at Jahad Square, Behesht-e Zahra Highway, ShahidRajaie Highway, Dowlatabad Highway (under construction) and Afsariyeh Three-way (Basij-e Mostazafin).Thus, the freeway is connected to Shahr-e Rey via ShahidRajaie Highway.Azadegan Freeway is located in North to South Zone and is stretched from Northwest to Southeast of Tehran.According to the traffic terminology, it is a super highway which is among the entrance and exit ways of Tehran.Azadegan Expressway is connected to Shahre-Rey through Shahid Rajaie Highway in order to generate a somehow fundamental way for passengers who intend to leave Tehran.Its traffic load is very heavy in vacations and weekends (Wikipedia, 2014).
This study aims at analyzing the effect of road pavement on how noise pollution is emitted.Reduction of empty space in pavement due to compression over time is the main reason why sound emissions patterns are changed after pavement installation (Department of Transportation and Road of Tehran, 2007).When there is wider empty space in the asphalt, the noise generated from tire rolling over the road pavement as well as that generated by the car's engine, which reaches the pavement surface, is trapped in the pavement's empty space, leading to a considerable loss of its energy.Now, if the empty space within the road pavement becomes smaller due to heavy car traffics and more compressed pavement, the sound wave will be further echoed and emitted around after colliding with the pavement surface.Accordingly, scientists started to fabricate a type of pavement with more porosity that can keep higher levels of sound.As a result, they have to invent porous asphalt, which indeed is very useful and practical in residential regions (Jalilzadeh et al., 2004).
Therefore, it can be concluded that both time and more durable pavements can affect the noise emissions level in the peripheral environment.The German standard sound emission model, that is, SMM1 will be analyzed in this study and pavement lifespan effect coefficient on the model will be considered in its coefficients based on the surveys made in the studied area.The interchange between Sattari Street and Azadegan Expressway in Tehran City was chosen as our study area.As mentioned before, this area was chosen because of applying new pavement for repairing a part of the street during this study.Therefore, comparing simultaneously the noise emission level in the repaired and unrepaired areas is very easy.As a result, the effect of new asphalt on sound emission can be studied (Figures 1 and 2).

RESULTS
The computational algorithm sample of the model is described in Table 2.For the case study area, all the parameters are explained, and for pavement age, Table 3 and the technical data in Figure 3 are used.The

Parameter
Equation Definition         and Figure 3.After few days of sampling date, we again referred to the site for analyzing the noise generated by the asphalt.It let us compare the new asphalt and old one quantitatively.Therefore, we again referred to the asphalt site on 6/26/2014 and measured the sound; the data gained were analyzed.

DISCUSSION
Road surface corrections are seen in the correction applied over the sound emission model, SMM1.The results of the studies are summarized in Table 4.We examined in this study the difference caused by correcting the old asphalt and replacing it with new one.It was due to more porosity in the new asphalt layer.Over time and heavy traffic flows make the pavement surface compressed and as a result its porosity percentage decreased.It continues until the current asphalt is replaced with a new one.In other words, the environment's noise pollution increases whenever the asphalt layer is compressed under vehicles' tires.However, one should note that the increasing trend of noise pollution is stronger in the initial phases of asphalt lifespan than its final phases; because initial phases of utilization are accompanied with more pounding and compression of the layer compared to the phases the layer experiences in final days.
Figure 4 shows two sets of measurements carried out in the site.The blue-colored diagram indicates the news asphalt, which has been spread and utilized recently, while the red one reflects the band with the old pavement, which has not been repaired so far.As it is clear, the average sound vibrations are lesser on the new asphalt in comparison to the old one.For the old pavement the average sound level or L eq is equal to 73 dB, whose maximum measure level stands at 97.1 dB.Compared to the other side of the street where a new pavement has been applied, the average noise level measured 69.9 dB with the maximum rate of 88.8 dB.
It can be seen that the new asphalt and the type of the asphalt used in the road are effective in minimizing noise pollution.On the other hand, the red curve shows the noise generated by the vehicles used the old asphalt.In this regional comparison, it became evident that the noise pollution decreased about by 3 dB, which is a  considerable amount, using the new asphalt.The decreased noise pollution in the environment can be assigned to more porosity in the new asphalt.Over time and due to using more vehicle of the street surface, the compression of the used pavement increased and hence its porosity decreased.Asphalt's pores trap the wave, generated by the engine, and also the noise, caused by rolling of tires, in the pavement, wasting its energy considerably.It is very effective in decreasing noise pollution.shows the 1 m distance to the sound origin, while the lowest line represents the 10-m distance to the noise origin, that is, axis of the street.As the diagrams suggest, noise initially is decreased considerably when the distance is increased, but when the distance is further increased, then the noise decline is ebbed away.However, lower noise is generated when the traffic load is increased and the average velocity is decreased.It suggests that not only number of vehicles, but also their velocity play a key role in generating noise; in that when the traffic load exceeds the standard capacity of the street, the generated noise is more despite fewer vehicles in the street.In this research, in order to reduce the computation time, the noise standard German first method was used, given the parameters of the mathematical method used to calculate the mean of the noise reduction parameters.The accuracy of the method of calculation of noise is obtained with the field testing.
Eventually, it can be concluded that there are various parameters than can affect the noise generated through the streets.Of them, the most important ones are type of pavement, asphalt lifespan, and distance to the voice origin.These parameters were used in the mathematical model of the study and tests carried out in the site confirmed the accuracy of the model.However, future studies can examine the exact effect of asphalt lifespan on noise emission in terms of mathematical viewpoint and when it can be considered serious in the noise pollution studies.It is necessary to do sufficient studies on traffic noise.
traffic volume in vehicles per hour and d is the distance from observation point to center of the traffic lane, in feet; no specification is included about vehicles and types of roads.The CoRTN procedure (Calculation of Road Traffic Noise) was developed by the Transport and Road Research Laboratory and Department of Transport of the United Kingdom in 1975 and was modified in 1988

Figure 2 .
Figure 2. Point of noise measuring in the study.

Figure 3 .
Figure 3. Grading diagram for asphalt mixed materials.

Figure 4 .
Figure 4. Observed noise level in studied area.

Figure 5 .
Figure 5. Equivalent noise level in different distances.

Figure 6 .
Figure 6.Equivalent noise level in different traffic volumes.
Figures 5 and 6 show how noise is emitted by various distances and traffic loads.The first figure indicates the ratio of equivalent noise level to different traffic loads.Parallel lines in this figure represent different distance to the sound origin.In this figure, the highest diagram

Figure 5
Figure5shows the relationship between distance and noise level.As the figure shows, there is a reverse relation between distance and noise level.Different lines of the diagram have been designed to show different traffic flows of the model.The highest line indicates a traffic flow with at least 4000 vehicles, while the lowest line shows a traffic load with 500 various light, medium and heavy vehicles per hour.As the figure shows there is a reverse relation between distance and diagram's gradient.It suggests that when the distance is increased, noise pollution decline is ebbed away.Several factors affect this including further hindrances in streets, physical properties of waves, trees and other obstacles around the streets, etc.In this research, in order to reduce the computation time, the noise standard German first method was used, given the parameters of the mathematical method used to calculate the mean of the noise reduction parameters.The accuracy of the method of calculation of noise is obtained with the field testing.Eventually, it can be concluded that there are various parameters than can affect the noise generated through the streets.Of them, the most important ones are type of pavement, asphalt lifespan, and distance to the voice origin.These parameters were used in the mathematical model of the study and tests carried out in the site confirmed the accuracy of the model.However, future studies can examine the exact effect of asphalt lifespan on noise emission in terms of mathematical viewpoint and when it can be considered serious in the noise pollution studies.It is necessary to do sufficient studies on traffic noise.

Table 1 .
Definition of equivalent noise level equation parameters.

Table 2 .
Computational algorithm model used in the case study.

Table 3 .
The technical specifications for traditional asphalt.
specifications are in Table3.For analysis of the area, initially we referred to the operation on Friday 6/11/2014 for taking the warm asphalt sample chosen for conducting grading tests and characterizing the asphalt.Examination showed that 5% bitumen has been used in the asphalt and grading test results are shown in Table3