Noise emissions from highway with the use of measuring and modeling

3.1. Noise diffusion model

From the studies on the models designed by different countries, the SMM1 model was selected for the study area (Tehran). This model was developed in Germany for noise modeling. In this country, two models are used for modeling noise pollution. The first model, which was used in this research, is simpler because it involves fewer calculation parameters. The advantages of using this model include the high speed and satisfactory precision of noise calculation. The second model involves many parameters which add to the complexity and difficulty of the model. This model is highly precise but it is time-consuming and expensive [8]. Since the calculation precision provided with the first method was suitable for the purpose of this model and since it provides for high-speed calculations, this model was used to calculate noise pollution diffusion in the study area. In this model, vehicles are divided into three categories: light-weight, medium-weight (e.g. uniaxial trucks and buses), and heavy-weight (e.g. multi-axial trucks and trailers). The first, second and third categories are shown by $l_v$, $m_v$, and $h_v$ in the set of traffic parameters, respectively. Each of the mathematical equations provided in the following incorporates signs and symbols. These symbols and abbreviations are also explained for the ease of understanding. The output of calculations is a figure equal to the equivalent level of noise. It is expressed in terms of decibels and is shown by $L_{eq}$.

The equivalent noise level is obtained as follows [11]:

In the above relation, the only unknown variable, which is of concern in this research, is the equivalent level of sound, which is shown by $L_{eq}$. Other parameters on the other side of the equation include parameters that can be calculated on the basis of traffic criteria. In the following, each parameter as well as the relevant calculation method is explained. In this equation, $E$ denotes the calculated noise, which normally falls in three different periods. The average value of $E$ is known as the average total noise per day. Following the calculations, +5 dB and +10 dB are added to the results obtained for evening and night, respectively. This modification is aimed to enhance the convenience of people of the region. The value of $E$ is calculated through the following relation:

where, $E_{lv}$, $E_{mv}$ and $E_{hv}$ refer to the noise spread by light-weight, medium-weight and heavy-weight vehicles, respectively. Each of these values can be calculated separately through the following equations:

where, $V_0$ is the speed limit (80 km/h for light weight vehicles and 70 km/h for medium- and heavy-weight vehicles). In addition, $V_{lv}$, $V_{mv}$ and $V_{hv}$ also stand for the average speed values regardless of time for light-, medium-, and heavy-weight vehicles, respectively. Finally, the value of $E$ or noise can be easily obtained following the aforementioned three calculations through modifications aimed to ensure comfort and convenience of the surroundings. On the other hand, $C_{wegdek}$ is also the correction factor for the type of asphalt, which is calculated according to the following Table 1.

The concern of the discussions included in this study and the result of observations suggest that this correction makes a difference when the asphalt is fresh or old. The difference is a result of the wider porous area in the case of fresh asphalt. Over time, as vehicles go over the superstructure, the concentration of the superstructure is increased and its porosity decreases. Consequently, the noise pollution in the environment escalates accordingly. However, in the early days of the life of the asphalt layer, the increasing trend of noise pollution is more than the ending days of the life of the asphalt layer. The reason is that at the beginning of operation of the superstructure and traffic of vehicles, the level of threshing and compaction is higher than the end of the life of layer.

$C_{optrek}$ is the correction resulting from the noise of braking or increased speed of the vehicle. This is also called the correction factor for acceleration of vehicles. This value is neglected as it is considered insignificant as compared to other sounds. Therefore, it is assumed to be zero in calculations. The reason is that since a highway in Tehran is considered to be the study area, the acceleration of vehicles is very low and automobiles almost move at a uniform speed. $C_{ref}$ denotes the noise diffusion resulting from the reflection of wave hitting the walls of surrounding buildings and sound barriers. This is obtained through the following relation:

where, $f_{obj}$ is the factor associated with the barriers with a value varying between 0 and 1. It is designed for barriers on the other side of the street at reasonable distances. Therefore, it is neglected for farther objects. These coefficients are a function of distance. In this research distance is shown by $r$:

where, $B$ is in the middle of the street and the point absorbing the noise (it varies between 0 and 1). Street and water surfaces reflect water while the trees and grass absorb it. In addition, $h_w$ and $h_{weg}$ denote the calculation point height and street height from the baseline (m), respectively.

3.2. Study area

The Azadegan Highway was selected for this study from the highways located in Tehran City. The reason was that this highway hosts the traffic of different types of vehicles (e.g. trailers and trucks) and thus it demonstrates a higher level of noise pollution. It is also considered among the inlets and outlets of the capital (Tehran). Azadegan Highway is connected to Shahr-e Rey through Rajai Highway. It is, therefore, a substantial route for passengers aiming to travel beyond Tehran and its boundaries. The main traffic load of this highway occurs in vacations and weekends.

3.3. Output of the research model

The noise of the region was measured using the traffic data obtained from the study area (including traffic volume) by the weight of vehicles and their average speed in the day, night and evening times in a one-month period. The information is show in Table 2.

The last column of the Table 2 shows the model output, which shows the equivalent noise level from the sound source (the study area) in decibels. Since the amount of data obtained in the one-time period was high, this table only presents the data obtained from a one-week period. Future study of diagrams and figures will include data resulting from the analysis of the whole one-month period.

These sound levels that show on illustration (Fig. 1), calculated for radius ($r=$3) that is measured from the centerline of road. For radius affection in noise level, different method was conducted. Volume and speed were assumed as constant values in model. The values were selected from Jul-23 that were taken before and sound level was calculated with different amount of “$r$”. The illustration (Fig. 2), shows the sound level changes in different distances.

As it is obvious, when distance increases, sound level will be decreased. As distance increases, sound level depends less than it was in closer distance.

Fig. 1Equivalent noise level variations for one month in studied highway

Fig. 2Equivalent noise level variations in different distances from source