Acoustic Simulator for Urban Noise Analysis José Francisco L. Naranjo

Acoustic Simulator for Urban Noise Analysis

José Francisco L. Naranjo ? Fernando C. Pinto ∗ Julio Cesar B. Torres ∗ Roberto A. Tenenbaum ? [email protected], [email protected] Universidade do Estado do Rio de Janeiro, IPRJ, Programa de Pós-Graduação em Modelagem Computacional, Nova Friburgo - RJ - Brasil ∗[email protected], [email protected] Universidade Federal do Rio de Janeiro, POLI, Programa de Engenharia Urbana, Cidade Universitária - Rio de Janeiro - RJ - Brasil -

Abstract. Acoustical simulation and auralization systems have been used in many areas, mainly for design and analysis of enclosed spaces (room acoustics). However, nowadays, there is growing demand for noise measuring systems, evaluation and control of the sound impact in the urban environment. Such simulators are developed to evaluate, for instance, the sound pressure level at a given place in the city, according to the sound sources characteristics (vehicles, highways, airports, entertainment etc) and to the sound propagation over the city areas. This work presents a acoustic simulator, named RAIOS, working for analysis of urban environment. There are presented the methods and models applied by the simulator as well as a comparison between measured and simulated data obtained with RAIOS and other simulator.

Keywords: Urban Engineering, Acoustic Simulation, Noise Control 1. INTRODUCTION Noise is a quite important environmental issue with the present amount of people living in cities, specially big ones with heavy traffic problems, the traffic noise being probably the most important factor. The increase of noise pollution may also be responsible for public health problems and, being unsustainable, demands measures to be reduced and contained. The assessment of noise in large cities therefore represents a necessary step in city planning but needs to take into account the combination of different noise sources contributing to the overall acoustic environment. Measuring the acoustic characteristics of a whole city or neighborhood may not be a fea- sible task due to the great number of measurement points and the statistical character of the sources like vehicle noise. In this context simulations through a prediction software, calculat- ing expected noise levels in specific places can provide the information needed in a fast way. Nevertheless, the simulation results must be compared to real measurements, that can also be used to verify the model accuracy or correctness. Some studies related to noise mapping have been conducted recently, actually made for most of the big cities, specially in the European countries since the European Community environmental policy incorporates the noise mapping as an important factor [1, 2, 3]. Urban noise, in Brazil, is beginning to be recognized as an environmental issue, although noise complaints are among the major issues dealt with by the environmental board as in the case of the Rio de Janeiro city. This work presents the details of a simulation software, RAIOS, being under development at the Federal University of Rio de Janeiro and the State University of Rio de Janeiro in Nova Friburgo. The software was initially applied to room acoustics but it was modified in order to deal with outdoor propagation for the case of urban problems. The simulation results are compared not only to those obtained by a commercial software for noise pollution assessment but also with actual measurements.

2. SIMULATION SCENARIO The region chosen simulates a small sector chosen to establish a comparison between the softwares and measured data. It is located in the central zone of the neighborhood of Copaca- bana, limited by the Atlantic Avenue and Toneleros Street and by streets Raimundo Corrêa and Siqueira Campos. The topography of the region is obtained from an available CAD model of the whole city of Rio de Janeiro. The database do not include only the topography of Copacabana, but also the building heights individually. Traffic volume data was informed by the traffic engineering company of the city of Rio de Janeiro. Since the actual conditions of the vehicle fleets between Brazil and European countries are quite different an adjustment based on measurements is mandatory to obtain accurate noise maps from Brazilian cities. The simulation was done for a height of 1,5m to allow a direct comparison with measurements obtained from portable sound level meters. Figure 1 shows an aerial photo of the sector indicating the street names. Details of the simulations and measurements can be found in [4].

3. SIMULATION WITH COMMERCIAL SOFTWARE There are several commercial softwares that might be used to generate noise maps of out- doors areas and to assess environmental impact. Examples are the codes Mithra, SoundPlan, CADNA-A, Predictor, IMMI, LIMA, ENM etc. An important feature and the main difference among then relies on the user interface and the possibility of directly import information from Figure 1: Aerial photo of the sector of Copacabana chosen for comparison (Google earth) [4] different CAD formats or GIS databases. The noise propagation is accounted for through the implementation of several procedures and national standards, being different from software to software. In this work in order to create a noise map for the sake of comparison with the code RAIOS the software CADNA-A was used as a code for calculation and presentation of environmental noise levels [5]. The simulation was done for the traffic noise in a restricted sector from Copacabana in the city of Rio de Janeiro, Brazil as described below and where measurements were done to obtain comparable actual noise levels [4]. Since the Brazilian standards do not establish a procedure for the traffic noise calculation it was performed based on the German Standard RLS90 [6]. Among other parameters it takes into account the type of paving, the amount of traffic, average speed etc. These data are translated according to the standardized procedure into a line source with defined sound power per unit length. As in the case of RAIOS this source is also split into a series of point sources along the traffic lanes. Details of the sound power considered are given in the next section. The topography of the region, and the individual buildings, are imported from the available database and modeled by the software. Thus, buildings and ground acting not only as barriers but also as reflecting planes, affecting sound propagation. The area under study is covered by a grid of points, with 5mx5m spacing, where the noise levels are calculated. The interpolation of these values are plotted as a noise map over the area. Using the same model, six point receivers are located in different positions where the actual measurements were done in order to allow an easy comparison between results. The obtained noise map is shown in Fig. 2 and the levels at each receiver can be found later in the comparison section (Sec. 7.). Figure 2: Noise Map of the sector

4. RAIOS SOFTWARE OVERVIEW Since 2002, the acoustical simulation software "RAIOS" has been developed [7, 8]. This simulation tool was initially developed for room acoustics and nowadays it is extended to simulate urban sound propagation. The main difference between the simulation of a room and a urban environment is the type of sources, which can be punctual, lines, areas, moving or static ones. Another differences are some specific models, not present in rooms, such as vegetation, waterfalls and wind. These models can be implemented in the same software and operate for both cases. In general, a simplified approach for these methods can lead to very consistent results, considering, for instance, average absorption and diffusion coefficients for facades and paving. The urban environment can be modeled, from the simulation point of view, as an enclosure space, by considering the open space as surfaces without reflections. The methods and characteristics of RAIOS will be briefly presented in the next subsections.

4.1 HYBRID PROPAGATION METHOD The simulation of the sound propagation for closed spaces in RAIOS is based on a hybrid method, where the specular reflections are obtained by an improved ray-tracing technique and the diffuse reflections are computed by using a modified energy transition method [8]. The main goal of combining these two methods is to generate a good estimation of the impulse response (IR) for a given source-receiver pair, from which the main acoustical parameters can be obtained as well as the auralization [9]. In the simulation sequence, the ray-tracing method is the first one. In this part of the process, the sound sources emit energy according to its directionality pattern and type (see Section 4.2), which is distributed in two ways: A pure specularly reflected energy and a diffuse one. The specular energy is used to achieve the Specular Impulse Responses (SIR) in the receivers and to accumulate energy in the surfaces to the next step, the Energy Transition Method. After running the specular process, the diffuse method consists of exchanging energy between the surfaces and through the receivers, such that a Diffuse Impulse Response (DIR) is obtained. The adequate superposition of the ray-tracing and energy-transition impulse responses leads to a realistic global IR that results in reliable values for the room acoustics parameters and Sound Pressure Level (SPL) at receiver positions along the time axis [10].

Ray-Tracing In this classical method, the sound wave is modeled by geometrical acoustics were the sound wave is assumed to propagate in straight sound rays [11] emitted from the sound source. Each one of these rays carry information about the power spectrum and the traveled route of the sound wave. Propagating in a straight line, the ray is submitted to the dissipation effects caused by the air viscosity and by the room contour surfaces, which can include phenomena such as surface absorption and specular reflection. The reflection which follows the Snell’s law is called specular, i.e., the incident ray, the normal to the surface and the reflected ray stay in the same plane with equal incident and reflected angles. The energy part of the incident sound wave which is reflected in directions different from the specular one is called diffuse reflection but, actually, it is a scattered reflection.

Energy Transition Method The energy transition method [7], also called the random walk approach [12], is based in the exchange of acoustic energy between the source and the surfaces of a enclosure and among the surfaces themselves, assuming that this exchange happens at a regular time interval equal to the room characteristic time,

4V τ = , (1) cS where V is the room volume, c is the sound propagation speed and S is the global boundary surface. This interval is also called the room transition time and corresponds to the time spent by the sound wave to cover the free mean path, lm, that is, 4V l = cτ = . (2) m s In other words, the enclosure is assumed, as proposed by Sabine, as totally diffuse. Obviously, such assumption is not true for most of rooms and for urban sound propagation, but the method has proved to be essential to obtain a good estimation of the IR [10] and we cannot neglect to consider the sound diffusion and scattering over surfaces.

4.2 SOURCE MODELING The sound sources modeled by RAIOS are divided in two groups: the ones used in the ray- tracing method, which can be punctual or connected straight lines – to simulate, for instance, a trafﬁc along roads and streets or a loudspeaker –, and that ones used by the energy transition method, which are punctual sources located at geometrical centers of the surface elements.

Ray-tracing Sources Ray-tracing sources are based on a spherical model of ray distribution. Virtual directional sound sources can be obtained by emitting a large quantity of rays over all directions around the point where the center of sound source is located and by ﬁltering the power spectrum according to its directional characteristics. Many models have been proposed to obtain the maximum of homogeneity in the angular distribution of rays emitted, since there exist no closed solution, of course, for dividing a sphere in more than 20 equal spaced directions. A method that proposes a randomly generated algorithm to generate ray directions is presented in [13]. The source modeling which presented the best results, according to the homogeneity crite- rion, is based on the geodesic subdivision of the regular icosahedron. Each one of the original triangular faces are then subdivided recursively in new regular triangles, as illustrated in Fig. 3.

Figure 3: Geodesic triangles subdivision (n = 8).

The recursive algorithm was described by Lewer [14]. After n subdivisions the number of rays is given by

2 NR =2+10 × n . (3)

Projecting the vertexes on a spherical surface with unitary radius, concentric to the icosahedron, we obtain the directions of the rays which will be emitted. Fig. 4 presents one of the triangles of the icosahedron after 65 divisions leading to approximately 40.000 directions, very close to an exact homogeneous distribution.

Figure 4: Source distribution for 65th order. Observe the good homogeneity

In RAIOS software, line sources are used to simulate the road noise. This type of source is implemented as a group of punctual sources, whose power is uniformly distributed along a polygonal line. The power of each individual point source of the line, is given by

P0 LW P = 10 10 , (4) i N where LW is the total sound power level of the line, P0 is the reference sound power (normally taken to be 10−12 watts) and N is the number of punctual sources of the line. For urban simulation the height of the line sources are kept constant about 0.5 meters over the ground. The power level of the source line is defined in decibels per meter (dB/m) and the total level depends on the source length. The level is defined by ISO Standards according to road pavement type, vehicle flow, road cross section etc. [6, 15]. The total power level of the source line is given by just adding 10 ∗ log(L)(where L is the length of the line) to the power level defined by the standard for the line source. The line source modeled with punctual sources are shown as red octahedrons in Fig. 7.

Energy Transition Sources The central idea of this modeling is that all surfaces irradiate sound to the remaining ones proportionally to the solid angle that the center of the surface sees each other, as can be seen in Fig. 5.

Figure 5: Solid angle ΩFS that the sound source center F sees a surface S

Therefore, the sound source distributes energy to the room surfaces and the surfaces irradi- ates its energy to the other ones, successively. The energy received by a surface from the sound source depends on the solid angle between the source and the boundaries of the surface, and is given by Ω E = E FS D e−γd, (5) S F 4π θφ where ΩFS is the solid angle of the surface S related to sound source center F . This is a statistical model for the sound propagation and once the energy is emitted from the sound source, the distribution over n surfaces can be represented by a row matrix in the E form 0 =(E01 , E02 ,...,E0n ), where E0i is the initial energy received in the ith surface.

4.3 RECEIVER MODELING The receiver model is decisive for the method accuracy. In the real world, a wave front intercepts the microphone surface, considered very small, but in the ray-tracing model a ray vector must ﬁnd a larger surface. This inversion in the mathematical treatment requires some care. All models found in the literature for the receiver are based in a volumetric expansion around the point under consideration. In the reception algorithms, this expansion is spherical and the expression for the computation of the energy ER retained by a spherical receiver is L ER = E, (6) cVR where L is the length of the ray portion crossing the receiver, VR is the receiver volume, and E is the ray energy. In spite of being widely used, Eq. (6) furnish rather inaccurate results. This can be con- ﬁrmed with a simple example, shown in Fig. 6. Using Eq. (6), the energy retained by receiver R1, with radius a, is 1.7 times the energy retained by receiver R2, with radius 3a. Figure 6: Example of two spherical receivers with radius a and 3a.

A better alternative is to convert the sound energy to intensity. Instead of a solid, a cir- cular plate of reception is considered. The reception disk rotates around its center so that the acoustical ray is always orthogonal to it. The intensity at the receiver, IR, at an instant t is

2 IR = X Ei/πr (7) i where Pi Ei is the sum of the energies of all incident rays at the same instant t, and r is the reception disk geometric radius. With this approach, the same acoustical intensity is obtained for both receivers, now two disks with radius a and 3a.

5. SIMULATION PARAMETERS In order to simulate this part of Copacabana a 3D model was generated in a DXF file (Drawing Exchange Format, open source format developed by AutoDesk) and then imported by the software RAIOS. This file contains information about planes, sources and receivers. Planes are categorized in Layers, according to the material to be applied on the surface. Line sources are defined as 3DPolylines, always separated by specific layer names to be imported from RAIOS. The receivers are Autodesk Points entities, also separated by layers. The main screen of software RAIOS, with the urban model loaded, is shown in Fig. 7. It is shown the facades, represented by vertical planes, the pavement with the line sources, the receivers (green spheres) and the top planes which simulate the open space (sky).

5.1 SURFACE MODELING The simulated environment must be contained in a closed space defined by plane surfaces. The surfaces are defined by a set of vertexes to provide geometrical information and to define the normal vectors used for reflection of rays. Each surface carry also information about absorption and diffusion coefficients related to the material that the plane represents. In the urban environment case, the surfaces needed to simulate the sound propagation are the facades, the ground floor (pavement) and the sky. The facades were made reflexive, since they are composed basically of a mix of cement, painted blocks and glassed windows. There- fore, mean absorption coefficients were used, whose values are presented in Table 1 for octave bands

5.2 SOUND SOURCE LEVEL ADJUSTMENT The levels of the line sources were obtained from the German Standard RLS90 [6], which is given in decibel per meter, according to several characteristics of the road. The level deﬁned by RLS90 Standard corresponds to the power measured at a point located at 25 meters from Figure 7: Main screen of RAIOS simulator, with the model loaded.

Table 1: Section properties

Absorption Coefficients per band (Hz) Diffusion Coefficients per band (Hz) Surface 125 250 500 1k 2k 4k 125 250 500 1k 2k 4k Pavement 0.02 0.03 0.03 0.03 0.03 0.02 0.10 0.12 0.15 0.2 0.25 0.30 Facades 0.10 0.05 0.06 0.07 0.09 0.08 0.10 0.12 0.15 0.2 0.25 0.30 Sky 1.00 1.00 1.00 1.00 1.00 1.00 0.01 0.01 0.01 0.01 0.01 0.01 an infinite line source in free field. Therefore, it is necessary to calculate the line source level which produces at the receiver point such power level. This can be done by considering the power of an infinite line considering a cylindrical surface with radius of 25 m [15]. The levels used by RAIOS and CADNA software were the same and are listed in Table 2

The RLS90 Standard also considers the emission spectrum of the sound source (road). However, at this stage of development, the simulator RAIOS hasn’t implemented such spectral compensation on the sound sources, being considered only the case of sound sources with white spectrum power.

6. MEASUREMENTS Experimental measurements from the area under investigation, used to verify simulation models, can be found in [4]. These are equivalent sound pressure levels (Leq) in receiving points, for a period of at least 5 minutes. From the referred measurements only a subset of 6 points, from the original ones, are used in the present paper. The measured levels are presented in Table 3 for point locations described in Fig. 2, numbered from 1 to 6. The measurements were made from three periods of time: morning, afternoon and night and in different days of the week in each point, but only morning Table 2: Source Levels used for simulation, according to the RLS90 Standard.

Road Name RLS90 Power Simulated Source Power Level dB/m Length (m) Level (dB) AnitaGaribaldi 54.6 161 90.0 BarataRibeiro 72.1 516 112.5 Figueiredo Magalhaes 68.5 407 107.9 N.S.Copacabana 72.6 588 113.6 SantaClara 65.7 345 104.4 period of time was chosen for simulation with RAIOS, since the power level of the sound sources were the same for both simulators.

7. COMPARISON OF RESULTS The results obtained from simulation with both programs are compared to the measured data, as described in Sec. 6.. The receivers were placed at the same location of the measuring microphones. The sound pressure levels achieved for the receivers are presented in Table 3 for day period.

Table 3: Comparison between simulated and measured data for six receivers.

Receiver Sound Pressure Level (dBA) Error (dBA) Number Location Measurement CADNA RAIOS CADNA RAIOS 01 Av. N.S.Copacabana 610 76.0 78.2 77.1 2.2 1.1 02 Av. N.S.Copacabana/Fig. Magalhães 74.3 74.7 77.1 0.4 2.8 03 Av. N.S.Copacabana/Santa Clara 73.5 73.6 76.3 0.1 2.8 04 Santa Clara 98 70.5 70.6 72.7 0.1 2.2 05 Av. Barata Ribeiro/Anita Garibaldi 71.8 73.3 75.4 1.5 3.6 06 Av. Barata Ribeiro 432 77.6 77.4 76.6 -0.2 -1.0

From Tab. 3 it is observed that the levels are similar, showing that the line source model and the acoustic properties of the surfaces were properly set up. The average level difference achieved from RAIOS to CADNA is about 2.2 dBA and it is probably due to level misadjust- ment between RAIOS and CADNA line sources. The levels shown in Tab. 2 for RAIOS are approximate and do not consider spectral characteristics of the road trafﬁc (which is considered by CADNA as described in RLS 90). This level difference may cause the difference of levels obtained in dBA.

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