Acoustic environmental impact of stadiums

Transcription

1 Acoustic environmental impact of stadiums C. Rougier 1, J. Defrance 1, N. Noé 2, J. Maillard 1, M. Baulac 1 1 Université Paris Est, Centre Scientifique et Technique du Bâtiment (CSTB), 24 rue Joseph Fourier, Saint Martin d hères, France christophe.rougier@cstb.fr, jerome.defrance@cstb.fr, julien.maillard@cstb.fr, marine.baulac@cstb.fr 2 Université Paris Est, Centre Scientifique et Technique du Bâtiment (CSTB), 11 rue Henri Picherit, Nantes Cedex 3, France nicolas.noe@cstb.fr Abstract Stadiums are usually built close to cities, or even sometimes into urban areas, mainly to make the access to the sport facilities easier. In this paper a methodology to predict the acoustic environmental impact of such buildings in the neighbourhood is presented. Firstly, a ray tracing simulation tool for complex spaces (ICARE) is used to characterize the stadium as an equivalent sound source, taking into account the geometry of the stadium, its material properties, and the sound sources present into the stadium. Then the equivalent sound source is implemented into the MITHRA ray-tracing code which takes into account topography, buildings, roads or meteorological conditions. At last the acoustic environmental impact of stadium can be evaluated. In this work the stadium can be used for football matches as well as for concerts. Keywords: environmental impact, stadium, ray tracing. 1 Introduction Urban noise has became a growing problem in every part of the cities, especially for the most important ones and for there suburbs. The noise emitted by existing or future equipments (roads or highways, railways, industry buildings, etc.) is becoming more and more important. These noises can be described with a common factor: they are all imposed to the inhabitants living nearby whom generally hardly, or even reject them. The entertaining areas and especially stadiums build close to big cities or their suburbs are part of this category of noisy equipments. That is why the acoustic environmental impact of such constructions is taken into account very seriously, at least just as the environmental problems are (like water pollution for instance). 1

2 Two main aspects lead to acoustic impact studies. Firstly there can be a building constructor's will to communicate to the neighbourhood about the project (what will be the noise emitted?, what the sound space will become?, etc.). On the other end the law imposes, at least in France, to the building constructor to make a global environmental impact study about the construction project including an acoustic impact study in which the maximum sound emergence must be quantify [1]. For both theses cases, there is need of a tool that allows an acoustic impact study of a stadium. Several computing tools exist to simulate the sound propagation of common sound source like railways, roads or highways [2, 3], but not directly the propagation of complex volumes building like stadiums. In this paper, we present a methodology to simulate this kind of buildings thanks to a combination of two simulation softwares based on ray tracing. Even if the methodology proposed can be applied to any use of a stadium, we will focus on two main uses for stadiums: a football match, and a concert. 2 Methodology of work The first step of the method consists in studying the stadium itself, taking into account the specificities of its complex volumes, the materials used for its construction, and the sound sources present inside the arena depending on the use of the stadium. This first step is achieved thanks to the ICARE software, an acoustic or electromagnetic propagation simulation tool for complex volumes using asymptotic methods (beam tracing) [4]. The second step consists in describing the stadium as a sound source. It can be a single sound source, or a sound source composed of many single ones. The final step consists in implementing this equivalent sound source in the MITHRA software (environmental acoustics) to calculate the noise levels in the area more or less closed to the stadium. This software allows us to take into account the topography, the buildings around, the roads, the railways and the meteorological conditions. In this methodology the characterisation of equivalent sound source of the stadium makes the link between the two other steps. 3 Predicting sound levels around the stadium 3.1 Use of stadiums for football matches Characterization of a supporter sound source for an existing stadium When a stadium is used to host football matches, the noise produced by this event is mainly due to the people in the stands and supporting their favourite team, even sometimes singing all together. Then this is the first and main sound source to take into account when modelling the stadium. The supporters sound source must be fully described with its sound power and frequency response. It would be then possible to implement it into the ICARE software to predict sound levels around the stadium. Acoustic measurements have been first done in and around a real stadium in Lyon, France to record typical sound levels during a football match. The measurement points are shown Figure 1. The Gerland stadium has been then modelled in ICARE with sound receivers located exactly at the same position around the stadium, and with a distribution of point sound sources in the stands to represents supporters, as shown in Figure 2. The results of 2

3 predictions have been compared to measurements to adjust precisely the characteristics of a typical supporter sound source. P3 P1 P2 Figure 1 Measurement points around and inside the Gerland stadium, Lyon, France Figure 2 Model of the Gerland stadium in ICARE, with three receivers positions P1-3 and a receiver located in the centre of the pitch (a), point sources distribution in the stands (b) Supporters have been modelled with a homogeneous distribution of point sound source with a maximal distance of 5m between two closed points, assuming that a point source models a group of supporters. For the Gerland stadium, it leads to 813 point sources distributed on m². The spectral characteristic of the supporter sound source has been determined after the analysis of the sounds recorded around the stadium. The mean spectral response has been applied to each point source in the simulation tool ICARE, and the sound power of each point source has been adjusted so that the acoustic levels at the 3 receivers in the ICARE software fit with those recorded during in situ measurements. The final supporter sound source characteristic is shown in the Table 1. 3

4 Table 1 - Supporter source characeristic. Sound pressure level in db at 1m of the source. Octave band L p (db) 63 86, , , , , , , ,0 db(a) 89,8 As the measurements have been done during a real football match, the noise emitted by the Public Address system of the stadium used for announcement and/or advertising is also included in the previous mean values of a supporter sound level Variations of sound power between stands, and depending on the match period The previous results give a good idea of a sound level emitted by an equivalent supporter. Nevertheless in order to have a better prediction of the sound levels of the stadium, we have considered more details in the model, and particularly a variation of sound power depending on the stand. Indeed, in-situ measurements between the pitch and the first stand (see mic positions 1 to 5 in Figure 1) have shown that the sound level of kop (or fan) stands is 8.5 db louder than other stands. This variation of sound power in the stadium has been introduced in the model, as the same time as absorbing characteristics of materials (chairs, etc.). Finally, two different types of supporter sound source have been defined, as shown in Table 2. These values are used in the following to model the different sound power in the stands of the stadium. Table 2 - Two types of supporter sound source characeristic, for a normal match period and for a "goal period". Sound pressure level in db at 1m of the source. Normal period Goal period Octave Band "kop" stands L p (db) other stands L p (db) "kop" stands L p (db) other stands L p (db) Global Level db(a) Calculation of sound pressure levels around the stadium As described in the previous paragraph, characteristics of the sound sources for a stadium used for football matches are known. Using the same repartition of point sound sources per 4

5 square meter in the stands with the right sound power level affected to the sources, the ICARE tool allows us to calculate sound pressure level around a stadium, using ray tracing. The Figure 3 presents an example of a typical semi-opened stadium (non-existing) with 2 stands and a pitch dimension of about 100m x 50m. This stadium has been used in this work to apply our methodology. The receivers around the stadium are placed on circle, every ten degree, at a distance of 200m from the centre of the pitch. It is also assumed that the lower stand is totally occupied by "kop" people who produce more sound pressure level than people located in the other stand above. Figure 3 Sound receiver around the stadium in green dots, and point sound source in stands in red. The ray tracing simulation finally gives the sound pressure level at the 36 point receivers around the stadium. It can be represented as a "directivity" of the stadium both in octave bands and in db(a), as shown in Figure 4. This directivity is symmetrical just as the shape of the stadium modelled above. Due to the frequency characteristics of the source, the most energetic octave bands of the sound produced by the stadium are 500 Hz and 1000 Hz. Figure 4 Mean directivity of the stadium in octave bands and in db(a), during a "normal" match period. 5

6 The directivity is given here for a "normal" period in a match (i.e. not a "goal" period). The directivity for a "goal" period is quite similar, with an increase of sound pressure levels of about 6-7 db(a) over all the frequency range. 3.2 Use of stadiums for concerts For this special use of a stadium, the loudest sound source is not the audience seated in the stands or standing on the pitch but the Public Address system used to broadcast the sounds of the show into the stadium. It is assumed that the loudspeakers are mounted in "line array" (which is a cluster of highly directive loudspeakers used to diffuse sound in a homogeneous way in a large surface) similar to those shown in Figure 5. The line arrays on the model presented in Figure 6 are each composed of eight speakers. Figure 5 Example of a "line array" (JBL) Figure 6 Inside view of the stadium used for a concert: line array loudspeakers position and principal axis of emission. A rock concert recorded in a stadium has been used to determine a typical spectrum for the line arrays. The calibration of the sound power of the loudspeakers is done in a way that the average sound pressure level (SPL) at receivers on the pitch (1m70 above the floor) is around 105 db(a), to respect the French regulation on noise levels in spaces where amplified music is played [5]. 6

7 When the sound power of the line arrays is known, calculations similar to those in paragraph are done. The directivity of the stadium can then also be represented. 4 The stadium as an equivalent sound source The stadium can be described thanks to a directivity whatever event is happening inside, as far as the sound sources inside the stadium are well defined. This naturally leads to consider the stadium as an equivalent sound source. The sound pressure emitted by the stadium is modelled as if 8 point sources distributed around the stadium were emitted the same sound pressure level as the stadium previously modelled in ICARE. The sound pressure level values of the ICARE simulation in 36 different points are used to identify the 8 point sources, thanks to the Source Identification Module of the MITHRA software. This module gives, for each octave band, the sound power level of point sources that lead to an imposed sound pressure level at some point receivers. This identification is done by using a mean square method and works for a number of receivers less than or equal to the number of sound sources, with a maximum of ten receivers and/or sources. The identification schematics for the stadium considered in this study is shown in Figure 7. Due to the limited number of receivers allowed by this method, the sound pressure level of the 36 receivers calculated in the ICARE model have been averaged by group of 4 or 5 to have 8 receivers R1 to R8. We can also notice that the stadium is represented here with its surrounding facade to simplify the identification. Figure 7 Identification of equivalent sound source for the stadium. Different layouts of sources and receivers for the identification have been tested. The use of point sources gives a good identification of point source if the stadium's exterior shape is not too complex and if the directivity is rather omnidirectionnal. For more complex shapes or more "directive" stadiums, the identification process should be improved. 5 Sound propagation in environment The last part of the acoustic impact study of the stadium consists in using the equivalent sound source of the stadium in the propagation modelling software MITHRA. This takes into 7

8 account factors such as building design, local topography, noise barriers, the ground type or even meteorological effects [6]. In this case study, focused on the prediction methodology, we only use a basic, non-dense topography with a simple layout of buildings without any meteorological effect. The example of topography used in the present study is shown in Figure 8. The site is composed of the stadium located 400m far from a road with an average hourly flow of 1000 vehicles; some houses at the west site of the road and the stadium (900m far from it); some houses at the east side of the stadium (700m far from it); and a hill. Figure 8 Typology of the simulated site, including the stadium. Figure 9 Horizontal noise map of the simulated site when the stadium is not used. 8

9 Figure 10 Horizontal noise map of the simulated site when the stadium is for a football match (in "normal" period). The results of noise prediction for this particular site configuration are shown on Figure 9 (respectively on Figure 10), without (respectively with) the stadium used for a football match. It is then possible to quantify the increase of sound pressure level at houses and buildings fronts due to the sound emission of the stadium. In this example, sound pressure level is increased by about 9 db(a) when there is a football match in the stadium (normal period). Although it is not represented in this paper, the different scenarios (football match in "goal" period, concert, or other) considered in the impact study can be also predicted in terms of sound propagation in environment. Moreover the result can be presented in terms of noise emergence. 6 Conclusions A methodology of prediction of acoustic environmental impact of stadiums has been described in this paper. The acoustic characterization of a typical semi-opened stadium has been done when it is used for football matches, and can be easily extended to its use for concerts. It has been achieved thanks to the combination of beam tracing simulations and real acoustic measurements. The identification of a "stadium" sound source composed of several single point sources has been then processed. This gives good results for this application but should be improved in a further work to give better results if the stadium has a more complex geometry. Finally the acoustic propagation prediction is done to quantify the sound pressure level at inhabitants' building fronts. However it is important to notice that each calculation step of the methodology implies some approximations that give calculation uncertainties. It is then possible to communicate about acoustics on a future project, to see if the project will fit with acoustics standards for noise annoyance, or even design in advance some acoustic solutions to prevent the neighbourhood from noise. 9

10 References [1] French Regulation, Décret n du 31 août 2006 relatif à la lutte contre les bruits de voisinage et modifiant le code de la santé publique, Journal officiel de la République française: Articles R to R , September 2006, 1 st. [2] Defrance, J.; Baulac, M.; Jean P.; Premat E. Modélisation de la propagation acoustique en milieu extérieur complexe : Effets de frontière, Acoustique & Technique 51, 18-24, 2008 [3] Defrance, J.; et al, Outdoor sound propagation reference model developed in the European Harmonoise project, Acta Acustica, 93(2), , 2007 [4] Noé, N.; Vermet M. A general ray-tracing solution to reflection on curved surfaces and diffraction by their bounding edges, Proceedings of the 9 th ICTCA, Dresden, 2009 [5] French Regulation, Décret n du 15 décembre 1998 relatif aux prescriptions applicables aux établissements ou locaux recevant du public et diffusant à titre habituel de la musique amplifiée, à l'exclusion des salles dont l'activité est réservée à l'enseignement de la musique et de la danse, Journal officiel de la République française: Article 2, October 2007, 16 th. [6] Aballéa, F. ; Defrance, J. ; Baulac, M. ; et al. Sensitivities of outdoor sound propagation predictions to environmental input parameters, Noise Control Engineering Journal, 55(1), 38-49,