ISO 1996 measurement procedure and the uncertainty associated in strategic noise maps

Transcription

1 PROCEEDINGS of the 22 nd International Congress on Acoustics Noise Mapping: Paper ICA ISO 1996 measurement procedure and the uncertainty associated in strategic noise maps David Montes González (a), Juan Miguel Barrigón Morillas (a), Guillermo Rey Gozalo (b), Pedro Atanasio Moraga (a), Rosendo Vílchez-Gómez (a), Juan Antonio Méndez Sierra a), Rubén Maderuelo Sanz (c) (a) Departamento de Física Aplicada, Universidad de Extremadura, Cáceres, Spain, (b) Facultad de Ciencias de la Salud, Universidad Autónoma de Chile, Talca, Chile (c) Departamento de Tecnologías y Construcción Sostenible, INTROMAC, Cáceres, Spain Abstract Strategic noise maps are an essential tool for the evaluation of the exposure of the population to noise pollution and the elaboration of Action Plans. In this regard, since in situ measures are required for the elaboration or the calibration and validation of noise maps, the Noise European Directive considers the standard ISO 1996 as a reference. On the one hand, this standard establishes in its normative part some corrections as a function of the distance between the microphone and the rear reflective surface. On the other hand, it contains an Annex B (informative) in which certain conditions are established for each case in order that the values obtained by in situ measurements are approximate to these corrections. This paper show a review of the scientific literature about this topic, in which an analysis of published results and a reflection about the accuracy of the strategic noise maps carried out under the European Noise Directive are made. Keywords: Noise map, ISO 1996, measurement procedure, urban noise.

2 ISO 1996 measurement procedure and the uncertainty associated in strategic noise maps. 1 Introduction The harmful effects of noise pollution on the health of humans has been shown in numerous studies, in which it was found that exposure to environmental noise can cause different kinds of health problems [1-3]. In this sense, any approach to improving this situation and search for solutions necessarily involves achieving knowledge of reality to reduce levels of noise pollution as far as possible. This approach has been considered by the European Community [4] and, therefore, by the countries that comprise, in particular by Spanish legislation [5]. Noise mapping is an important option to be considered for studies about noise pollution and its effects on the population and for the approach of possible solutions [4-6]. Noise maps, as is stipulated in the Noise European Directive [4], are the principal instrument to confront environmental noise. For this reason, its development both nationally and internationally is important. Different methodologies can be considered for the realization of a noise map. Usually, studies use computational methods and in situ measurements. Two of the references for noise mapping are ISO and ISO international standards [7, 6], which have served as a basis for development of national and international legislation because, among other things, they define aspects associated with the calculation and measurement methodology of sound pressure level outdoors. If somebody wishes to know experimentally the noise dose received by citizens in their homes, the fundamental problem is to evaluate the noise incident on the façade. In this regard, it depends on temporal and spatial factors. Therefore, not only the features of the sound source would be necessary to be considered for a proper evaluation. In addition, the situation of the measurement point relative to the source and the specific urban environment of each street or façade to be evaluated would be important. This means to take into account the effect of the different elements or configurations of the urban environment on the results of the measurements. In this form, for each measurement configuration, the sound level value that is finally associated to each measure assess, as accurately as possible, the sound energy incident on the façade of the house under consideration. However, ISO standard, as will be discussed below, contains some inaccuracies and lacks of definition in the measurement procedure in outdoor environments, and in corrections to be applied, which could be decisive in the results obtained in the development of noise maps through measures and, therefore, the approach of possible solutions to reduce the levels of noise in cities. In this paper, firstly, it is analyzed to what extent and how these aspects are considered in ISO standard. Secondly, a review of the literature is made to know the studies concerning these aspects and conclusions reached. 2

3 2 Revision of ISO standard 2.1 Normative considerations of ISO The first aspect that may be of interest is the fact that ISO standard explicitly does not establish the distance relative to the reflecting surface at which the microphone should be placed, which means that this decision would be based on the criteria of the technician. This is not necessarily a problem. It could even be considered as recognition of urban reality. Given the urban planning of many streets of our cities, it is difficult to indicate a reference distance for measurements that, at the same time, does not involve a complex assembly. Associated with this topic, the standard suggests some corrections that should be applied on the values of the measured noise levels. The values for these corrections are given depending on the distance from the façade, with the aim of correcting the noise level increase that reflection implies respect to sound field effectively incident on façade (free field) and which is really of interest. The standard considers three cases in which corrections should be used: a) A position with the microphone flush mounted on the reflecting surface: 6 db. b) A position with the microphone located between 0.5 and 2 m in front of the reflecting surface: 3 db. c) A free field position (reference condition): 0 db. The corrections indicated by the standard shown a certain lack of definition, because as the standard itself suggests, these corrections proposed above may not match the results of measurement in real conditions in urban environments. For example, in the case of microphone flush mounted on the reflective surface, the standard indicates that the difference of 6 db between a microphone placed on a façade and one in free field is an ideal case, occurring in practice deviations lower than this value. In the same direction, when the microphone is placed between 0.5 and 2 m in front of the reflective surface, the standard states that the difference between the sound pressure level in a microphone located 2 m in front of the façade and a microphone free field approaches 3 db for an ideal case without any vertical reflective obstacle influencing sound propagation to the receiver. However, this difference may be greater in complex situations, for example, sites with a high density of buildings, streets, etc. In addition, for this same case, it indicated that under grazing incidence deviations may be higher. Finally, for the position of the microphone in free-field, the standard states that either a real or theoretical case, the sound pressure level corresponding to the free field incident on a building is calculated by measurements made near the building. This means that those measurements made in front of the building, in which is verified the free field condition for distance, would not be covered either in the standard. This fact seems to be clear, since, in this case, the measured sound field would not be incident on the façade. 3

4 2.2 Informative considerations of ISO Although ISO standard makes some vague references in its normative part to the conditions in which the proposed corrections are verified, different considerations are shown in detail in Annex B (informative) that should be taken into account. They are going to be analyzed for each of the three cases described above. In case of microphone mounted directly on a reflective surface, the annex establishes as a first option to place it on a plate on the surface or with the microphone membrane flush with the surface of the mounting plate. For assembly, certain conditions must be respected. In relation to the façade, it must be flat within 1.0 m from the microphone, with a tolerance of ±0.05 m, and the distance from the microphone to the edges of the surface must be higher than 1.0 m. Another aspect to be considered is that the plate should not be thicker than 25 mm and not less than 0.5 m x 0.7 m dimensions. It must be made of a rigid material and acoustically hard. Finally, it indicates that the distance from the microphone to the edges and to the axes of symmetry of the plate must be greater than 0.1 m. For this measurement position, the second possible option is to place the microphone directly on the wall, without the plate, in case of the surface is made of concrete, stone, glass, wood or similar hard materials. In this case, the reflecting surface must be flat within 1 m from the microphone, with a tolerance of ±0.01 m. In addition, the standard states in Annex B that a microphone of 13 mm in diameter or smaller should be used for measurements without the plate in octave bands and, if the frequency range exceeds the 4 khz, a microphone of 6 mm. When the microphone is placed near the reflecting surface (between 0.5 and 2 m in front of this), Annex B of the standard indicates that the façade must be flat with a tolerance of ±0.3 m. Furthermore, in order to avoid edge effects, minimum distances are established (Figure 1) from point O to the closest edges of the reflecting surface: b (horizontal distance) and c (vertical distance). These distances must meet conditions given by equations 1 and 2: b 4d, (1) c 2d, (2) where d is the perpendicular distance from the microphone to the façade. Moreover, the annex of the standard states that to ensure that the incident and reflected sounds have the same magnitude; the criterion of equation 3 must be met in the case of the extended source. It relates a' and d', taking these distances along the dividing line of vision angle, as can be seen in Figure 1. Considering that M' is the point on the dividing line of angle at a distance d from the façade, d' can be defined as the distance between M' and the façade and a' as the distance between M' and the sound source: d' 0.1a'. (3) 4

5 Figure 1: Microphone near the reflecting surface [8] In practice, considering a reflective surface parallel to the linear sound source, in the case of receiver located between 0.5 and 2 m in front of the façade, this condition means that the perpendicular distance between the microphone and the sound source (a') should range at least between 5 and 20 m. This means that for sources located at distances lower than 5.5 m from the façade, if the indications of Annex B are followed, measurements on façade should only be conducted. Moreover, only for sources placed at distances greater than 22 m from the façade, it would be possible to carry out measurements at any of the distances from the façade considered in the standard. Another aspect of Annex B of ISO standard to be considered is that, to ensure that the microphone is placed at enough distance from the region of +6 db next to the façade, in case of extended source it should be taken into account the criterion of equation 4 when a study of global sound pressure levels is realized or, the criterion of equation 5, if it is carried out in octave frequency bands: d' 0.5 m, (4) d' 1.6 m. (5) On the other hand, if we wish to perform measurements more than 2 m from the façade, Annex B of the standard indicates a criterion as a requirement for considering free field conditions. It indicates that the distance from the microphone to any reflective surface, not including the ground, should be at least twice the distance from the microphone to the dominant part of the sound source (equation 6): d' 2a'. (6) 5

6 If these measurements are performed in front the façade, it must be taken into account that if the microphone is in free field conditions, the measured noise level is not representative directly of the incident level on the façade. Thus, a correction would be needed. However, no correction concerning sound propagation is provided in the standard. For these three measurement positions described above, Annex B of the standard makes a distinction of the type of sound source under study. Depending on the viewing angle () of the microphone over the source (Figure 1), it is considered as extended source when is greater than or equal to 60 and, as a point source if the angle is smaller. 3 Literature review Differences between the corrections stated by ISO standard and empirical studies [9-11] could be motivated by various factors, and they seem to be associated with the complex configuration of the urban environment of cities. Some papers show a study of the variation of the sound pressure level in front of reflecting surfaces [12-18]. They suggest the occurrence of noise level fluctuations near reflective surfaces due to the combination of diffraction and interference effects of sound waves. In this line, Hall et al. [12] compare the measured noise level on façades and 2.0 m from them considering traffic noise as sound source. Outcomes show fluctuations above and below 3 db, especially for frequency bands under 200 Hz. In connection with this results, the work published by Quirt [13] indicate that the assumption that the energy is doubled (+3 db) at 2 m from the surface of the building is a reasonable approximation for an extended source such as road traffic and for third octave bands above 100 Hz. It also concludes that, on average, pressure doubling (+6 db) is a good approximation when the microphone is placed very close to flat building surfaces. Hopkins et al. [14] study the above mentioned effects using a microphone placed on a reflecting surface and other microphone in the range between 0.1 and 2.0 m from it. For this purpose, measurements of sound pressure level are carried out in a scale model into a semi-anechoic chamber using a point source and different sizes of reflective surface. The results show differences between the finite and semi-infinite reflectors, particularly at frequencies below 300 Hz. This becomes apparent because of the appearance of a comb filter effect whose maximum and minimum occur at different frequencies and at different levels. It is also noted as the comb filter effect moves toward lower frequencies as increases the distance between the microphone and the reflective surface. Berardi et al. [15] research about the interference effects in field measurements of airborne sound insulation of building façades. Using a loudspeaker as a sound source, experimental results show the existence of destructive interferences at frequency bands below 125 Hz. Further investigations are recommended to better understand the possible influence of different materials and decorations of the façade in modifying the interference pattern. Olafsen [16] indicate that if calculations are made with a perfectly reflecting façade, no other reflecting surfaces and a perfect point source in a single position generating the noise, the comb filter effect would show up at around 5000 Hz, with a microphone at a distance of 0.03 m 6

7 Buenoss Aires 5 to 9 September, 2016 Acousticss for the 21 st Century in front of the façade. This type of calculation indicates that the lowest frequency where comb filter effects should be expected will go down as the distance from the façade is increased. Even at 2 m distance in front of the façade, these calculations show a pattern of interference effects in 1/3 octave bands, to some extent influencing the whole building acousticss frequency range from 50 to 5000 Hz. Beradi [17] use a point source to study the position of the instruments for the sound insulation measurement of building façades. The results of the investigation suggest averaging the extereffects of interference nal SPL measurements among different positions in order to reduce the in front of the façade. In this regard, the paper also cites the incidence angle of sound relative to the reflective surface as a factor to be considered in this kind of studies, as real sound sources cause different angles of incidence on the façade. Another study is done by Olafsen et al. [18] where field measurements of façade sound insula- tion are carried out using a loudspeaker as a sound source. It concludes that, when possible, microphone positions on the façade should be preferred. If positions on the façade are not available, acceptable results can be achieved using microphone positionss in front of the façade. The measurement positions cannot be directly compared. Until further knowledge is collected, it is suggested that the two positions on or in front are considered to give the same result at freis considered to give 3 quencies up to and including 160 Hz, and that the position on the façade db higher level than in front from 200 Hz upwards. 4 Conclusions The results published to datee and which may have a significant impact on the results obtained so far in the implementation of the European Directive for noise mapping are summarized be- low: Some papers study differences between the corrections proposed by ISO standard and experimental results. They show a disparity in values that could involve differences up to 2 db relative to the 6 db correction and 1 db relative to the 3 db correction. Other works suggest the occurrence of noise level fluctuations near reflective surfaces due to the combination of diffraction and interference effects, especially in the low-frequency range. It may involve that the 3 db correction would not be uniform in all the frequency bands. Acknowledgments This work was partially supported by the project TRA R (MINECO/FEDER, UE); Junta de Extremadura, Consejería de Economía e Infraestructura (GR15063); European Re- and Technological gional Development Fund (ERDF) and the National Commission for Scientific Research (CONICYT) through Nacional Fund for Scientific and Technological Development (FONDECYT) for research initiation (Nº ). 7

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9 [18] Olafsen, S.; Bard, D.; Strand, M.K.; Fernández Espejo, T. Methods of field measurements of façade sound insulation. Noise Control Engineering Journal, Vol 63 (5), 2015, pp