The influence of microphone location on the results of urban noise measurements
Introduction
In recent decades, society has become conscious of the drawbacks that modern development has introduced in the daily lives of its citizens. Among these is noise, which is a secondary effect of the important increase in the population of our cities and in the communications necessary between their citizens. Due to its known significant effects on population [23], [22], [7], [10] and considering that noise pollution affects a large part of the world population, noise represents a risk to our health and quality of life [5], [28].
Thus, noise must be studied and characterized in order to assist the authorities in protecting the citizens. As the measurement methodology can have an important influence on the noise level measured, standards have been proposed to ensure reproducible and comparable results in the measurements carried out by different technicians or in different locations. In that way, the ISO 1996 standards [13], [14] must be a clear reference. Many studies that use this international standard to measure and assess environmental noise can be found in the scientific literature [21], [19], [8].
In the ISO 1996-2 standard [14] some considerations referring to the location of the microphone relative to reflecting surfaces are included in Annex B. In the standard, no restrictions are given about the distance of the microphone from a reflecting surface. However, because the reflexion of noise from a surface can increase the noise level measured, some corrections in the measured noise levels are proposed as a function of the position of the microphone with respect to the façade of the buildings.
In previous research, some authors have made comparisons between these proposed corrections and the results obtained in actual measurement conditions [11], [25], [24], [20], [16], [18].
First, Hall carried out a study [11] regarding the differences between sound pressure levels of traffic noise at 2.0 m from a façade and the sound levels at its surface, for a series of measurements at 33 different houses. It was suggested that a 3 dB correction between measurement locations was appropriate on average.
A subsequent study conducted by Quirt [25] using two kinds of sound sources, road traffic noise (outdoors) and a loudspeaker (in anechoic room), indicated that in many practical measurement situations, the 3 dB and 6 dB approximations were not appropriated. However, the assumption of energy doubling at 2 m from a building’s surface is a reasonable approximation for a distributed source, such as road traffic.
Recently, Memoli et al. [20] studied the corrections for reflections on a façade. The sound levels from microphones placed at distances of 0.5 m, 1.0 m and 2.0 m from the façade wall, mounted directly on the façade wall and placed in free field, were compared for road traffic noise. The differences between the average sound levels obtained from the microphones placed near from façade wall (0.5 m, 1.0 m and 2.0 m) or mounted directly on the façade was 3 dB. These results are similar to the results obtained in Jagniatinskis and Fiks’ study [16] on long-term environmental noise assessment. In the study by Jagniatinskis and Fiks, the microphone near from façade was placed only two meters from the reflecting surface.
Also, in Memoli’s study [20], the sound levels obtained from the microphone mounted directly on the façade wall and from the microphone placed in the free field were compared. The façade correction was 5.8 ± 0.9 dB and the inferior limit of this confidence interval coincides with the results obtained in a similar study [18]. Mateus et al. indicated that, especially for large distances between the noise source and the receiver, the assumed value of 6 dB might introduce significant errors in the results.
The studies by Jagniatinskis and Fiks [16] and Mateus et al. [18] only evaluate the corrections of a microphone mounted directly on the façade wall at distances of 150 m and 250 m from road traffic noise sources respectively. Relative to the urban forms of southern European cities (narrow streets, reflective façades, construction density, etc.), the façades are closer to road traffic noise sources, and this feature could influence the sound propagation in urban areas. In relation to this topic, Picaut analyses in an experimental work [24] how the sound is propagated in an urban street using an impulsive sound source, reaching the conclusion that the sound field is uniform within a cross section of the street.
Moreover, the ISO 1996-2 standard [14] provides different possibilities as to the height at which you can place the microphone to make noise maps. However, it makes no mention as to the corrections to be applied in each of these cases, nor does the European Directive [4].
In connection with this issue, different studies [26], [27] use the corrections proposed by the French standard “Guide du Bruit des Transports Terrestrial: Prevision des Niveaux Sonores” [3] and ISO 9613-2 standard [15] in order to normalize the acoustic long-term measurements performed on balconies of dwellings situated at higher altitudes to four meters.
The aim of the present work is to study the differences that may exist between the values of the sound levels measured outdoors with the measuring equipment in different positions with regard to the façade or the ground. Accordingly, we analyse the necessary corrections to ensure reproducible and comparable results in the urban noise measurements carried out by different technicians or in different locations.
For this work, six different locations were chosen in the city of Cáceres (Spain), all of them different with respect to the street and traffic flow characteristics. The measurements were carried out simultaneously with two sound-level meters to analyse the ISO 1996-2 corrections. To that end, measurements were performed at different distances from the façade (0 to 3.0 m) and at different heights (1.2 to 6.0 m) to study the effect of outdoor reflexion.
Section 2 describes the methods used. Section 3 presents the results and discussion. Finally, Section 4 gives the principal conclusions of the study.
Section snippets
Sampling points selection
For the purpose of the present study, sampling locations were selected throughout the city of Cáceres (Spain). In previous studies, information about the acoustical levels in both the historic center and the entire city can be found [1], [2], [9], [27].
In the selection of the sampling points we pay special attention to the considerations of Annex B of the ISO 1996-2 standard (microphone positions from reflective surfaces) with regard to the following:
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Size and structural features of the façade:
Results and discussion
As stated below, the results are shown as a function of the measurement groups A, B and C. In each of these groups, we analysed first the differences in the mean values of the bandwidth equivalent sound level and then the differences in the mean values per octave bands frequency.
Conclusions
For normal measurement situations in urban areas, in terms of meeting the requirements specified by the ISO 1996-2 standard in Annex B, we have studied the necessary corrections that must be made in order for the measurements carried out at different equipment positions with respect to the façade to be comparable between different measurement points.
To study the effect of reflexion from the façade, a reference distance to the façade of 2.0 meters has been considered, and to study the effect of
Acknowledgements
This work was partially supported by the European Regional Development Fund (ERDF) and the Autonomous Region of Extremadura, GR10175. The authors wish to thank Mr. C. Montes for assisting in carrying out the experiments.
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