## 1 Introduction  
In the last decade, distributed acoustic monitoring systems started to appear in urban contexts, due to lowering costs of electronic components and to cheaper and smaller hardware for data transfer. Triggered by the European Directive 2002/49/EC and connected to the assessment and management of environmental noise (END) [1], this interest has grown due to the awareness that noise maps represent a powerful tool for determining the population exposure to environmental noise. The recent END revision [2] has updated the current situation on the application of the END. Noise pollution continues to be a major health problem in Europe, with a host of health effects that can be summarized as follows: annoyance [3], sleep disorders with awakenings [4], learning impairment [5; 6; 7], and hypertension ischemic heart disease [8; 9; 10]. In this context, the END's prescription for noise maps and action plans,have prompted many efforts by the member states and the scientific community to high level study and propose new mitigation systems for the main sources of noise: road traffic [11; 12], railway traffic [13; 14], airports [15; 16], industrial centers [17; 18], plus other sources recognized as highly annoying [19; 20]. In particular, mitigation measures in urban context need to be identified according to a realistic picture of noise distribution over urban areas. This requirement demands for real-time measurements and processing to assess the acoustic impact of noise sources. A number of projects have been devoted to making extended noise measurements. To cite a few: the SENSEable Pisa project [21] was aimed at developing a network of noise sensors located at the residences of volunteer citizens to produce a detailed real-time acoustic overview of the territory. In the same way, the NoiseTube [22] and Smart Citizen [23] research projects proposed a participative approach to monitoring noise pollution by involving the citizenship. The Harmonic project [24] suggested the creation of a simple, a-dimensional noise index, closer to the feelings of the populations than the usual averaged indicators, similar to the ones used for air quality. Noisemote [25], a real-time service for environmental noise control based on wireless capabilities operates in such a way that sensors constantly monitor the environment by collecting data and sending real-time alerts if noise levels exceed the prescribed threshold. H.U.S.H. project [26] (Harmonization of Urban noise reduction Strategies for Homogeneous action plans) had the general goal of contributing to the harmonization of the national laws to the European ones for the noise management contained in Directive 49/2002, starting from the realization of a study and an intervention in the city of Florence, considered as a pilot case. The project QUADMAP [27], which stands for QUiet Areas Definition and Management in Action Plans, delivered a method and guidelines for the identification, delineation, characterization, improvement, and management of quiet areas in urban regions as per the Environmental Noise Directive 2002/49/EC. The idea that a limited number of real-time noise measurements could be used to build up a noise map and could be representative of a large area found its realization in the Dynamap project [28]. Dynamap is a co-financed project by the European Commission through the Life+ 2013 program designed to produce dynamic noise maps within a large urban area. The method is described for a given zone in the city of Milan (Zone 9, illustrated in Figure 1), consisting of about 2000 road arches, using 24 continuous measuring stations [29]. Figure 1: The Urban Zone 9 of the city of Milan, the pilot area chosen for the deployment of the acoustic sensors for Dynamap. The number of recording stations has been fixed at 24. The idea that noise emission from a street generally depends on its activity, its use in the urban context rather than strictly from its geometric characteristics, suggested a different approach to monitoring sampling based on stratified sampling [30; 31].