# Edge-Computing Meshed Wireless Acoustic Sensor Network for Indoor Sound Monitoring
## 2 State of the Art  
Nowadays, in the literature, several designs of WASNs can be found, all of them with different strengths and weaknesses, depending on their goal and focus in terms of application. One can find networks for indoor localization purposes, or WASN to be deployed in a big city such as New York to evaluate the \(L_{Aeq}\) and even try to detect certain pretrained events. In the recent years, there has been an increase in health concerns together with advances in sensing technologies. These have led WASNs to take a predominant place amongst the tools to survey around the acoustic health of the population living in urban areas [10], and also to monitor the biodiversity conservation in forests [11]. Not only in outdoor environments, but also at home, several WASNs have been tested to include networks on indoor environments to promote ageing at home with high quality of life [12] in the ambient assisted living paradigm, promoting active and healthy ageing at home. The advantage of WASN against other monitoring systems is that they respect the privacy of the users more than other technologies [13] (e.g., video surveillance), especially when the raw acoustic data are processed in the proper sensor, so that only labels travel to other nodes. The number of connected nodes, as well as the distance and connectivity between them, is another of the issues that varies substantially depending on the application; finally, the computational capability of each of the nodes and the central server--if there is one--usually defines the requirements list of these kind of designs, especially when conducted using commercial hardware pieces. Several projects can be reviewed that have developedsensor networks that attend to different features and categories. In the context of the IDEA project [14], [PERSON] et al. [15] propose the usage of low-cost nodes (with a cost of around EUR 50) to monitor outdoor environments that actively auto-check the frequency response of the microphone of each node by embedding a low-cost speaker that generates a periodical frequency sweep. Another example of an outdoor acoustic sensor network is the MESSAGE project, which is explained in [8]. In their work, [PERSON] and [PERSON] present the results obtained on a WASN of 50 nodes in which, apart from a noise detector module, each node incorporates traffic count and chemical sensor modules, using a microcontroller with low processing capabilities to do so. Regarding indoor WASNs, the homeSound project [1; 12] proposes a network architecture with several sensing nodes that send their information to a concentrator node composed of a GPU with parallel computing capabilities, and later, the same team developed a new sensor with low-cost capabilities and more possibilities of real-time algorithm computing [16]. WASNs have been understood as a group of wireless microphone nodes spatially distributed over either an indoor or outdoor environment. The design has to take into account the scalability of the network, the delay of the acoustic signal, the synchronization of the nodes and the decision of where the computing of the data is conducted--if either in the cloud or locally in the sensors [17]. The low invasiveness of these systems made them more attractive for indoor applications, where patients or users are explicitly involved [18], especially when there is a clear intention of health monitoring or remote health tracking. Nevertheless, these sensors can also be used for surveillance applications when taking care of the elderly or disabled people [19]. In [18], the authors present an acoustic event detector system focused on a low-cost platform, recording and processing the sounds indoors. The requirements of the latter applications both in indoor and outdoor environmental projects have led the priorities of the designs into three main issues: the low cost of the sensor design, the acoustic qualities of the designed sensor and the connectivity of the nodes, especially when there is no predetermined network to enable the link of all the nodes.