## 2 Related Work
### Environmental Audio Databases  
During the last decade, diverse online digital repositories containing environmental audio recordings have been developed. Among them, several online platforms have been designed as a query-based content supplier service where the user is also allowed to upload new audio clips (e.g., Freesound ([[https://www.freesound.org](https://www.freesound.org)]([https://www.freesound.org](https://www.freesound.org))), Soundcloud ([[https://soundcloud.com](https://soundcloud.com)]([https://soundcloud.com](https://soundcloud.com))), AudioHero ([[http://www.audioero.com](http://www.audioero.com)]([http://www.audioero.com](http://www.audioero.com))), StockMusic ([[http://stockmusic.com/](http://stockmusic.com/)]([http://stockmusic.com/](http://stockmusic.com/))), etc.). Other online repositories are devoted to sharing audio information from ecological and/or cultural perspectives with the aim of providing a testimony of environmental sounds of nature and cultures (e.g., EarthEar ([[http://www.earthear.com](http://www.earthear.com)]([http://www.earthear.com](http://www.earthear.com))) or AcousticEcology ([[http://www.acousticecology.org](http://www.acousticecology.org)]([http://www.acousticecology.org](http://www.acousticecology.org)))). Similarly, some sites provide long recordings of environmental soundscapes (e.g., Listen to Africa ([[http://www.listentoafrica.com/audio](http://www.listentoafrica.com/audio)]([http://www.listentoafrica.com/audio](http://www.listentoafrica.com/audio))) or Open Sound New Orleans ([[http://www.opensoundneworleans.com](http://www.opensoundneworleans.com)]([http://www.opensoundneworleans.com](http://www.opensoundneworleans.com)))), while other sites have been designed as digital libraries of sound events (e.g., Macaulay Library ([[http://macaulaylibrary.org](http://macaulaylibrary.org)]([http://macaulaylibrary.org](http://macaulaylibrary.org))) for animal sounds or Xeno Canto ([[http://www.xeno-canto.org](http://www.xeno-canto.org)]([http://www.xeno-canto.org](http://www.xeno-canto.org))), specifically devoted to bird songs). However, since they are either oriented to provide audio clips for aesthetic purposes (e.g., for a soundtrack of a film production) or intangible culture heritage (e.g, saving a particular soundscape), almost no attention is given to validating the quality of the uploaded audio files and their corresponding meta-data (e.g., audio format, sound sources, recording equipment, etc. ), which are key aspects in ensuring data reliability for possible derived scientific studies. During the same period, the environmental audio research community has built some standardized databases for comparative purposes. Some examples of this attempt are the following: Freefield1010 [29], which integrated samples from the FreeSound database, MIVIA road Audio Events Dataset ([[http://mivia.unisa.it/datasets/audio-analysis/mivia-road-audio-events-data-set](http://mivia.unisa.it/datasets/audio-analysis/mivia-road-audio-events-data-set)]([http://mivia.unisa.it/datasets/audio-analysis/mivia-road-audio-events-data-set](http://mivia.unisa.it/datasets/audio-analysis/mivia-road-audio-events-data-set))) [19], containing sound events, namely tire skidding and car crashes, for road surveillance applications, and the \"ESC: Dataset for Environmental Sound Classification\" [30], including 250,000 recordings extracted from FreeSound (tagged as \"field recording\"). Last but not least, in [16] a 19h audio database covering 10 audio contexts of indoor and outdoor environments (including transport and leisure scenarios) was described and evaluated for context-dependent sound event detection (The reader is referred to [[http://www.cs.tut.fi/~heitolt/datasets](http://www.cs.tut.fi/~heitolt/datasets)]([http://www.cs.tut.fi/~heitolt/datasets](http://www.cs.tut.fi/~heitolt/datasets)) for a complete collection of this kind of databases).