Sound Sensors & Pattern Recognition
These can be highly accurate and measure volume (dbl), tone, pitch, and/or frequency, or simply detect the existence sound above a certain threshold. They all use microphones but not all deliver insights related to the nature of sound or sound identification which is enabled by using existing libraries or training with machine learning. Typically noise levels are recorded in public spaces and visualised with a graph in a dashboard.
For more information about all the different sorts of sensors being used in several projects in Amsterdam to create data visualisations, download the Crowd Monitoring Systems Amsterdam (CMSA) project report PDF (link)