The isobel map derives from the contour maps of geographers and meteorologists, and consists of hundreds or thousands of readings on a sound level meter averaged out to produce bars of equal intensity, projected as if the observer were above the field of study. On such a map the quietest and noisiest sections of a territory can be immediately identified. … Material for the events map would have to be limited to a specific period of time and would be gathered by walking over or around the selected location.

Schafer, Murray R, The Soundscape- Our Sonic E…, loc. 452