Isobel Maps

The isobel map is a specific type of contour map used in many scientific disciplines to show how some variable changes over an area. The most familiar examples are the altitude contour map that shows the height or altitude variations of a geographical area, or the barometric map in meteorology that joins points of equal pressure (isobars).

Besides documenting the spatial variation of specific data, the density of contour lines indicates steep or gentle gradients, depending on how far apart they are. In the 1970s, the World Soundscape Project attempted using this form of representation of sound level data in what they called isobel maps, that is, showing points of equal decibel readings.

Of course, time variation of levels is a major issue for the isobel map. In the European examples, the team took their measurements in late evening when the villages were quiet, and therefore the isobel map shows mainly the constant sources, which could be regarded as keynote sounds because of their constancy.

In the case of Stanley Park in Vancouver, a much larger area, sound level readings (in dBA) were averaged between 3 readings taken 10 seconds apart on a particular weekday during daytime hours with similar weather conditions.

Today, more sophisticated graphic methods are routinely used in acoustical engineering, such as using colour gradients, or 3-dimensional modelling. In terms of time variation, Equivalent Energy Level measurements, Leq, may be used for energy averages over a period of time, converted to decibels, as covered in the Noise Measurement module.

Click to enlarge. More details about the data in the Isobel link

In this map, the effect of the road running through the middle of the park is easily seen in the density of isobel lines, in the 65-70 dBA range, compared with the smaller roads running around the perimeter that were measured in the 50-55 dBA range. The quietest areas of the park are those farthest away from the roads, which measured in the 40 dBA range.

Isobel map for Skruv, Sweden, with levels taken in the late evening, reflecting the factory hums, a shopping plaza and a stream.

Isobel map for Cembra, Italy, with levels taken in the late evening, reflecting the various fountains.
The slightly higher levels at the perimeter of the village reflect the river and sounds from the valley below the village.