This isobel map of Stanley Park in Vancouver, British Columbia, shows average sound levels in different locations. Sound level measurements were taken on the footpaths at intervals of about 100 yards, between the hours of 10 a.m. and 4 p.m. on several successive Wednesdays during May, June and July of 1973. The weather each day was similar—clear and bright with temperatures in the middle 60s and 70s F. At each point three readings were taken, ten seconds apart, and later averaged together for the construction of the isobels.

Schafer, Murray R, The Soundscape- Our Sonic E…, loc. 884