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Equal SOUND PRESSURE LEVEL (SPL). Locations where equal SPL readings are taken, or for which they are extrapolated from other readings, may be joined together in isobel contour lines, similar to the geographer's equal altitude contours.
An isobel map shows variations in sound pressure over a given area by connecting those points for which the measured levels are equal. Compare the sound profile map under ACOUSTIC SPACE.
See: SONOGRAPHY. Compare: ACOUSTIC HORIZON.
Isobel map of Stanley Park, Vancouver, B.C. (from The Vancouver Soundscape, No. 2, Music of the Environment series, World Soundscape Project, 1974).Note: Sound level measurements were taken on the footpaths at intervals of about 100 yards, between the hours of 10 am and 4 pm, on several successive Wednesdays during May, June and July, 1973. The weather each day was similar: clear and bright, with temperatures in the middle 60's and 70's °F. At each point, three readings were taken on the A-scale, fast meter speed, 10 seconds apart, and were later averaged together for the construction of the isobels.
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