![]() The basis for the map is a series of sound level measurements taken by the National Park Service in national parks across the U.S. A dBA of zero indicates the threshold of normal human hearing while rustling leaves might have a dBA of 20, a babbling brook 40, regular conversation 60, an alarm clock 80, a rock band 110 and a jet engine at take-off 140. On the map this is indicated by the notation L50 SPL, which refers to the median (50th percentile) sound pressure level (SPL). Clearly sound levels are not constant so the median dBA is used, which is to say the sound level that sits in the middle so that it is less than the actual sound level half the time and greater than the actual sound level for the other half. The actual measure used for this map is the A-weighed decibel (dBA), which is intended to account for the relative loudness perceived by the human ear, which is less sensitive to low audio frequencies. The decibel scale is logarithmic so that a 10 dB increase corresponds to a tenfold increase in sound energy. A decibel is one-tenth of a bel, a unit named for Alexander Graham Bell, the inventor of the telephone.ĭecibels measure the intensity of the pressure wave created by the sound relative to a normal or standard pressure. Sound is normally measured in decibels (dB). The loudest are found in wetter areas with more vegetation, flowing water and many birds, insects and amphibians, such as the Mississippi Valley and South Florida.īy contrast, mountainous areas and deserts that are relatively devoid of life are much quieter. The map shows that natural sound levels are linked to topography. On top of this are sharper and more intermittent sounds made by birds, insects, amphibians and mammals.Īlthough the natural world is generally peaceful some of its sounds can be remarkably loud, such as the deafening trill of chorus frogs or the piercing whine of cicadas. Some such as the sound of running water, the wind rustling through the leaves and falling rain provide a steady background of noise. It shows the distribution of the loudness of natural sounds (sounds that would be heard if human influence were taken out of the picture). This remarkable map was developed by the Natural Sounds and Night Skies Division of the U.S. ![]() Source: Natural Sounds and Night Skies Division of the U.S. ![]()
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