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Using Decibels Addition and subtraction of decibels is often necessary for estimating total noise levels or background noise. Because decibels are measured using a logarithmic scale, conventional linear mathematics cannot be used. The most convenient way to perform simple arithmetic functions involving logarithmic measurements is to use doubling rules. These rules provide an accurate estimate of the effect distance and multiple sources have on measured sound pressure level. Distance Attenuation Estimations Line When the distance is doubled from a source the sound level decreases three decibels. Example: If a sound level is: 70 decibels at 50 feet it will be 67 decibels at 100 feet, and 64 decibels at 200 feet. Point When the distance is doubled from a source the sound level decreases six decibels. Example: If a sound level is: 95 decibels at 50 feet it will be 89 decibels at 100 feet, and 83 decibels at 200 feet. Decibel Levels of Common Noise Sources Sound Pressure Level (dBA) Noise Source 140 ----------------------------------Jet Engine (at 25 meters) 130 ----------------------------------Jet Aircraft (at 100 meters) 120 ---------------------------------- Rock and Roll Concert 110 ---------------------------------- Pneumatic Chipper 100 ---------------------------------- Jointer/Planer 90 ----------------------------------- Chainsaw 80 ----------------------------------- Heavy Truck Traffic 70 ----------------------------------- Business Office 60 ----------------------------------- Conversational Speech 50 ----------------------------------- Library 40 ----------------------------------- Bedroom 30 ----------------------------------- Secluded Woods 20 ----------------------------------- Whisper D = DISTANCE FROM CENTER OF GENERATOR S = SOUND PRODUCED IN Db D / 2 = S – 6 Noise similar to: 100ft = 60dB Conversational Speech 200ft = 54dB Conversational Speech – Library 400ft = 48dB Library 800ft = 42dB Bedroom 1600ft = 36dB Bedroom - Secluded Woods This information is taken from pages 5 and 6 of Minnesota Pollution Control Agency “A Guide to Noise Control in Minnesota” revised 3/99 and GENERAC Power Systems, Inc.