Laserfiche WebLink
Distance attenuation estimations <br />Decibel levels of common noise sources <br />Sound pressure level (dBA) Noise source <br />140 Jet Engine (at 25 meters) <br />130 Jet Aircraft (at 100 meters) <br />120 Rock Concert <br />110 Pneumatic Chipper <br />100 Jackhammer (at 1 meter) <br />90 Chainsaw, Lawn Mower (at 1 meter) <br />80 Heavy Truck Traffic <br />70 Business Office, Vacuum Cleaner <br />60 Conversational Speech, Typical TV Volume <br />50 Library <br />40 Bedroom <br />30 Secluded Woods <br />20 Whisper <br />Figure 3 <br />Using Decibel Measurements <br />Addition and subtraction of decibels is often necessary for estimating total noise levels or background noise. <br />Because decibels are measured using a logarithmic scale, conventional linear mathematics cannot be used. The <br />most convenient way to perform simple arithmetic functions involving logarithmic measurements is to use <br />doubling rules. These rules provide an accurate estimate of the effect distance and multiple sources have on <br />measured sound pressure level. <br />Sound propagation and sources <br />Sources of sound can be defined as point or line sources, based on the way sound pressure waves spread away <br />from the source. Sound waves propagate from sources in a way similar to waves traveling away from a rock <br />dropped in a pond. A point source, like a factory, emits sound that spreads out in a sphere. A line source, like a <br />busy highway, emits sound that spreads out in a cylinder. Knowing the sources of sounds makes it possible to <br />make assumptions about how the sound behaves. <br />Over distance, sound attenuates, or is reduced in amplitude, and is perceived as the sound becoming quieter. <br />This occurs as the sound travels outward to an increasingly larger sphere or cylinder, and the energy per unit of <br />area decreases. These basic principles allow us to make generalized assumptions about sound. <br />When the distance is doubled from a line source, the sound level decreases three decibels. <br />Example: If a sound level is: 70 decibels at 50 feet it will be <br />67 decibels at 100 feet, and <br />64 decibels at 200 feet. <br />When the distance is doubled from a point source, the sound level decreases six decibels. <br />Example: If a sound level is: 95 decibels at 50 feet it will be <br />89 decibels at 100 feet, and <br />83 decibels at 200 feet. <br />A Guide to Noise Control in <br />Minnesota • October 2008 Minnesota Pollution Control Agency <br />4 <br />