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Appendix H <br /> The maximum sound level(I.)and the minimum sound level(Lmi; )are the <br /> maximum and minimum sound levels respectively,measured during the <br /> measurement period. When a sound meter is set to the slow response setting as is <br /> typical for most community noise measurements,the Lmax and L . values are <br /> the maximum and minimum levels measured over a one second period. <br /> Ambient Sound <br /> Ambient sound is the all-encompassing sound associated with a given <br /> community site,usually being a composite of sounds from many sources,near <br /> and far,with no particular sound being dominant. <br /> Equivalencies between Various Sound Descriptors <br /> The Ldn value at a site calculated from a set of measurements taken over a given — <br /> 24-hour period will be slightly lower than the CNEL value calculated over the <br /> same period. Except in situations where unusually high evening sound levels <br /> occur,the CNEL value will be within about 1.5 dB of the Ldn value for the same <br /> set of sound measurements. <br /> The relationship between peak hourly Ley values and associated L&values _ <br /> depends on the distribution of traffic over the entire day. There is no precise way <br /> to convert a peak hourly L,,value to an L&value. However, in urban areas near <br /> heavy traffic,the peak hourly Leq value is typically 2-4 dB lower than the daily <br /> Ldn value. In less heavily developed areas,the peak hourly Leq is often equal to <br /> the daily Ldn value. For rural areas with little nighttime traffic, the peak hourly <br /> Leq value will often be 3-4 dB greater than the daily Ldn value. <br /> Working with Decibel Values <br /> The nature of the decibel scale is such that the individual sound levels for <br /> different sound sources cannot be added directly to give the combined sound <br /> level of these sources. Two sound sources producing equal sound levels at a <br /> given location will produce a composite sound level that is 3 dB greater than — <br /> either sound alone. When two sound sources differ by 10 dB, the composite <br /> sound level will be only 0.4 dB greater than the louder source alone. <br /> Most people have difficulty distinguishing the louder of two sound sources if <br /> they differ by less than 1.5-2.0 dB. Research into the human perception of <br /> changes in sound level indicates the following: <br /> ■ a 3-dB change is just perceptible, <br /> ■ a 5-dB change is clearly perceptible,and <br /> ■ a 10-dB change is perceived as being twice or half as loud. <br /> J <br /> H-4 <br />