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20 <br />Attachment 3 <br />Environmental Sound Page 5 <br />DNL is most useful for planning purposes when noise does not change much from day to day. It faits to <br />identify some problems due to intermittent loud periods our undesirable sound qualities, Most typically, <br />daytime average levels aze close to or a few decibels less than the day-night leveh Night-time average <br />levels are usually 5 to IS dB less than the day-night leveh If the noise is dominated by a major highway, <br />the DNL is usually within one dB of the average level for the loudest hour, As with any average, <br />communities with very different actual noise chazacteristics can have the same day-night level. <br />Typical Community Sound Levels <br />Our world is filled with sound. People in various places have become accustomed to a wide range of <br />sound levels with widely varying frequency and temporal characteristics. We usually do not even notice <br />these background sounds. We may notice but accept certain identifiable sounds such as properly muffled <br />automobiles or lawn mowers. Problems occur when loud sounds regulazly interfere with activities, There <br />also can be annoyance when the frequency content, time pattern, information content, or concentration of <br />sound from a given direction makes it especially noticeable. When the sound is undesirable, we call it <br />noise.. Noise is by definition undesired sound.. Depending on circumstances, one person's music maybe <br />another's noise. Even the same sound may be music or noise to the same person at different times, <br />The figure on the next page shows ten minutes of sound measured for the US EPA on what is described <br />as a "quietsuburban street.." However, many southeastern US azeas would consider this an urban azea by <br />local standards, While the levelmuch of'the trine is below 50 dBA, it sometimes rises above 60 dBA. The <br />average level, I,EQ, forthis period is azound 58 dBA. Many people have a tendency to consider only the <br />quietest background sounds as the typical sound of their community. this is not realistic. A common <br />occurrence often overlooked is high-pitched insect sound in the range of 55 to 60 dBA. The average level <br />often provides a better indicator of'the sound. However, you must be cazeful. The average level can be <br />raised very much by a single, short, very loud sound, Thus, average levels also may not represent <br />conditions normally expected. Knowing the full statistical picture of the sound is helpful. The L90 indicates <br />the background level, azound 44 dBA ur the figure, and the LOl indicates the loudest levels, around 68 <br />dBA in the figure, <br />