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Note that these four cumulative descriptors , all in the first <br /> category, do not answer the question "How loud are the airplanes?" They <br /> simply describe the total amount of noise an area receives during a day. <br /> • <br /> Tine Above Threshold (TA) <br /> One airport descriptor does answer the question "H w loud?" It <br /> also answers the question "How long at that level?" To describe the <br /> total environment - an environment that , particularly at an airport , is <br /> made up of many levels of sound - a series of times above several sound <br /> levels is required . This description of the environment is i3 referred to <br /> as time above threshold (TA) . <br /> TA is amultiple-threshold system that adds together ingle-event <br /> durations of noise above a number of sound levels - typically 65, 75 , <br /> 85 , 95 , 105 , and 115 dBA - for 24-hour and for evening a d nighttime 410 <br /> periods . For full use of this method , a series of numbers is required <br /> to describe the environment . However , in some instances , a single level <br /> will be of primary concern ; for example , a level representing the onset <br /> of speech interference . <br /> We discuss the use of the TA descriptor in the segment on Annoyance <br /> and Community Reaction . <br /> S <br /> 22 <br /> _,e'l'gry'r,J-Tvin'Tolgarmwg <br />