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108 Aircraft Noise: A Toolkit for Managing Community Expectations to accommodate the system if the cost is less than furnace replacement. For homes with a hot water system, the existing boiler system will not be replaced, but a separate condensing system would be installed. Electrical systems will be upgraded as necessary to accommodate the new systems. Kitchen and bathroom vents will be treated as necessary. This may include replacing with a non- recirculating fan, baffling the fan exhaust, or rerouting the ductwork thru a ceiling or attic space. In most cases, roof, gable, soffit, and ridge vents are baffled per standard details to reduce noise infiltration into the house. Exterior Noise Abatement The abatement of noise outside a structure may be accomplished through quieting the source or making it less frequent. These actions may be accomplished through modifications of flight tracks, runway usage programs, changes in aircraft run-up locations or hours, or changes in flight procedures. Of comparable value are noise mitigation actions that manage the develop- ment of noise-sensitive properties in areas exposed to significant levels of aircraft noise. The land itself may be designated for uses that are deemed compatible with aviation noise sources (indus- trial, commercial, agricultural, and open space uses are often selected as the most suitable to accomplish this end). Chapters 7 and 8 of this report discuss a number of opportunities for noise abatement through aircraft operation or land use management. Contour and Impact Area Change Over Time and the Differences Between Federal and Local Response to Change During the years since the introduction of noise contours, the area within the contours of sig- nificant noise exposure (65 DNL) have generally shrunk around airports where its mission and character of its operations have remained unchanged through time. In 2007 the FAA reported that the requirement of increasingly more restrictive 14 CFR Part 36 noise level requirements during the last quarter of the 20th century resulted in a reduction of the number of persons within the 65 DNL contour around the nation's airports by 90 percent between 1975 and 2000. When the last of the Stage 2 jets were retired from the United States continental fleet at the end of 1999, the population within the 65 DNL pattern at U.S. airports was estimated to have been reduced from over 7 million to less than 700,000. Subsequent information suggests an additional reduction of approximately 27 percent from year 2000 levels, or to less than 500,000 today. (More is available at: http://www.faa.gov/about/plans_reports/Performance/quarter_scorecard/media/ Noise%20Exposure%20Detail.pdf) Even though the number of large aircraft operating today is much greater than was present more than 30 years ago, the patterns of significant noise levels have been reduced through a combina- tion of improvement in aircraft design, engine design, flying techniques, and air traffic actions. The reductions are being maintained through the management of land uses in areas exposed to high noise levels to assure that the risks to the continued operation of the nation's airports remains low. Federal Response With the gradual shrinking of the noise exposure patterns of 65 DNL or more has been the reduction of the area in which the FAA will fund land mitigation programs. The FAA has made it a practice to limit its funding of acquisition and sound insulation programs to areas within the 65 DNL contour, often prioritizing such programs to mitigate the most severely (loudest) affected areas first. Consequently, some programs implemented in areas around large airports find that by the time the schedule of improvements has reached the outlying program areas, the noise levels are no longer sufficiently high to justify the mitigation action because the contours have shrunk.