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128 Aircraft Noise: A Toolkit for Managing Community Expectations Noise Barriers/Berms/Shielding Purpose: The noise impact of an aircraft on the ground is usually confined to nearby areas where the line of sight between the source and the receiver is interrupted by the barrier. An effec- tive method to mitigate this type of noise impact is through the use of sound barriers or berms. Hush houses (Ground Run-up Enclosures, GRE) are a special type of barrier that may be appro- priate in engine maintenance areas where noise events are long and often conducted at night. Strategic placement of airfield buildings has been used to interrupt the flow of noise from the source to sensitive receivers nearby. A barrier of any type does not stop the transmission of noise, but rather reflects, absorbs, or redirects parts of the noise energy. Limitations: Construction of a barrier or GRE can be cost prohibitive. Any such facility requires a considerable land footprint on the airfield to construct. Implemented by: Airport. Public reaction: The public reaction is usually positive to the noise level reduction produced by a GRE (there are documented cases where the noise level reduction at the airport boundary from a GRE is as much as 20 decibels). Lesser benefits are achieved through the provision of sound walls or earthen berms between aircraft noise sources and noise-sensitive uses. These too generally yield positive public reaction, although there may be some complaints about their structural design being unaesthetic. Example of technique for public information: Single Event Run-up Contour Proposed GRE Location 727 Stage 3 (171) 8-8 Restrict Apron/Gate Power Purpose: The airport may limit the airlines and other users from utilizing aircraft engines to power aircraft at the gate or at parking positions on the apron. Airports may encourage ground power units, auxiliary power units, or the installation of power and air conditioning at the gate to minimize the noise effects associated with idling aircraft. Limitations: Requires investment in auxiliary power units, ground power units, or gate elec- trical power by airport or airlines. Implemented by: Airport and aircraft operators. Public reaction: Generally positive, but may not be noticeable beyond the immediate environs of the airport. Enhanced Navigational Aids Purpose: The improvement of navigation aids may lead to better defined or predictable routes of flight, as well as enhanced safety at the airport. The introduction of point-to-point navigation systems on aircraft, coupled with ground transmitting equipment has led to the development of radar navigation system approaches and departure routes that may be crafted to more consis- tently overfly noise-compatible areas than previously used headings or vector routes. A wide array of equipment is available that may be used to allow the airport to be used in poorer weather conditions, as well as to safely navigate through the area along specific flight paths. Limitations: Requires investment in equipment. Often subject to long delays in rising on FAA priority lists for installation. Relatively long evaluation periods are required for more complex procedures. Implemented by: FAA with the assistance of the Airport and participation of the users.