Because aircraft have operating lives of 20 or more years, it takes decades for airline fleets to catch up to the latest and quietest technology. A new Boeing 787 has approximately one-tenth the noise “signature” on departure as a Boeing 727-200, yet it carries more than twice the number of passengers. This information does not imply that reductions achieved will not be maintained, only that the continued improvements may be less dramatic. Small evolutionary changes may be occurring, but the technological noise reduction achieved through higher engine bypass ratios has a limit, and that limit is being approached. However, the reduction in noise with each new generation of engine is not as dramatic as previous generations. Dramatic reductions in engine noise have occurred since the early 1980s. The current production of civilian aircraft is markedly quieter than the older technology aircraft. What is a noise contour map and how is it produced? The CNEL is a 24-hour noise dose average (on an energy basis) of all SELs recorded. The overall airport noise environment is described and defined by the Community Noise Equivalent Level (CNEL) metric.

These are commonly referred to as single event noises and are described by sound exposure levels (SELs). Aircraft noise in the vicinity of SAN is made up of many events, including aircraft takeoffs, landings, overflights, and ground noise (the noise generated by aircraft on taxiways and gate areas). In hearing individual aircraft (a single event), a change of 3 dB is usually discernable by most people, a change of a single event of 10 dB is significant. Airport Noise Mitigation’s NMTs use A-weighting. Sound pressure level is a measure of the sound pressure of a given noise source relative to a standard reference value, where the reference value is set as the typical threshold of perception of an average human. The decibel (dB) a metric that is commonly used in acoustics as a unit of sound pressure level. Finally, downloaded information from the 23 NMTs is used to develop a noise contour map that defines the area of noise-impacted communities. Additionally, stored information is used to confirm departure curfew violations and to respond to aircraft noise complaints from community members. The system also helps staff monitor noise levels and aircraft flight tracks. The data is evaluated and compiled to meet State requirements, and periodic reports are submitted to the County of San Diego and the State of California, Division of Aeronautics. These sites, which are acoustic microphones mounted on tall poles, measure noise and transmit the data, twenty-four hours a day, to a computerized noise monitoring and flight tracking system located at the Airport. There are currently 23 noise monitoring terminals (NMTs) surrounding San Diego International Airport (SAN) in communities ranging from Golden Hill, Uptown, Loma Portal, Point Loma, Ocean Beach, and Mission Beach.

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