Environmental Assessment June 1989

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2.0 PROBABLE ENVIRONMENTAL IMPACTS 2.1 NOISE Introduction One of the most conspicuous aspects of airplanes is the noise they produce. Noise has been a major point of objection among people living in the vicinity of busy airports. Clearly, specific types of human activity may be incompatible with certain levels of noise. Therefore, one of the key ingredients in appraising the compatibility of an existing or proposed airport or an airport improvement in an area must be the eval- uation of the volume, frequency and distributionof noise that is generated by that airport at future levels of operation. There are basically four contributing factors to the generation of noise impacts at airports: 1. The type of aircraft 2. The number of operations 3. The time of day 4.- The percent use of different runways and associated flight paths Aircraft noise levels can be determined by a number of different methodologies and then plotted on appropriate maps to indicate exposure levels. The noise levels at Gateway North Industrial Airport were determined by the Day -Night Level (Ldn) method as detailed in FAA report FAA-AS-75-1, "Developing Noise Exposure Contours for General Aviation Airports". All of the aforemen- tioned contributing factors are included in the analysis. The Ldn contours illustrate a single number measure of the impact of noise intensity over a 24 hour period. Ldn (Day -Night Level) provides a cumulative measure of the noise intensity of all events in a 24 hour period in units of dBA (decibels on the A - weighted scale) with a 10 dB penalty applied to nighttime opera- tions (10 p.m. to 7 a.m.). The Ldn technique provides a quantity which can be measured directly using portable moni- toring equipment. It was developed by the Environmental Protection Agency for noise exposure surveillance and as an aid in land use planning. Because Ldn is a measure of the cumula- tive noise level occurring within a 24 hour period, with a weighted value for nighttime operations, it cannot be converted to single event level dBA which measures only the peak intensity of sound during one aircraft operation. Table 2-1 lists a number of common sounds, dBA, and their relative loudness. Note that the relative sound energy involved is logarithmic. Thus, a difference of 10 dBA in sound corresponds to a 10 fold increase (or decrease) in relative sound energy, and the perceived relative loudness of the sound doubles (or halves). 2-1