What is a noise contour?

A noise contour is a line surrounding an airport that encloses a geographic region, which is exposed to a particular level of noise generated by aircraft arrivals and departures. Annual contours are used to determine whether certain types of zoning or land uses are allowable under government-mandated maximum noise levels. The Federal Aviation Administration has determined that 65 dB DNL (65 decibels, Day-Night Noise Level) is the level at which residential land use becomes incompatible in relation to aircraft operations at an airport.