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**Aircraft Noise Modeling**

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Noise analysis requires a combination of physical description of sound and potential responses to it. Human response to noise depends on many factors: loudness, number and duration of events, time of day, ambient background noise levels, interference with certain activities (such as sleep or conversation) and previous experiences.The Navy prepares comprehensive noise studies to understand the noise associated with airfield operations at a specific airfield location. Primarily, aircraft noise is associated with aircraft takeoffs and landings, and during engine maintenance operations (or engine run-ups). Takeoffs and landings are intermittent sounds which occur in short duration; while, engine run-ups are continuous sounds which occur for longer durations for maintenance actions and engine testing.Noise from aircraft operations, which exceed ambient background sound levels, typically occurs: 
                        -  beneath the arrival and departure flight tracks 
                        -  in aircraft flight patterns around the airfield 
                        -  in areas immediately adjacent to aircraft parking ramps, aircraft staging areas and engine maintenance areas 

As an aircraft in-flight gains altitude, its noise contribution drops to lower levels, often becoming indistinguishable from ambient background sound levels.**How is noise assessed?**The U.S Environmental Protection Agency (EPA), Federal Aviation Administration (FAA) and Department of Defense (DoD) measure community impacts of aircraft noise levels in decibels (dB) using several common metrics:
                        -  Day-Night Average Sound Level (DNL) 
                        -  Sound Exposure Level (SEL) 
                        -  Maximum Sound Level (Lmax) 

DNL is the federal standard for determining community noise impacts. Studies have found good correlation in the percentage of people highly annoyed and the level of average noise exposure measured in DNL. DNL represents the average sound energy of events over a 24-hour period, with 10-dB added to night-time (10 p.m. to 7 a.m.) events. This 10-dB penalty accounts for the added intrusiveness of noise when ambient background noise levels are low and noise-sensitive activities such as sleep take place.DNL is depicted as a continuous line around a noise source (i.e., a noise contour line) and is usually shown in 5-DNL increments (e.g. 65 DNL, 70 DNL, and 75 DNL). DNL takes all of those factors that influence our perception of noise – loudness, number and duration of events, and time of day – and includes them in one metric that is used to identify land uses that are compatible with specific noise zones.SEL combines the loudness and duration of a time varying sound even such as an aircraft flyover. It provides a measure of total sound energy of the entire event, but does not directly represent the sound level heard at any given time.  It is a composite metric that represents the total noise energy of a single event as if it occurred in one second. Lmax is the highest dB level measured during a single event where the sound level changes value with time, like an aircraft flyover.  It defines the maximum sound level occurring for a fraction of a second (typically 1/8 second for aircraft noise).  For sound from aircraft overflights, SEL is typically greater than Lmax because the individual takes several seconds, and Lmax occurs instantaneously.A comprehensive noise study may include supplemental analyses and metrics to improve public understanding of noise exposure and compatible land uses around installations, and to evaluate a specific concern such as annoyance, sleep disturbance, classroom speech interference, noise-induced hearing impairment and non-auditory health effects of noise.**How Is Noise Modeled?**Noise is modeled using a computer program called NOISEMAP, which considers the number and type of flight operations planned over the course of a year to establish noise contours for all types of aircraft operating at a specific airfield location. Input includes: types of aircraft, number of operations, flight tracks, altitude, engine power settings, aircraft speed, terrain, temperature and relative humidity. Engine maintenance operations and testing are also included. Results are presented on installation land use maps in the form of DNL noise contours.Computer modeling is the most accurate and useful means for comparing “before-and-after” noise levels and evaluating different scenarios and ensuring consistent calculations and results.

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