NOISE EXPOSURE FORECAST (NEF)
A method, developed by the U.S. Federal Aviation Agency to predict the degree of community annoyance from aircraft noise (and airports) on the basis of various acoustical and operational data. As such it is not a consistent index for all environmental health problems. It is applied to determine acceptable levels for various community zoning regions.
For instance, a residential area should have a NEF of 30 or less; between 30 and 40 NEF is stated as suitable for multiple family housing, although it is not clear why a higher tolerance level should apply in this case; above NEF 40, the area is suitable for industrial and recreational purposes only.
The calculation is based on EFFECTIVE PERCEIVED NOISE LEVELs (in EPNdB) for various aircraft, and considers all aspects of flight operation and time of day (weighting night occurrences heavier than daytime ones). However, weather conditions and background noise levels are not considered as yet. Increased public awareness, and subsequent decrease in tolerance of aircraft and other environmental noise, demands continual reassessment of methods such as the NEF.
See: JET PAUSE, SOUND INTRUSION.
The measurement is based on the following equation:
NEF = EPNL + 10 log10 (ND + 16.7 NN ) - 88 (dB)
where EPNL is the energy mean value of the EPNL (see EFFECTIVE PERCEIVED NOISE LEVEL for method of tone and duration correction) and ND and NN are the number of flights during the day (0700 to 2200) and night (2200 to 0700) respectively. The factor 16.7 represents a 10-to-1 weighting of night flights over day ones.
In some places, the NEF has superseded the CNR system (see COMPOSITE NOISE RATING) but because it requires complex computer calculations, another system, the CNEL (see COMMUNITY NOISE EQUIVALENT LEVEL), has been developed based on normal dBA readings. In practice, CNEL values are higher than NEF by 35 ± 2 dB.
The approximate relation of NEF to CNR and NNI (see NOISE AND NUMBER INDEX) is:
NEF = CNR - 72
NEF = NNI - 16