Evaluation of Aircraft Noise

The noise levels around airports are of serious concern in many countries. Several attempts have been made to produce measures to predict and assess the annoyance caused by aircraft noise in the community. A study of rating measures in 1994 showed 11 different measures in use in the 16 countries studied [29]. The following measures merit brief discussion.

6.12.1 Composite Noise Rating

The CNR has a long history dating back to the early 1950s [36–38]. Originally, the basic measure it used was the level rank – a set of curves placed about 5 dB apart in the midfrequency range, rather similar to the NC and NR curves described earlier. The level rank was obtained by plotting the noise spectrum on the curves and finding the highest zone into which the spectrum protruded. The rank found initially plus the algebraic addition of corrections gave the CNR. The corrections [36] were for spectrum character, peak factor, repetitive character, level of background noise, time of day, adjustment to exposure, and public relations. The value of CNR obtained was associated with a range of community annoyance categories found from case histories – ranging from no annoyance, through mild annoyance, mild complaints, strong complaints, and threats of legal action, to vigorous community response.

In the late 1950s, the CNR was adapted to apply to the noise of military jet aircraft [39] and later of commercial aircraft [40]. The calculation was further modified later when applied to commercial aircraft by using the PNL instead of the level rank or the SPL just referred to.

The final version of CNR does not contain any corrections for background noise, previous experience, public relations, or other factors such as the presence of pure tones. Although CNR is no longer used, it is discussed here for completeness and because it has formed the basis for some of the other noise measures and descriptors, such as NEF, which follow.

6.12.2 Noise Exposure Forecast

The NEF is a similar measure to CNR, but it uses the EPNL instead of PNL [41, 42]. Thus NEF automatically takes account of the annoying effects of pure tones and the duration of the flight events. The use of NEF has been superseded in the United States and most other countries by the day–night level L_dn or the day–evening–night level L_den.

6.12.3 Noise and Number Index

The noise and number index (NNI) is a subjective measure of aircraft noise annoyance first developed and used in the United Kingdom. The NNI was the outcome of surveys in 1961 and 1967 of noise in the residential districts within 10 miles of London (Heathrow) Airport [43, 44]. NNI is based on a summation of (PNL)_N terms weighted by aircraft movements. (PNL)_N is the average peak noise level of all aircraft operating during a day, and N is the number of aircraft movements. Here PNL is the peak PNL produced by an individual aircraft during the day and N is the number of aircraft operations of that type over a 24‐hour period.

In 1988 NNI was superseded in the United Kingdom by a measure based on the A‐weighted L_eq. The L_eq is determined over the period 07:00–23:00 hours. Noise at night is evaluated in terms of the size of the 90 SEL footprint of individual aircraft movements, or, less commonly, using L_eq determined over the period 23:00–07:00 hours.