Noise Basics

Supplemental Noise Metrics

Number-of-events Above a Specified Level, NA

The Number-of-events Above a Specified Level, NA, metric is a useful metric, but is not currently available in existing aircraft noise computer programs. This metric is usually denoted by NAL(X)'. The NA metric calculates the total number of aircraft events (X) that exceed a selected sound level threshold (L) during a specified time period. The threshold is usually defined using either the SEL or Lmax metric. The time period can be any time interval, such as a 24-hour day, nighttime, or daytime.

  • STRENGTHS: The NA metric is the only metric that combines single-event noise levels with the number of aircraft operations. This metric is used to answer the question, "How many aircraft fly over a given area above a selected noise level?" The NA metric is a good indicator of the effects that airport noise will have on human activities, such as speech or sleep.
  • 'WEAKNESSES' One drawback of the NA metric is that does not measure the maximum sound level that occurs during the given time period. NA analysis requires more software than is currently available in existing aircraft noise computer programs. Figures 1 and 2 depict various NA analyses.
Number of Events Above 65 dB Contours

Figure 1. An Example of Number-of-events Above (NA) Contours Above 65 dB Lmax

(Source: Noise Study for the St. Petersburg-Clearwater International Airport, Wyle Laboratories Report WR 05-15, December 2005)

Number of Events Above 70 dB Contours

Figure 2. An Example of Number-of-Events Contours Lmax Above 70 dB - Comparison of Existing and Proposed Conditions

(Source: Buffalo Niagara International Airport, FAR Part 150 Noise Compatibility Study, Niagara Frontier Transportation Authority (NFTA), PB Aviation and Wyle Laboratories, October 2004)