• STRENGTHS: The L_max metric measures the maximum level of noise during a single event. It provides some measure of how intrusive the noise event is. It is also one of the few metrics than people can actually experience and measure.
• WEAKNESSES: The L_max metric does not measure the duration of the noise. Thus, a noise with a low Lmax and a long duration can be just as intrusive as a noise with a high L_max and a short duration.

Sound Exposure Level, SEL

The Sound Exposure Level, SEL, represents the duration and the magnitude of a time-varying noise event. It measures how loud the noise is, and how long the noise lasts. The SEL metric is used to describe the noise exposure of a single aircraft event. The SEL metric measures the entire event and therefore, does not directly represent the sound level heard at any given time. SEL contour maps are used to show the noise footprint of an individual aircraft event. These maps are often used to compare noise levels produced by different aircraft. SEL contour maps can be used to help the public understand the DNL metric. An SEL contour map is shown below [Figure 1].

Figure 1. An Example of a Sound Exposure Level (SEL) Contour Map

(Source: Aircraft Noise Study for Eastern WV Regional Airport/Shepherd Field, Martinsburg, West Virginia, Environmental Impact Statement (EIS) independent review for the Air National Guard Readiness Center (ANGRC) C-5 Aircraft Conversion, Wyle Technical Note TN 03-03, July 2003)

Equivalent Sound Level, L_eq

The Equivalent Sound Level, L_eq, is used to determine the average sound level that occurs over a specified period of time. L_eq can be calculated for any period of time, such as an hour, a school day, daytime, or nighttime. L_eq is the constant sound level that contains the same sound energy as an actual time-varying sound level over the identical time period. L_eq can also be graphed on contour maps. Figure 2 depicts the L_eq during the daytime, while Figure 3 depicts the L_eq during the nighttime.

• STRENGTHS: The L_eq metric is useful in explaining the total aircraft noise exposure over an extended period of time. It can be used to show the variation in sound level from hour-to-hour.
• WEAKNESSES: Because 'L_eq_ is an average sound level over a specified period of time which could include periods of no noise, it does not represent a sound level experienced at any time. Also, since it does not take into account heightened sensitivity to nighttime noise (unlike DNL), it may not correlate as well with human response to a 24-hour noise exposure when events occur both in the day and nighttime hours.

Figure 2. An Example of Daytime Average Sound Level (DL, Leq, 15h) Noise Contours

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

Figure 3. An Example of Nighttime Average Sound Level (NL, LAeq,9h) Noise Contours

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

The Time Above a Specified Level, TA, metric measures the total time or percentage of time that the A-weighted aircraft noise level exceeds a defined sound level threshold (L) over the desired time period (X). The TA metric can be calculated for any period of time, such as daytime or nighttime. The time period is usually defined in minutes. The TA metric is usually described by TAL(X). For example, a TA65(60) calculated over a 24-hour day describes an area within which the noise level exceeds 65 dB for 60 minutes or more in a 24-hour day. Other TA based metrics include the Time Above Ambient Level (TALA) and Time Audible (TAUD). The TA metric can also be displayed on contour maps [Figure 4].

• STRENGTHS: The TA metric can be shown on contour maps. It can also be displayed in tables to show multiple noise-sensitive locations at different sound level thresholds.
• WEAKNESSES: The '''TA metric does not provide the maximum noise level reached during the given time period. TALA or TAUD do not distinguish between noise levels produced, so the same TALA or TAUD could represent either very loud noise events or quieter but audible noise events produced over the same time period.

Figure 4. An Example of Annual Average Day Time-Above 65 dB Contours [TA65(x) for 10, 20, and 40 min per day]

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