Percentile SPLs

The equivalent SPL discussed above accounts for the fluctuation in noise level of an unsteady noise by forming an average SPL resulting in an equivalent steady A‐weighted SPL. There is, however, some evidence that unsteady noise (e.g. from noise sources such as passing road vehicles or aircraft movements) is more disturbing than steady noise. To try to better account for fluctuations in noise level and the intermittent character of some noises, A‐weighted percentile SPLs are used in some measures, in particular those for community and traffic noise [27, 34, 35]. The level L_n is defined to represent the SPL exceeded n% of the time, and thus L₁₀, for example, represents the SPL exceeded 10% of the time.

Figure 6.12 gives an example of L₁₀, L₅₀, and L₉₀ levels and a cumulative distribution. It is seen in this schematic example figure that the A‐weighted level exceeded 10% of the time L₁₀ is 85 dB. L₅₀ is sometimes termed the median noise level, since for half the time the fluctuating noise level is greater than L₅₀ and for the other half it is less. L₅₀ is used in Japan for road traffic noise. Levels such as L₁ or L₁₀ are used to represent the more intense short‐duration noise events. L₁₀ is used in Australia and the United Kingdom (over an 18‐hour 06:00 to 24:00 period) as a target value for new roads and for insulation regulations for new roads. Levels such as L₉₀ or L₉₉ are often used to represent the minimum noise level, the residual level from a graphic level recorder, or the average minimum readings observed when reading a sound level meter. Figure 6.13 shows the outdoor A‐weighted SPLs recorded in 1971 at 18 locations in the United States. Values of L₉₉, L₉₀, L₅₀, L₁₀, and L₁ are shown for the period 07:00 to 19:00 hours [27, 32]. The small range in levels in recordings 1 and 4 (urban situations) and the large range in levels in recordings 6, 11, 13, and 18 (situations involving aircraft overflights) are very evident. Obviously road traffic usually creates more steady noise, while aircraft movements lead to more extreme variations in noise levels.