THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

332 13. Criteria and Regulations for Noise ControlFigure 13.6. Example of a cumulative distribution and its percentile sound levels.13.8 Percentile Sound LevelsMuch of environmental noise tends to have a great deal of fluctuations in soundlevels. There have been indications that unsteady noise, which occurs from noisesources such as passing aircraft or ground vehicles, is more disturbing than steadynoise. In order to consider fluctuations in noise levels and the intermittent characteristicsof some noises, percentile sound levels are used internationally asdescriptors of traffic and community noise. The level L n represents the soundlevel exceeded n% of the time. For example, L 20 represents the sound level exceeded20% of the time. In Figure 13.6 a cumulative distribution is given withexamples of L 10 , L 50 , and L 90 levels. In this example, L 10 = 85 dB(A) denotesthat 85 dB(A) is the level exceeded 10% of the time. L 50 = 75 dB(A),and this is termed the median noise level, because half the time the fluctuatingnoise level is greater than this value and the other half of the time the noise isless.In Japan the median noise level is used to describe road traffic noise. Levelssuch as L 1 or L 10 are used to describe the more intense short-duration noises. InAustralia and in the United Kingdom, L 10 is applied (over an 18-h period, from0600 to 2400 h) as target values for new roads and for insulation regulations fornew roads. High percentage levels such as L 90 or L 99 are usually used to denotethe minimum noise level or residual or background noise level.13.9 Rating of Aircraft NoiseAircraft noise is a cause of increasing concern in many nations. A considerableamount of effort has gone into developing the means for predicting and evaluatingthe annoyance caused by aircraft noise in communities. A 1995 survey (Gottlob,1995) of rating measures revealed that 11 measures are in use in 16 countriessurveyed. We discuss a few of these measures below.