Gracey & Associates
Noise and Vibration Measurement Basics
Statistical Noise Levels
Because noise levels often fluctuate over a wide range and over time, no single value describes the noise accurately.
A common used parameter is the Leq - equivalent level, which we deal with under a separate heading - for details click here.
Another useful set of parameters are the Ln values which we briefly describe here.
If we sample the fluctuating noise levels and store the results once a second, then at the end of an hour we would have 3600 samples. We can then use these sample to determine some helpful statistics. For example if add up all the samples and divide by 3600 then we will get the average or L50% value of the noise over the hour. In practice, modern sound levels meters sample at more than ten times a second to improve the statistical accuracy if the noise level are very intermittent.
You could of course calculate the L23.5% value for instance or any value between 0.01% and 99.99%, but the most common levels used are the L10 and the L90 or L95.
It is also good practice to state the period of time the measurements were taken over - for example 70 dBA L10(18-hour) ties it down pretty well and shows the measurements were much more extensive than 70 dBA L10 (5-minutes).
The L10 was found over a long period of time to be a useful descriptor of road traffic noise as it correlated quite well with the disturbance people felt when in close proximity to busy roads as well as more rural situations. By definition the L10 value is the level just exceeded for 10% of the time so it is biased towards the upper end and takes account of the annoying peaks of noise. It is widely used for planning applications for new traffic schemes and the L10 over an 18-hour week day period is also used when compensation or grants for double glazing are being considered.
When assessing nuisance from industrial sources it is common practice to compare the levels from the factory at the residents house with the background or preferably the pre-existing levels before the alleged nuisance was introduced. Now the background fluctuates as well, so the L90 or the L95 have been widely adopted to quantify background noise levels.
The well-known BS 4142 Method for rating industrial noise affecting mixed residential and industrial areas recommends using the L90 level. You might think that BS 4142 would compare the L50 level with the L90 to assess the noise impact. But it was found that the Leq, usually a higher figure than the L50, agreed more closely with the majority of the population's subjective response, over many years.
Click here for more information on the Leq
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