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Sound and Vibration Basics

Statistical Noise Levels, Ln, L10, L90%, etc.


Because noise levels often fluctuate over a wide range and over time, a single value descriptor like the Leq - Equivalent Level is essential.

Another useful set of parameters are the Ln values (Statistical Noise Levels) which we describe here.


If we sample any 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 sound levels fluctuate widely over time or are intermittent.

Any statistical value between 0.01% and 99.99% may be calculated, but the widely used ones 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 and statistically more reliable than a 70 dBA L10 (5-minutes) sample.

The L10 has been found over the years to be a useful descriptor of road traffic noise as it correlates quite well with the disturbance people feel when close to busy roads as well as more rural situations. By definition the L10 value is the level just exceeded for 10% of the time and takes account of any annoying peaks of noise. L10 calculated levels are widely used when planning new traffic schemes and the L10 measured 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 measure the noise levels from the factory at the complainants house and compare them with the background or ideally the pre-existing levels before the alleged nuisance was introduced. However the background levels fluctuates as well, so the L90 or the L95 have been widely adopted to quantify background noise levels.

The long established BS 4142 Method for rating and assessing industrial and commercial sound, recommends using the L90 level. Before the advent of Leq meters, BS 4142 would compare the L50 level with the L90 to assess the likely noise nuisance with a 5dB penalty added for distinct noise sources. More recently the Leq - Equivalent Sound Level, based on an energy average rather than the L50 statistical average, was found to correlate better with the majority of the population's subjective response.

More information on the Leq - Equivalent Continuous Noise Level.

Certified Statistical Sound Level Meters