Statistical noise levels LN are commonly used for environmental noise monitoring, such as road traffic or community noise assessments. In these applications, noise levels fluctuate significantly over time, and level statistics help to better understand what's going on.
For example L90 describes the level which was exceeded for 90% of the time. That is, for 90% of the time for the period measured, the noise level was at least above the L90 level. The L90 level is often used as approximation of the background noise level. On the other side of the scale, L01 describes the level which was exceeded for only 1% of the time. That is, for 1% of the time for the period measured, the noise level was at least above the L01 level. The L01 level is sometimes used as approximation of the maximum level Lmax.
To demonstrate visually how LN values are calculated, the following chart explains the L10 level.
Statistical noise level L10
How does the XL2 measure statistics?
The XL2 samples the selected input values once every millisecond and stores each result. To optimize the memory required to store this much data, values are rounded (e.g. to 1 dB) and then only the number of occurrences of a certain value is stored (and not the value itself). The memory which stores the occurrence is called the "class" and the rounding of the input value the "class width".
The measured parameter used to calculate statistics has to be chosen before the measurement has been started, e.g. LAF or LAeq, and cannot be changed afterwards. The choice of Level Statistic Ln (e.g. L33 instead of L90) may be changed even after the measurement has been finished.
Measured levels every millisecond (ms)
Number of occurences
Level Statistics Screenshot of XL2 Data Explorer Software
Level Statistics of 100 Hz Band (Screenshot of XL2 Data Explorer Software)