Level Statistics with PULSE
Levels statistics is a way to classify and describe fluctuating noise such as industrial noise and traffic noise associated with roads, railways and airports. We have incorporated level statistics into PULSE, starting with PULSE 15, as a part of the overall level analyzer in Types 7700, 7770 and 7771. Level statistics are already an integral part of Bruel & Kjær sound level meters and are well known to our sound level meter customers.
Level statistics – The background
The level statistics analysis of a signal is carried out in four major steps:
- Sample the time signal
- Divide the samples into classes according to level
- Add the distribution levels to get the cumulative distribution
- Calculate the percentile levels from this curve
Once the signal has been sampled, it is divided into small sections called classes (e.g., of 1 dB), and every time a sample falls within each class, a counter is incremented. The resulting curve as a percentage of the total number of samples (counters) is the level distribution of the noise.
Noise Level Distribution
In this example you can see that the class width is about 2 dB and that the signal is mostly between 60.0 - 63 dB. The signal is never above 75 dB and never below 45 dB.
Cumulative Distribution
The level distribution can now be used to create a cumulative distribution by taking the contents of each class from the top of the measurement range and working downwards, adding the results.
This result is a series of percentile levels (LN values) at which you can say that the noise was above them for N% of the time. In this example you can see that the signal was above 70 dB for 10% of the time, above 60 dB for 50% of the time and above 50 dB for 90% of the time.
Percentile Levels
Finally the information is transferred back to the original signal to give the percentile levels (LN values) and form a statistical description of the time history of the noise.
Percentile levels are often used for describing traffic noise, background noise and extremes of noise (L1 approximates the maximum RMS level, L99 approximates the minimum RMS level). Frequently-used percentile levels are: L10, L90 and L95. The percentile L90 is often used to represent the background noise level. L10 is used to represent noise occurrences of very high levels.
Setting up Level Statistics in PULSE
Level Statistics is a part of the Overall Level Analyzer. To set up the Level Statistics, first select the Overall Analyzer. Then select Measurement Modes Exp + Statistics in the properties of the Overall Analyzer, as shown below.
To calculate a selected percentage value, first select the Function Organiser, then show the properties page for Exp. + Statistics, and enter the desired percentile value.
Multiple percentile levels can be calculated, and by inserting more functions in the Level Statistics function group, and setting the required percentile level in each of the functions, it is possible to display all the calculated percentile levels in the same display – giving an overview of the variance of the signal.
