There are different time weightings and frequency weightings when measuring sound pressure. 

When it comes to frequency weighting, the best known is the “A” weighting (dB(A)). This adapts the sound level values ​​at different frequencies to the human hearing sensitivity. Maximum permissible sound pressure values ​​are usually given in dB(A). 

When evaluating the time, either the average of the sound pressure level is determined over 1 s (“Slow”), 125 ms (“Fast”) or 35 ms (“Peak”). Depending on the type of sound signal, these values ​​can vary significantly. It is obvious that averaging over different time ranges will produce completely different values. 
Especially when measuring low volumes, these are often overshadowed by background noise. This could be traffic noise, a refrigerator, birdsong, breathing, police sirens or similar. 

In order to eliminate this effect, when making such sound measurements we always consider the sound signals that are evaluated at different times. Over a period of e.g. B. 30 s then the maximum and minimum values ​​are determined: 

Since ventilation systems as well as heat pumps always generate a kind of background noise that is continuously present and is hardly subject to fluctuations, the value LAFMinT is particularly important for measuring the sound of such systems, i.e. the lowest sound pressure level (over an evaluation period of 125 ms). 30s. 

Short noises shorter than 30 s do not affect this value. However, long-lasting disruptive noises (such as other continuous noises or aircraft overflights, etc.) cannot be eliminated.

Example of the time course of a sound signal with noise and different design periods