We may all be familiar with the term decibel as it applies to sound level, but what does it really mean? And what does it have to do with EMFs?
Decibel is not really a unit of measurement. Rather, it is a way to describe the ratio of one intensity measurement to another. Expressing it this way is a useful for engineers to make comparisons. Using decibels we can compare the field intensity before and after an event, or compare a measured intensity to a known reference. We can use it to compare electric fields, magnetic fields or even radiofrequency intensity.
One formula for calculating decibels when the frequency remains constant is:
 field* measurement before event 
dB = 20 log  

 field measurement after event 
* Field measurements such as mG, V/m etc.
We can use this formula to compare the effectiveness of an EMF shield. Also using this formula, we can generate the following graph which allows us to easily convert from dB to % shielding (notice that because of the logarithmic relationship, values in excess of 20 dB represent field strength reduction of more than 90%!):
Furthermore, when comparing power ratios, the formula is modified slightly, but this yields a big difference in results:
 power* measurement before event 
dB = 10 log  

 power measurement after event 
** Power measurements such as mW/cm² etc.
Notice that using this formula, values in excess of 20 dB represent field strength reduction of more than 99%!):
