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 and is much simpler than using percentages.

For example: it is much easier to say a material shields the power from a radio tower by 120 dB (decibels) than the equivalent 99.9999999999%.

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.

We can use decibels for describing the ratio of both fields and powers. But which one should we use and why"

Fields
One formula for calculating decibels, when describing fields, is:

* 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%!):

Powers
Furthermore, when comparing power ratios, the formula is modified slightly, but this yields a big difference in results:

** 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%!):