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dB tutorial
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What is a Decibel?
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%!):

dB vs. % graph


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%!):

dB vs. % graph


POWER
FIELD
dBPower
Ratio
%
Attenuation
%
Transmission
Field
Ratio
%
Attenuation
%
Transmission
01.00001001.000100
11.25921791.121189
21.58537631.262179
32.00050501.412971
42.51260401.583763
53.16268321.784456
64.00075252.005050
75.01380202.245545
86.31084162.516040
97.94187132.826535
101090103.166832
1531.696.83.25.628218
20100991109010
2531699.7.317.894.45.6
30100099.9.131.6973
35316299.97.0356.298.21.8
4010,00099.99.01100991
50100,00099.999.00131699.7.3
601,000,00099.9999.0001100099.9.1
80100,000,00099.999999.0000110,00099.99.01
1001,000,000,00099.99999999.000001100,00099.999.001

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