Shielding Effectiveness (SE): Definition and Decibel Conversion
To judge a shield or an absorbing product, we need a single quantity that says how much the field is reduced once the shield is in place. That quantity is shielding effectiveness (SE).
The formula
SE = 20lg(E1 / E2), in decibels (dB). E1 is the field strength without the shield and E2 is the field strength with it. Substituting magnetic field strength gives the magnetic-field figure, and substituting electric field strength gives the electric-field figure.
Decibels and attenuation
| Unshielded field | Shielded field | Attenuation | SE |
|---|---|---|---|
| 1 | 0.1 | 90% | 20lg(1/0.1) → 20 dB |
| 1 | 0.01 | 99% | 20lg(1/0.01) → 40 dB |
| 1 | 0.001 | 99.9% | 20lg(1/0.001) → 60 dB |
| 1 | 0.0001 | 99.99% | 20lg(1/0.0001) → 80 dB |
| 1 | 0.00001 | 99.999% | 20lg(1/0.00001) → 100 dB |
Every additional 20 dB reduces the field strength to one tenth of the previous value.
Typical requirements
Enclosures for ordinary consumer products generally provide below 40 dB. Military equipment cabinets usually need about 60 dB, and shielded cabinets for TEMPEST equipment need 80 dB or more. Shielded rooms and compartments often have to reach 100 dB. Shields above 100 dB are very difficult and expensive to build, so choose materials according to the figure actually required rather than simply pursuing a high number.