The last 20 dB#
Shielding a room is a purchase, not a decision. What does each additional 10 dB cost, and where on that curve does everyone who is not a government stop buying?
Why the emission is reconstructible#
A display drives its image on a fixed schedule and repeats the frame sixty times a second. A receiver that knows the schedule can add the repeats together.
That is the whole reason this is a picture rather than a hiss: the signal is periodic and the noise is not.
What the target actually is#
Attenuation is not measured against audibility. It is measured against how long an attacker is willing to sit outside.
Coherent averaging of N repeats buys back 10·log₁₀(N) dB. Sixty frames a second is the exchange rate.
The exchange rate, written out#
| Attenuation | Repeats needed to undo it | Time on a 60 Hz display |
|---|---|---|
| 10 dB | 10 | 0.17 s |
| 20 dB | 100 | 1.7 s |
| 40 dB | 10,000 | 2.8 min |
| 60 dB | 1,000,000 | 4.6 hours |
| 80 dB | 100,000,000 | 19 days |
The first two rows are free to the attacker. The defender pays for all five.
What each band costs to build#
| Target | What buys it | Where the leak has moved |
|---|---|---|
| 20 dB | conductive paint or foil on the walls | the door |
| 40 dB | mesh through every wall, floor and ceiling | ventilation and cable entries |
| 60 dB | waveguide vents, filtered mains, RF door gasket | the gasket’s maintenance schedule |
| 80 dB | welded enclosure, isolated power, no penetrations | the people who have to work inside it |
Each row closes the dominant leak and promotes the next one.
The shape, not the total#
The curve decides who pays#
A build to 80 dB costs five times a build to 60 dB. The last 20 dB is four-fifths of the bill.
TEMPEST is still a government practice, not because the mechanism stayed secret — van Eck published in 1985 — but because nobody else was ever going to buy the right-hand end of the curve.
Before the Bench#
Bring a stated attenuation target and the willingness to cost a room instead of capture one: mesh, seams, penetrations, door.
The Bench asks for a bill of materials and one sentence separating what the room costs from what the device costs. Compare curves with another pair afterwards — your totals will disagree and your shapes should not.