Filter to Prevent Eavesdropping through Control Wiring 1

[DRWeig]

Request from client today:

"Do you know of a filter that can be installed on control cable to eliminate noise, not electrical noise but eavesdropping? Apparently there is a new government spec out there that certain conference rooms have to have eavesdropping filters installed on all wiring entering the room. Wireless is out of the question because of the way the rooms are constructed and the relationship to where equipment can be installed."

He's referring to typical HVAC control cable. Generally these are headed for thermistors, RTDs, 4-20 mA devices, 0-10V devices, and others. They may also be RS-485 or Ethernet lines. Others may be from 24 VDC or 24 VAC power supplies, to power controllers in the room. I have asked for clarification from the client as to whether this new spec refers to:

The use of the control wires to connect a microphone inside the room to a listening device outside the room;
The use of the control wires to power a wireless-listening or fixed-recording device inside the room;
Sensing actual acoustic vibration transmitted down the conductors or cable jacket from inside the room; or,
Listening to what goes on in the room through the other end of the conduit, or through the hole around the wall penetration.

Until I get answers (which I'll post here promptly), has anyone already run into this spec and found a way to meet it?

Many thanks.


Best to you,

Goober Dave

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[DRWeig]

Yeeesh! Spies aren't fun, sexy, suave, daring, and exotic when you're building things for them.

Thanks for the conduit seal / duct seal / firestop suggestions, those might work too. Our client might combine most of the ideas above and make a submittal. The submittal rejecter (if rejected) will have to at least give us hints. I think we have some conduit seals and firestop in stock, will have to check with the product managers.

Still no word on if or when we'll see the spec. I don't even know which agency this thing is for, or in which hemisphere it's located. All I know is "Federal Government." They're probably reading this as I type and having a good laugh.

Best to you,

Goober Dave

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[VEBill]

"...noisemakers are used in lots of classified facilities."

We usually call them "attendees".

There's a non-hardening goop sold in tubes that (in Canada) we often call "Acoustical Sealant". The intended purpose is to smear on studs before mounting the drywall so as to reduce sound transmission. It's also very commonly used to seal-up 6-mil ultra poly+ sheets to achieve a permanent air-tight vapour barrier. This sort of product could be useful.

If one was paranoid, one might wish to open up the cables and dampen the individual wires.

There's a concept of 'inspectable area'. If the adjacent rooms are under control (inspected and locked), then one shouldn't need to deal with the risk that an enemy agency sets up a rack of monitoring equipment in the next room.

 

[DRWeig]

Thanks VE1BLL.

That's another one to add to the list. Our client is submitting tomorrow, a combination of all the good suggestions here and some others they got from somewhere else. The good news is that none of the other subcontractors know what to do either. There's supposed to be some government lady coming to give them the requirements verbally.

Best to you,

Goober Dave

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[VEBill]

"...give them the requirements verbally."

That's sort-of ironic.

 

[controlsdude]

I thought using fiber optic cable would prevent anyone from listening in by inductive methods on cables. As far as a safe room like that is not using some kind of lead paint or screen on walls stop people from electronic listening thru walls?

 

[MikeHalloran]

Fiber is not secure, given physical access to a modest segment anywhere along its length.
Remove or slit the jacket, bend the core a little, and detect the light that leaks out.



Mike Halloran
Pembroke Pines, FL, USA

 

[DRWeig]

Remember, controlsdude -- this isn't about sensing electrical (or optical) signals, it's about stopping the transmission of vibration along a physical path from inside the room to outside. You're right about the walls themselves. I don't know how they prevent listening-thru with the technology available today, but I'm betting they've done it.

That's interesting, Mike. Now you've got me wanting to experiment.



Best to you,

Goober Dave

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[IRstuff]

Again, audio white noise generators are used specifically for this purpose.

TTFN
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[TomDOT]

Throw an electronic solution at the problem. Don't let the wire directly connect. Run the first wire to a hub inside the wall, which will then retransmit on a different wire.

 

[DRWeig]

Thanks TomDOT, another version.

I hope I'll get to find out what the gummint wants...

Best to you,

Goober Dave

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[140Airpower]

This is an interesting thread. The problem with afterthought bandaids to isolate a room to a high standard of security when the room was not designed that way in the first place is that you are not likely to patch all the holes. It depends on how valuable the confidential data is and how serious the spy is.

 

[Sparweb]

I can't help but think of the plastic room from the X-men movie

How about no physical contact at all between the room and its surrounding structure? Make the secure room itself a self-contained self-supporting box, floating on magnets like a maglev train. The entrance door has an airlock that retracts in the surrounding room. The surrounding room is climate controlled so you need less fans in the isolated room. Or go the opposite route, and give the secure room self-contained atmosphere and evacuate the surrounding room. In space, no on can hear you share state secrets. Power (for lights, coffee maker, etc.) can be transmitted through inductive coils under the floor, picked up by secondary coils in the floor, like a flat air-gap transformer. Self-contained power is also an option; run the lights off batteries. If you need to send out/in any data signals, convert them to optical (laser LED) signals in one side of the wall, shining on receptors on the other side of the wall panel. Narrow beams with covers around them so that no one else can see the light except the intended target.
This is very sci-fi. Inefficient but you've got a government budget to spend.


STF

 

[MikeHalloran]

You also have to secure the maglev circuits.



Mike Halloran
Pembroke Pines, FL, USA

 

[DRWeig]

Just an update -- they're not talking.

Our client is stuck with running a twisted pair through the wall, different room construction is not on the table.

It would be interesting to make a soundproof room from scratch, though. Perhaps something super-heavy like a bank vault, surrounded by a larger concrete room, with IRstuff's noise generators in the interstice. Maybe the vault could have its own foundation, floating on water or an air cushion.

Add armed guards around the perimeter, and you're done. Stick a fork in it.

Note that I said soundproof -- not spyproof. If the bad guy is sitting at the conference table with you, you're toast.

You could also do it like the North Koreans are supposed to have done it. 30 stories underground, with soldiers everywhere above.

I'll keep trying to pry the info out of our client. They haven't said the word "classified" yet.


Best to you,

Goober Dave

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[TomDOT]

Dave, they haven't said "classified" because it is sooo seekrit that the fact it even is a seekrit is classified. So you would have to be cleared in order to be told that you can't be told.

 

[stevenal]

they're not talking

Problem solved, eavesdroppers have nothing to hear.

 

[cranky108]

Just use the cone of silance, and forget about the other stuff.

 

[IRstuff]

If it's just a twisted pair that they're worried about, then any of the original suggestions would work, i.e.; provide lots of slack and hang a weight on the cable. So long as the cable is not taut, the damping from the insulation should significantly attenuate any acoustic vibrations impressed on the cable. It's already at an absurdly low level to begin with, given the low capture cross-section for collecting the vibrations in the first place.

TTFN
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[DRWeig]

Agree, IRstuff.

Best to you,

Goober Dave

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