Eavesdropping Through a Wall

From The New Scientist:

With half a century’s experience of listening to feeble radio signals from space, NASA is helping US security services squeeze super-weak bugging data from Earth-bound buildings.

It is easy to defeat ordinary audio eavesdropping, just by sound-proofing a room. And simply drawing the curtains can defeat newer systems, which shine a laser beam onto a glass window and decode any modulation of the reflected beam caused by sound vibrations in the room.

So the new “through-the-wall audio surveillance system” uses a powerful beam of very high frequency radio waves instead of light. Radio can penetrate walls – if they didn’t, portable radios wouldn’t work inside a house.

The system uses a horn antenna to radiate a beam of microwave energy –between 30 and 100 gigahertz – through a building wall. If people are speaking inside the room, any flimsy surface, such as clothing, will be vibrating. This modulates the radio beam reflected from the surface.

Although the radio reflection that passes back through the wall is extremely faint, the kind of electronic extraction and signal cleaning tricks used by NASA to decode signals in space can be used to extract speech.

Here’s the patent, and here’s a Slashdot thread on the topic.

Wow. (If it works, that is.)

Posted on October 26, 2005 at 3:12 PM40 Comments


Ari Heikkinen October 26, 2005 3:44 PM

Seems like basicly the same idea as pointing a laser beam to a window, but using RF instead. Sounds feasible, ofcourse. There’s endless possibilities, atleast in theory, when transmitting either a laser beam or RF and then checking what reflects back.

Mithrandir October 26, 2005 4:22 PM

How does closing the blinds defeat a laser mic? The blinds are generally on the inside, and not sound-proof.

Still, a neat trick. Probably easy to pick up on with the right equipment though, and easy to jam.

obd October 26, 2005 4:25 PM

Didn’t russians offer a gift comprising a hollow metallic sphere to the US embassy in Moscow, then irradiate said sphere with a microwave beam to listen in with that same technique some twenty years ago ? With the embassy personnel frequently getting leukemia as a result…

Davi Ottenheimer October 26, 2005 4:35 PM

Interesting. I guess NASA’s search for intelligent life continues.

I’ve found it quite amusing lately to run interference with all the wireless signals I seem to be constantly bombarded with. It started with simple curiousity about stopping rogue WAPs from broadcasting WiFi into my private space…and I’ve found it quite trivial to disrupt other signals and render data useless, and not much harder to choose a signal you want to specifically target (let alone source).

I wonder if NASA’s listening/cleaning is able to handle intentional signal tampering.

Jake October 26, 2005 6:54 PM


Rogue WAPs? Do mean your neighbor’s wireless networks? Are you are jamming your neighbor’s wireless networks just because the signals are bothering you?

Davi Ottenheimer October 26, 2005 8:06 PM

@ Jake

Actually, if it was just the neighbors I might go over for a hot cup of cocoa and discuss our signal to noise ratio.

But it’s becoming quite clear to me that people on the street are sophisticated enough to broadcast bogus access point beacon frames into my internal airspace. This is quite a bit more disturbing that the usual null probe traffic because it means someone may be masquerading as a WAP. I’ve also seen a spike in suspicious associations, to the point where it looks like poorly-crafted attempts to saturate APs.

I guess you could say those activities bother the people who may potentially have their wireless network hijacked or disrupted by interference.

In terms of the NASA listening technology, I assume they haven’t had to deal with hostile aliens trying to disrupt and jam their antennas (and if they did, they’d prob just say “Eureka!”).

So they might turn on their microwave emitter and know how to interpret water-vapor and vibrations of things in room, etc. but what if some wiseguy is manipulating an internal microwave/PIR to generate noise to disrupt listening?

Incidentally I don’t think drawing the curtains inside gets you much in terms of a laser detecting vibrations off a window. Outside shutters would do the trick, but if the window is exposed it can still be read like a resonant barrier.

jammit October 26, 2005 9:13 PM

The Russians did offer gifts that were good at reflecting microwaves. One of the better ones is where an American embassy was made with local bricks, and the bricks had small aluminun cones mixed in. The walls were the microphone. I’m pretty sure you could use a frequency hopping xmitter to cloak your self. If the reciever is pretty good and sensitive, or the room just happens to have good reflecting materials in it, you can beat frequency local radio stations against the reflected signal.

Thomas Sprinkmeier October 26, 2005 9:52 PM

Anyone know here I can get a “Cone of Silence” ala “Get Smart”?

“So Now we must ALL learn sign language!!!”

lip-reading will do.

Roger October 26, 2005 11:19 PM

Que? How exactly is this news, or patentable? In “Spycatcher”, Peter Wright discussed doing this in the 1950s! The first publicly known version, as someone noted above, was the Russian bug in the Great Seal, which used a specially tuned resonator, but once Wright had figured out how the basic idea worked the British quickly extended it to stuff like the sides of filing cabinets.

The one surprising thing to me is that these researchers claim that the human chest provides an adequate reflector. BTW, contrary to the NS summary, they do NOT claim that cloth provides an adequate reflector, and unless it was metallised I would think it would be just about the least useful material.

“what if some wiseguy is manipulating an internal microwave/PIR to generate noise to disrupt listening?”

The “obvious” precaution against that sort of thing is to spread the outgoing beam with a cryptographic spreading code. Similar techniques are already used (with Galois codes rather than cryptographic ones) to eliminate the effects of accidental/natural interference in interplanetary radar and similar weak signal microwave stuff. I would be surprised if cryptographic codes haven’t already been used for military radar, as a counter-measure to jamming.

Other spread spectrum techniques could also be used, e.g. frequency hopping, chirping, and ultra-wideband. Come to think of it, UWB would be especially suited here.

“Incidentally I don’t think drawing the curtains inside gets you much in terms of a laser detecting vibrations off a window.”

True, but they may reduce the vibrations in the first place. Bear in mind that for most windows, the strongest impinging sounds are not internal but external (wind, traffic etc.), so anything which reduces the volume of interior noise just in front of the glass will probably make things quite tricky. And heavy drapes can absorb quite a lot of sound. Hanging very heavy drapes one to two inches in front of a wall is already a recommended method to counteract “spike mikes”. However much more effective measures quickly come to mind.

RonK October 27, 2005 1:51 AM

“So Now we must ALL learn sign language!!!”

Uh, no. It’s already well known that it is possible to locate objects using a technique similar to radar through walls. One interesting use of that is when trying to organize an armed assault on a hostage situation.

The resolution I remember stated for that wasn’t enough to read sign language but that was a while back.

RonK October 27, 2005 1:57 AM

“So Now we must ALL learn sign language!!!”

On second thought, if you mean letter signing as opposed to gestural signing, I’d guess you still have some window of safety, still. People had looked like large blurry blobs in the demo image shown in the Internet article.

I don’t find the original article, see
(no images).

Tanuki October 27, 2005 4:48 AM

And of course even the old standby of a Faraday-cage isn’t going to work because the Faraday-cage itself will vibrate and reflect voice-modulated energy.

Methinks the best defense against this sort of technique is to have the ability to detect when your location is subject to high levels of ‘unexpected’ RF and to move elsewhere.

Hmmm… I wonder if the listening technique can also be used passively – by depending on existing TV/radio transmitters to provide the RF ‘illumination’. OK, the frequencies are different, which would pose a problem since the vibration-induced movement would be a very small fraction of the wavelength of a BC-band signal – but given enough signal-processing… ?

lawnsea October 27, 2005 6:25 AM

Perhaps I am being obtuse, but why not just play a CD of innocuous conversation at normal speaking volume, sit close to your co-conspirator, and speak in a low voice? A CD (perhaps a book on CD?) and boombox would be a lot easier to explain to the secret police than a Faraday cage or other EM interference/attenuation device. A bit more portable, too…

VWM October 27, 2005 6:44 AM


Whispering while listening to loud Music, TV, etc. will probably make eavesdropping more difficult. That will work regardless which technique is used (Laser, RF, good old Bugs, …).

But Music is predictable, an attacker can use the same CD to recover your whispering. So better add some random noise like letting the shower run.

shoelover October 27, 2005 7:16 AM

We are coming up with a kevlar helmet with lead lining for when they develop a brain wave eavesdropping technology.

Better safe than sorry


steve October 27, 2005 7:34 AM

A Faraday-cage should be sound-proof on the inside… or why not detect the frequency and modulate white noise onto it? Or the latest chart hits?
Does anyone know how hard it would be to detect these signals (even if they use frequency hopping etc.?)

Kevin Davidson October 27, 2005 8:02 AM

Reminds me of what the Soviets did to bug the American Embassy in Moscow. They embedded a microwave reflector in the Great Seal in the conference room and bounced a microwave beam off it. Only now, the listener never has to enter the room to plant a device.

I think for my grandchildren, “privacy” will be nothing more than a quaint historical notion.

Jack Hammer October 27, 2005 8:38 AM

An interesting quote from the above mentioned spybusters.com article:

“The wife of the Italian ambassador in Moscow during 1927-30 said: ‘Spying on the part of the authorities was so common as not even to be thought of as spying.'”

How quickly the world and the U.S. inparticular is moving in that direction.

Davi Ottenheimer October 27, 2005 10:45 AM

“the best defense against this sort of technique is to have the ability to detect when your location is subject to high levels of ‘unexpected’ RF and to move elsewhere.”

I suppose, but that’s the ‘blacklist’ model, which is far more costly to maintain, with higher risk, than a ‘whitelist’.

Moreover, as Roger pointed out, the laser method is susceptible to noise on both sides of the window. So if you attach something to the windows to make them vibrate irregularly (foreign language recording through a cheap diaphram taped to the window) you will probably raise the cost of effective laser eavesdropping beyond even sophisticated spies.

Microwaves seem to be defeatable in a similar fashion, through generating sufficient interference with the signals themselves, or the original sounds, to raise the bar/cost of deciphering.

another_bruce October 27, 2005 11:50 PM

if you duct-tape a transistor radio to your window and tune it to a rap station, nobody will be able to figure out what people are saying in your office, whether they use lasers, rf, tempest scans, whatever.

Anonymous October 28, 2005 6:35 AM

I’ve been thinking about the issue of drawing curtains on the inside of the window as a tactic for defeating the LASER approach to eavesdropping.

The attack is usually used to bounce light off of a vibrating waterglass or similar surface isn’t it? Closing suitable curtains stops the light getting there, so would be effective.

The second attack is to detect the vibrations in the window itself. If the curtains are thick enough they would absorb a large proportion of the sound before it got to the window. This would stop (limit) the reasonance of the window itself.

Harrold October 28, 2005 10:52 AM

I like all of the counter measures suggested. I’m sure if you were planning something particularly nasty, or you are already a spy, then you might take such precautions. But most people “foolishly” believe that their conversations at home are private.

They should be private, of course. But the threat continues to grow. They can watch you with heat, hear you through walls, tap your phones and ISP, install keyloggers, take your picture whenever you are outside, etc. (Can they see you through those small holes or gaps in blinds and drapes without being on your property the way you can if you approach the window up close?)

It seems we have a war against liberty and a war against privacy. This escalation simply means everyone loses as we pay top tax dollars to outfit the police so they can snoop against us. After all, any technique that works for spys one day will become commonplace in the near future for law enforcement.

RSaunders October 28, 2005 11:58 AM

This seems like a movie plot to me. There are lots of patents for perpetual motion machines. Yes, the Great Seal device worked, but it was a well engineered resonator. The issues of signal to noise can’t be “cleaned up” with “special secret sauce” software. Your “beam” of microwaves is going to reflect off lots of things. The reason lasers are better is that you can literally see what they are reflecting off and choose appropriate items to produce good signal response. You’re going to receive the sum of all those signals, with no referents to null out individual sources. Engineers have been working problems like this in Sonar, Radar, radio mapping, for decades with remarkably few successes.

It’s only a Wow if it works.

Tauroid October 29, 2005 12:14 PM

What about the concept of “sound proofing” a room? If you were to take sheets of, lets say, 18 gauge aluminum and cover the walls, floor and ceiling, the metal will do what it likes to do, absorb the energy of the microwaves, resulting in an induced current in the aluminum. The very reason that you dont nuke soup in a tin can.

RonK October 30, 2005 2:12 AM


(Can they see you through those small holes or gaps in blinds
and drapes without being on your property the way you can if
you approach the window up close?)

If they can have a “large enough” camera mosaic (its size being dependent on their distance from you, the size of the slits in your blinds, and the spacing and number of those slits if they are periodic, as is usual), the answer is yes.

At least theoretically. I don’t have any information on a real attack like this.


The reason it might work better than you think is that (1) you only look for signals within a certain frequency band and (2) the microwaves can be modulated so that the reflected signal you are interested in may possibly be separable from other reflected signals with different time delays.

I get the feeling you’re thinking about a microwave signal which is a pure, constant frequency. I’m not a big expert, but I don’t think that that’s what it’s about — see the comment by Roger, above…

Anonymous October 31, 2005 9:26 AM

I’m all for old fashioned microwave bugging within the walls of buldings and maintained by BT plc

dws August 1, 2011 9:59 PM

I believe the Moscow Great Seal was a modulator not reflector. you can do the same on AM with a crystal set tuned to a strong signal and a sound source in place of the headphones. I does work!

Rita patterson July 28, 2012 2:42 PM

I am suspicious of my neighbor lady that lives next door to me. Her bedroom windows and my bedroom windows are across from each other. At night after I go to bed, these lights start showing up on my walls and ceiling like they are searching for something. All of my blinds are drawn but I had some very small places that wasn’t covering all of my window. Her windows are the only place the light could come from. I thought I was going crazy until one night I saw her at her window and she was holding something. I don’t know how to explain all of this but something is going on. This goes on until 4 or 5 in the morning. when she turns the light off, it leaves a bright streak across one wall of my bedroom. Would you be able to tell me what kind of light she is using and is it harmful to the body?
Thank you,
Rita Patterson

JB Smith April 16, 2013 3:57 PM

Virginia State Police and the Newport News Police are using this technique to take away vets 2nd amendment rights to bear arms. They sleep deprive and electronically tase you into “excited delirium” to cause them to act out so they can put them in stabilization wards (gitmos) and continue to torture them. They declare them mentally ill so they can disarm them. The techology works like the audiospotlight and the diablo flashlight. It is excruciatingly painful.

Billie October 7, 2015 4:41 AM

My experience seems similar to what Rita describes. Small round light pulses across ceiling and top of walls during early hours to around 3 am regularly, as if mapping. Then neighbour seems to watch footage on cctv 6 to 7 am.

Billie October 7, 2015 5:09 AM

Further to my post above, if anyone can give information or thoughts on the purpose, nature, counteraction etc. please can you post them. I was thinking the chap would say it was his home security system simply over-running, which I doubt. In UK where we can now watch CSI Cyber. Explains some of it.

Billie November 4, 2015 12:03 PM

I have outlined the circumstances and some thoughts about intrusion via lights through at least one window described above, on a blog http://targatt.blogspot.co.uk

Also see @jayebill on Twitter.

If I get info or comfirmation of parts of it I will post. It is a residential park with bright street lamps that can come through curtains.

Paul Green July 29, 2016 9:31 AM

The real question is laser audio legal in Ontario and why it might now be located at #406-100 Ridout Street South , in London , Canada. Who wants to live near a jabbering idiot who maintains and needs to repeat every thought overheard within proximity, with this technology when the equipment includes a recorder. Another important point is how businesses in Canada can protect proprietary info , including remote neural sensing of employees of tapping into their vision and hearing to spy inside corporations is the greatest threat to companies in the decades ahead. We are all aware of municipal streets or companies posting signs of visual surveillance and not for laser audio , or thought reading technologies unknown to the public. If the public was fully aware of this intrusive tech stuff they would be yelling for tougher privacy laws beyond intrusion to seclusion .

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