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.