Since the mid‐1960's Washington repeatedly protested to Moscow on radiation bombardment. The exact purpose of the activity never has been clear. American officials theorized that the microwaves could have been intended to jam American electronic‐intelligence equipment or to trigger electronic monitor devices concealed in the embassy.
https://en.wikipedia.org/wiki/The_Thing_(listening_device)
The Thing, also known as the Great Seal bug, was one of the first covert listening devices (or "bugs") to use passive techniques to transmit an audio signal. It was concealed inside a gift given by the Soviet Union to W. Averell Harriman, the United States Ambassador to the Soviet Union, on August 4, 1945.The device, a passive cavity resonator, became active only when a radio signal of the correct frequency was sent to the device from an external transmitter. This is currently referred in NSA parlance as "illuminating" a passive device. Sound waves (from voices inside the ambassador's office) passed through the thin wood case, striking the membrane and causing it to vibrate. The movement of the membrane varied the capacitance "seen" by the antenna, which in turn modulated the radio waves that struck and were re-transmitted by the Thing. A receiver demodulated the signal so that sound picked up by the microphone could be heard, just as an ordinary radio receiver demodulates radio signals and outputs sound.
So basically, the Soviets beamed a massive MW signal toward one side of the building. The people talking in the building vibrated a tuned box which caused the MW signal to be changed by the voice. The Soviets had a receiver on the other side of the building that subtracted the MW signal and what was left was the voice signal of the Ambassador speaking.
That was seventy years ago. With the digital world of the past 30 or 40 years a whole new age of intrusive spying opened up.
Isn't that extraordinary. Have we ever really had secrets?
If you can take video of a room where a conversation is happening, you can probably get audio intel from vibrations of objects in the room. This has been demonstrated by filming a potato chip bag.
http://news.mit.edu/2014/algorithm-recovers-speech-from-vibrations-0804