The seal worked like a radio-controlled microphone
Hidden inside a carved wooden Great Seal was a thin vibrating membrane connected to a small metal chamber. Sound in the room made the membrane move, changing the chamber’s shape by tiny amounts. The device contained no power source of its own; it was a passive resonator, meaning it responded to an outside radio signal rather than broadcasting continuously.
When a nearby transmitter sent the right frequency toward the seal, the chamber reflected some of that radio energy back. The membrane’s movement altered the reflected signal in step with the sounds it picked up. A receiver could turn those changes back into audio. In effect, the radio beam supplied the energy, while the room’s speech supplied the pattern.
Silence made it difficult to detect
Because the device did not transmit on its own, ordinary radio monitoring had little to hear while the activating signal was absent. Its wooden hiding place also looked like a decorative object, not an electronic assembly. Detecting it required noticing the signal used to interrogate the room—and recognizing that its faint return carried speech.
That design traded convenience for concealment. The operators had to be close enough to illuminate the seal with the correct radio frequency, but the listening device itself needed no battery to run down and could remain dormant between activations.
A change in what “bugging” could mean
The seal demonstrated that a listening device did not need to behave like a miniature radio transmitter. It could instead act as a reflector whose response was shaped by sound. That distinction matters: a room can be monitored by equipment that leaves no steady signal for a conventional receiver to pick up. The wooden emblem was not merely a hiding place; it was part of the device’s antenna-and-resonator system.
