New idea for espionage: monitoring the vibrations of a light bulb
It's called the Lamphone, and it can work from 25 meters (82 ft) away. Unlike many other similar eavesdropping techniques, this one doesn't require any access to the actual room being monitored, can be used in real time, and is passive (so no direct interaction with the object acting as the "microphone" – in this case the light bulb – is needed).
The system makes use of kit that's relatively easy to source as well. The team behind the Lamphone says that the system comprises an electro-optical sensor and a telescope, put together with a laptop and some special audio processing software.
Using a custom-made algorithm and four separate processing stages, speech and singing can be decoded from the vibrations of the light bulb being monitored. The audio fidelity isn't perfect, but it's good enough to work out what's being said or the song that's being sung.
The whole setup is possible because sound waves from speech cause very tiny vibrations around the rest of the room, including ones that affect the light bulb in the ceiling. Even the slightest of shakes can reveal what's being said in the room.
"The hanging bulb acts as a diaphragm that vibrates when sound waves hit the bulb," explains the research team in an accompanying video. "In addition, when the bulb is turned on it acts as a transducer, converting air pressure from sound to small changes of light."
Before you start blacking out windows, there are some limitations to bear in mind. The spoken and sung audio was played through speakers at a higher volume than normal for human conversation, and for now the technique only works on hanging light bulbs (not those fixed into the ceiling or inside a lamp).
The researchers themselves suggest some ways of guarding against an eavesdropping attack of this kind – by using lower energy bulbs, for example, or heavier bulbs, both of which would reduce the strength of the vibrations. They emphasize that their intention is to raise awareness of these types of attacks, rather than to enable anyone to carry them out.
"As a future research direction, we suggest analyzing whether sound can be recovered via other light sources," write the researchers in their paper. "One interesting example is to examine whether it is possible to recover sound from decorative LED flowers instead of a light bulb."
This is only the first attack attempt of its kind, so it's likely that the Lamphone and devices like it are going to get better over time, giving you another reason to keep your curtains drawn. There's an interesting game of cat and mouse going on in the field of surveillance, as researchers develop both more ways to eavesdrop and more ways to stop it.