Fiber‑optic networks have become a tool for espionage: eavesdropping in walls, listening to intimate conversations

How fiber‑optic cables can “eavesdrop” on people

The distributed acoustic sensing (DAS) technology uses ordinary optical fibers to record seismic waves, but researchers at the University of Edinburgh show that its capabilities are much broader.

How DAS works
1. The interrogator sends a laser pulse down the cable and records the reflected signals.
2. Microscopic glass defects inside the fiber stretch or compress as a seismic wave passes through, changing the phase of the reflected light.
3. These changes allow reconstruction of an earthquake’s picture.

Thus one cable becomes a “chain” of thousands of mini seismometers that can record not only earthquakes but also vibrations from volcanoes, cars, or even orchestral instruments.

Eavesdropping on people
To test whether DAS can transmit sounds, scientists placed a speaker next to the cable.
- Pure tones and music are easily recorded without additional processing.
- Human speech (200–5000 Hz) is also captured: low frequencies are read directly, while high frequencies require subsequent analysis by OpenAI’s Whisper neural network.

Method limitations
Parameter Limiting Factor Cable position Works only for coiled cables lying on the surface. Distance to sound source No more than 5 m. Burial depth At a depth of 20 cm and above, speech quality noticeably degrades. Straight cable Even when placed next to a speaker it does not provide high‑quality recording.

What this means
- Parallel monitoring: eavesdropping can be performed simultaneously with earthquake observation.
- Military applications: DAS measurements at depth can detect movements of submarines and ships, opening prospects for military intelligence.

Thus fiber‑optic cables traditionally used in seismology demonstrate potential both as “acoustic sensors” and as a tool for audio analysis.