Wi-Fi Can Count People Through Walls

It sounds like a gadget from a spy film, but the wireless signals that deliver your internet can also act as a rudimentary form of radar. A Wi-Fi router floods a space with radio waves, and when this area is empty, the signal strength received by a device or dedicated sensor remains relatively stable. The human body, however, is about 60% water and is excellent at absorbing and scattering these signals. As a person moves through a room, they create subtle fluctuations and "shadows" in this invisible Wi-Fi field, causing the signal strength to dip and change.

Researchers, like the team at UC Santa Barbara, have harnessed this principle to do more than just detect motion; they can accurately count the number of individuals present. Their system typically uses a transmitter and a receiver on opposite sides of a space. By analyzing the precise changes in the Received Signal Strength Indication (RSSI), sophisticated algorithms can distinguish the disturbances caused by one, two, or more people. Because radio waves at Wi-Fi frequencies can penetrate common building materials like wood and drywall, this count can be performed without a direct line of sight, making it possible to sense presence through walls. This technology could lead to smarter buildings that adjust heating based on occupancy or security systems that detect intruders without privacy-invading cameras.