Sonic Eye

The Sonic Eye is a sensor that displays nearby sound sources as animated symbols.

Usage
During the 1974 Peace Walker Incident, Big Boss and his private military group, the Militaires Sans Frontieres, managed to procure design specs for the Sonic Eye while attempting to intercept a train suspected of carrying nukes in El Cadaso. Afterwards, MSF's R&D team managed to develop a prototype. The prototype stage was not affected by surrounding objects, and could only work when the user was lying down or against a wall. The Sonic Eye under further modifications was able to display the nearby sound sources regardless of the operative's current body position. Afterwards, the R&D Team primarily modified the sensor to consume less battery life until it utilized barely any battery life at all to function.

Behind the scenes
"Sensor that displays nearby sound sources as animated symbols. (Rank 1) Not affected by surrounding objects. A prototype, it will only work when lying down or against a wall. (Rank 2) Symbols are displayed regardless of body position. (Ranks 3 and 4) Modified to extend battery life. (Rank 5) Modified to consume virtually no battery power at all."

- Sonic Eye description from Metal Gear Solid: Peace Walker