Building in Beyond Earth
|Notes||Prevents Alien units from approaching within 2 tiles of this City.|
Once you build your first Ultrasonic Fence you will get a quest, "SAFE BEHIND WALLS", which will give you the choice of either extending the fence's range to 3 tiles, or preventing Trade Units from being attacked by Alien units.
By two tiles of the city, it means of the center city tile. So aliens will still walk on tiles you claimed and are trying to work, they just can't get too close to the city itself.
It was known from planetfall that the indigenous wildlife could sense and react to sound and light wavelengths beyond normal human perception. One of the first efforts taken by most colonial settlements to passively deter their depredations was the installation of ultrasonic fences. Ultrasonic fences were used on Old Earth for livestock and wildlife control for many decades before the Seeding; while the colonists added ultrasonic transducers to the design, the basic configuration has remained unchanged. The transducers pick up high-frequency sounds (above 18,000 hertz) of passage of an alien lifeform, amplifies this and narrow-beams it back. In virtually all cases, this causes the animal to veer away from its current course to avoid the source location; it has been found that Siege Worms are particularly susceptible when travelling underground due to their sensitivity to vibration. However, the fences have proved effective against all indigenous lifeforms, even the marine species such as the Kraken. The passive nature of the deterrence is one of the major advantages to ultrasonic systems, impacting the ecology minimally. Although labor intensive – the inspection and servicing of the transducers is a daily chore – and demanding energy reserves often at a premium in small settlements, most colonial administrations invested in ultrasonic fences around their perimeters, and often extended these outward as the settlements grew.