The high-frequency sonar fence is a continuous stream of high-intensity sound waves generated by a series of evenly-spaced pylons surrounding the Barracks. It was built by the DHARMA Initiative. The range of the waves reaches only as high as the top of the pylons. With the potential to render someone walking through the fence unconcious, or even killing them, the waves can cause severe cerebral hemorrhaging to humans who pass through them. Although humans can physically breach the barrier before the sonic waves trigger side effects, the Monster cannot pass between the pylons while the fence is active.
The sonar fence was first mentioned in the DHARMA Initiative Barracks video, seen by Ben Linus when he first arrived on the Island. The film described the fence as serving to protect the Barracks area from the Island's abundant wildlife. Ben encountered the sonic barrier as a child, while running out of his home. He later obtained the code to deactivate the sonic barrier and, as a test, sent a rabbit through the barrier before crossing it himself.
Kate, Locke, Sayid and Rousseau encountered the sonar fence on their mission to rescue Jack. According to Mikhail Bakunin, their prisoner, and the DHARMA cabling map discovered by Sayid in the Flame, the fence extends all the way around the area containing the Barracks.
A keypad is used to enable the fence. The actual keypad is a HID model RKL55.Locke appeared to kill Mikhail by shoving him through the barrier. After passing through it, he began shaking violently, foaming at the mouth, and bleeding from the ears. He collapsed on the ground and appeared to have died. However, Mikhail did actually survive, and later helped save Naomi after encountering Desmond and the others who were attempting to find her. After he returned to the Others' camp, Ben commented, "I thought you were dead." Mikhail responded, "I am fortunate the pylons were not set to a lethal level."
The fence can be set to a lethal level.After seeing what happened to Mikhail, Locke, Kate, Sayid and Rousseau made a ramp out of a tree trunk in order to scale the barrier and bypass it. In doing so, they did not break the connection between each barrier pylon, which Sayid guessed was the trigger for the sonic waves.
The Monster cannot pass the sonar fence. As shown by Ben in his flashback, the barrier can be deactivated. Kate and Juliet encountered the inactive fence while running from the Monster. After crossing to the other side, Juliet entered the code 1623 into a data pad, and after lifting a panel and turning a knob, the fence was reactivated. The Monster then attempted to attack the pair, but was repelled by the sonar fence, which appeared to act as a physical barrier against the smoke. After the Monster retreated, Juliet explained, "We don't know what it is but we know it doesn't like our fences."
When a young Ben deactivated the fence, he used the code 54439. This code was changed every day according to the orientation film he and Roger watched upon arrival to the island. The fence intensity seems to be controllable at a continuous scale going from red to green to black, where green is presumably a repelling but less harmful effect.
The fence was seen in excellent condition after the final skip in time of the Island. Juliet shouted for Daniel to immediately stop as he almost walked through it. Amy faked disabling the fence by using earplugs. Sawyer, Jin, Miles, Daniel and Juliet walked through the fence, which rendered them unconscious. Richard Alpert managed to get past the fence, and claimed that his people could do so with ease.
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