Calm Control (CC), also called the White Noise, is a sound at a frequency that only PSI persons can hear. Adults, who cannot hear it, use it to subdue children throughout the series. It is one weapon that they have absolutely no defenses against.
At the beginning of The Darkest Minds, an unprecedented burst of White Noise comes on while Ruby is working in the Garden. She begins to bleed from her nose and head and she blacks out. Cate Connor explains later that the Camp Controllers wanted to root out those who had been mis-categorized by playing a frequency that only certain color classifications could hear. (To an Orange, for example, the Green frequency buzzes like a bee and nothing more.)
Several times throughout the series, PSFs use the Calm Control against the kids. It is unconfirmed whether or not they found a way to altogether resist it, though Ruby managed to sit through a bit of it in In the Afterlight.
The Camp Controllers tote around hand-held devices that can play the White Noise. The Children's League has smaller versions of these devices.