These ferrite cores work to erase noise that is generated by a portable device or equipment that would otherwise be propagated around them. They also work as a filter to get rid of those noises generated by other devices so that your notebook PC does not get those noises that would otherwise cause malfunction for example. Ferrite cores erase and filter noises, outgoing and incoming noises at the same time. There are more than a dozen different ferrite cores are offered varied by materials, identified by two digits numbers. The Material 31 or, sometimes called Mix 31 is the latest development for noise suppression that covers the broadest frequency range of 1 - 1000 MHz to respond to every growing electromagnetic noise pollution today. The ferrite cores come in two split halves, and clamp on to the cable whether it has a connector, or is hard wired to equipment. Install the ferrite core at the equipment side of the cable. If a cable has equipment that needs to be protected on both sides, install one at each end. Make a loop of cable over the ferrite core. Make as many loops as you can and/or install multiple ferrite cores until you successfully suppress offending noises. One loop of cable over the core has 4 times the increased erasing/filtering power, or you could use 4 cores for the same effectiveness. When there is known offending noise present, making multiple loops of cable over the core is generally necessary. You can install the ferrite cores at affected devices, as well as at offending devices. Depending upon circumstances, identifying and troubleshooting both of affected and offending devices are necessary for successful installation. U.S. engineered with the latest material technology. 1pc/order unless noted as a multi-quantities pack, bulk pack.