Ferrite beads are a type of filter used to filter out higher frequency noise or energy. A ferrite bead is an effective filter that helps to increase the impedance over a certain band of frequencies. It essentially has dynamic impedance as you increase in frequency -- thus being a more effective filter across certain frequency bands than others. Ferrites are a high permeable material. This implies they exhibit the quality of collecting magnetic fields such that they can increase the inductance and effectively the impedance of a piece of wire or coil. Because they can collect the magnetic fields, they limit the ability for the fields to change rapidly thus acting as a filter of high energy.

Ferrite beads exhibit three electrical behaviors across their frequency bands. They exhibit an inductive region or band, where the impedance linearly increases vs. frequency. They exhibit a resistive region or band, which is located in the mid-band range of the bead, which is also where the impedance is the highest. Thus, ferrite beads can act as a great loss mechanism at their mid-band frequency. Then, at the later part of their band on the high end, they start to act capacitive as they come out of the restive band as frequency increases.

Ferrite beads are useful for EMI suppression, where energy can be coupled onto a long cable that can act as an antenna and radiate the energy coupled onto the cable. For this reason, it is common to see ferrite beads encased on long cables connected to electrical systems where higher frequency energy can be found. They can also act as an electrical choke on a power supply, so that noise is not coupled or conducted onto power supplies at the input to the system, from some other exterior noise source.