Ferrite beads are used to suppress unwanted signals that can interfere with electrical devices such as DC supplies, transmission lines and cables. They provide attenuation of selected frequency bands. The physical shape of ferrite beads is similar to a toroidal inductor, but the beads have a greater length to diameter ratio and usually a greater outside to inside diameter ratio than most toroid cores. Different size / shape beads of the same material have different degrees of suppression.  The type of ferrite material used to manufacture the bead determines the range of frequencies for suppression purposes, and the physical size and shape of the bead determines the amount of attenuation. In general the impedance is directly proportional to the length of the ferrite beads.