Ever notice a that big cylinder-shaped piece that is bigger than the cable at the end of some cables and wondered what is that? Those are called ferrite beads or sometime known as ferrite chokes.
A ferrite bead is a simple hollow bead or cylinder made of ferrite to suppress the amount of high frequency (EMI) electromagnetic noise in electronic circuits. The ferrite is a black cylinder shaped semi-magnetic substance made from iron oxide mixed with other metals. The ferrite are encased in plastic or rubber to house the ferrite bead and keep it in place. Usually when the cable is made, a ferrite bead is slipped over the cable; though it can be snapped on after the cable has been made.
When signals are traveled through the cable, they act as antennas as they can broadcast the signals and interfere with other devices as well as receive signals from other devices as well, which can be a problem. While the case or wrapping of a device can reduce most of the noise, they cannot completely eliminate the electromagnetic noise it creates. The functionality of a ferrite bead is to absorb the energy from electromagnetic noise and returns it as heat; a reason they are at the end of the cable.
One example of a noticeable effect of a ferrite bead is in unshielded speakers. If you use are calling from a GSM cell phone and near an unshielded speaker, you will hear a noise similar to the video shown below. When you apply a ferrite bead to the end of the speaker wire, just before the speaker, the ferrite bead will absorb the high frequency electromagnetic interference caused by the GSM cell phone and prevent that noise you hear.
You can purchase ferrite bead in some local electronics stores and online through sites like Amazon and electronics supply stores.