Shield
Overview
The term “shield” is used most frequently in audio to describe a conductor that takes the form of a tube surrounding the signal conductor or conductors of a shielded cable. This serves as a form of “Faraday cage” around the signal conductors to reduce their pick-up of electrostatic and some forms of electromagnetic interference (or “noise”).
Basics
In audio cables the shield may be constructed of stranded copper or steel which may be braided or simply wound in a spiral manner. In other cases; metalized plastic or aluminum foil are used with an additional “drain” wire in contact with them. The drain wire is bare (not insulated) so that it conducts to the shield material along the length of the cable and provides a means of soldering the conductor to the connectors at the ends of the cable.
Most cable shields are made of non-ferrous metals such as copper or aluminum; which limits the shields effectiveness to electromagnetic interference to frequencies higher than those used in AC power systems. In balanced audio; power frequency “hum” pickup is addressed by the balanced input and twisting together the center conductors to help insure they are in virtually the same physical relationship to the “hum field.”
Applications
In some cases; not connecting the shield of a shielded cable at the receiving end can provide better results than connecting the shield at both ends. This approach can be used to eliminate a form of “ground loop” where the shield’s connection at both ends provides an additional path for ground currents that flow between the two audio devices. This can result in “hum” at the input of the receiving device.
This approach works with contemporary audio devices that are grounded to a common ground by something other than the audio cable, such as both devices having a 3 prong AC cord plugged into a common AC source. It is NOT recommended unless there is a ground connection other than the audio cable. Please see adapter cable for more specific information.