Whereas electrostatic interference is generated by the presence of alternating voltage, electromagnetic interference is generated by the presence of alternating current. In an electrical circuit, there is a relationship between voltage, current and the circuit resistance. All electrical circuits carrying alternating current generate both electrostatic and electromagnetic interference. Some generate predominantly electrostatic interference whilst others predominantly electromagnetic interference.
Consider two parallel wires in proximity with each other. One wire is connected to an ac power source, whilst the other one is connected to an amplifier input, as shown below.
The alternating current flowing in the source wire generates an alternating magnetic field. Even though the wires do not touch, the alternating magnetic field will cause a corresponding alternating current to flow in the wire connected to the amplifier.
The wires in fact form a crude transformer and are referred to as being inductively coupled. The amount of inductive coupling between the two wires is governed by the following factors:
As far as audio equipment is concerned, the most common problems with electromagnetic interference occur at low frequencies, caused by power transformers. Power transformers usually generate strong magnetic fields at mains frequencies, whilst they are relatively poor at generating or transmitting high frequencies.
Note that two wire circuits such as balanced audio cables, speaker cables and mains cables do not radiate electromagnetic interference! This is because current flows in an equal and opposite direction in the two wires. The magnetic fields generated by the two currents are therefore equal and opposite and tend to cancel each other. Twisting the two wires together helps to reduce the effects separate cables have on each other, even when tied together in a loom.
Electrostatic shields, such as those used in shielded audio cables, have no effect on electromagnetic interference. Electrostatic shields are electrically conductive, but are often made of non-magnetic material, such as copper or aluminium.
The most effective magnetic screen in mu-metal, a highly magnetically conductive material. Mu-metal is often used for head shields in magnetic tape recorders, since tape heads are highly sensitive to magnetic fields.
Mu-metal is very expensive and is therefore not used widely in as a means of magnetic screening in audio equipment. Ferrous metals, such as steel can be used for magnetic screening in sensitive audio equipment. It is impractical to make magnetically shielded audio cables. We therefore must rely on other means to reduce the effects of electromagnetic interference.
Note that using thick wires has no effect on electromagnetic interference. A 6" square solid copper buss bar is just as susceptible to electromagnetic interference as a hair thickness wire!
Sources of electromagnetic interference include:
Electromagnetic interference is generated by the presence of alternating current in a conductor causing a corresponding alternating current to flow in an adjacent conductor.
Electromagnetic interference can be minimised by:
| © 2005 Colin Abrahams, Studio Connections, Sydney, Australia |  |