Wiring a Studio: THE GROUND RULES

3.  Electromagnetic Interference

3.1  Electromagnetic v Electrostatic Interference

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 resistance.  All electrical circuits carrying alternating current generate both electrostatic and electromagnetic interference.  Some generate predominantly electrostatic interference whilst others predominantly electromagnetic interference.

3.2  How Electromagnetic Interference Works

Consider two parallel wires in proximity with each other.  One wire is connected to an ac signal source, whilst the other one is connected to an amplifier input, as shown below.

electromagnetic coupling

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.  A classic case is that 110 volt step-down transformer sitting at the bottom of your rack or under your console!  Power transformers usually generate strong magnetic fields at mains frequencies.  Toroidal power transformers are more efficient and generate less electromagnetic radiation than conventional power transformers.

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 further reduce electromagnetic radiation.  High voltage power lines in Australia are "3 phase" - the current in each wire is 120° out of phase with each other and the current in each wire is equal.  The net result is that they cancel each other out to minimise electromagnetic radiation.  If you're ever doing a long country drive you will notice that every kilometer or so the wires cross over, effectively slowly twisting them to further reduce electromagnetic radiation.  Less electromagnetic radiation means higher efficiency so that less power is wasted.  Contrary to what many people believe, high voltage power lines produce very little electromagnetic radiation and therefore their presence is not a concern for recording studios.

3.3  Screening out Electromagnetic Interference

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 is mu-metal, a highly magnetically conductive material.  Mu-metal is often used for head shields in tape recorders, since tape heads are highly sensitive to magnetic fields.  It is also sometimes used in microphone transformers.

Mu-metal is very expensive and is therefore not widely used as a means of magnetic screening in audio equipment.  Ferrous metals (which are magnetic), such as steel can be partially effective for magnetic screening.  It is impractical to make magnetically shielded audio cables!

In general, magnetic screening is not an option for reducing electromagnetic interference.  We must therefore rely on other means...

3.4  Sources of Electromagnetic Interference

Sources of electromagnetic interference include:

3.5  Electromagnetic Interference Summary

Magnetic 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: