Posted on 15/03/2018

Audio distortion due to the Proximity Effect

Audio distortion due to the Proximity Effect

Audio distortion due to the Proximity Effect

The Proximity Effect

When a wire carries an alternating current, such as may be carried on audio cables, it creates an associated alternating magnetic field around it. The alternating magnetic field induces eddy currents in adjacent conductors.

If there is an electric current flowing in an adjacent conductor, the eddy current caused by the first cable alters the overall distribution of current flowing through the adjacent conductor. The eddy current will reinforce the main current on the side facing away from the first wire, and oppose the main current on the side facing the first wire

This is called the Proximity Effect.

If two conductors have the same direction of their current, then the current in the wires is concentrated in the areas of the conductor furthest away from the other conductor.

Conversely if, as is the case with loudspeaker cables and power cables, the current in each conductor is flowing in opposite directions, the current in the wires is concentrated in the sides closest to the other conductor.

 

The proximity effect can significantly increase the resistance to alternating current of an adjacent conductor compared to its resistance to DC current. This effect increases with frequency.How the Proximity effect causes audio distortion

Most loudspeakers are complex reactive loads and, as a result, the voltage in either wire at any time is not in phase with the current.

The result is that the magnetic field that is altering the resistance of the wires is not in phase with the voltage in the wires at any time.

Put another way the current in one wire is modulating the output of the other wire at a level that has no relationship (at any point in time) with the voltage in the wire. Because of the phase difference the small change of the voltage to the loudspeakers due to the ‘proximity Effect’ is heard as random noise unrelated to the signal. This form of distortion is highly noticeable.

As an example of this effect, it has on occasions been claimed by loudspeaker designers that their new speaker is more ‘musical’ because the crossover has less components. It is possible that they may be saying that “because their loudspeakers have less components, there is less difference in phase between the current and voltage in the speaker cable and hence the ‘Transient Phase Distortion’ in the speaker is reduced.”

The Proximity Effect decreases as the distance between the two conductors increases.

Cable with thin insulation. The horizontal line shows the level of the magnetic field at the centre of the other conductor. Cable with thick insulation. By using thicker insulation the magnetic field at the centre of the second core is much reduced. This results in a lower level of ‘Proximity Effect’ audio distortion.

 

As the diagram above shows, increasing the separation between conductors by thicker insulation reduces the magnetic field strength that the current in each conductor causes in the other conductor.

Reducing the magnetic field strength reduces the audible distortion caused by the magnetic fields.

A typical speaker cable (left) with insulation thickness of 0.6mm alongside Black Rhodium Twist which has a 1.2mm insulation thickness. Both cables have the same 24 x 0.2mm diameter conductors

The pictures above demonstrate how cables can be built with thicker insulation. In practice, loudspeaker cables with thicker insulation, such as Black Rhodium Twist, Twirl and Samba, have proved this point by their extensive awards for the sound quality.