The speaker coil or loudspeaker voice coil is a coil of wire attached to the apex of a loudspeaker cone. Current passing through the speaker coil provides the motive force to the cone by the reaction of a magnetic field to the current passing through it.
The coil, although it may appear to be simple in many aspects has some demanding requirements to meet.
Speaker coil impedance
The impedance of the speaker coil consists of both resistive and reactive elements.
Typically the DC resistance is around two thirds of the stated impedance which can be considered to be a minimum impedance as it varies over the range, rising to a peak at the speaker resonant frequency. This is normally not an issue because the drive amplifier will be designed to have a very low source impedance.
It should be remembered that for multi-speaker units, there may be several peaks in the impedance. Also the overall speaker impedance may be mainly reactive at some frequencies resulting in the amplifier voltage and current being out of phase. This may result in high current levels flowing at some frequencies that can cause the amplifier to limit if it does not have sufficient capability - it is always wise to run systems with margin for best operation.
Typical speaker coil impedance levels are 4 and 8 Ω for modern systems. Older systems had a standard 3 Ω impedance with high quality speakers having an impedance of 15 &Omega.
Speaker coil heat dissipation
One of the key issues for the speaker coil is that of heat dissipation. With high powers often being applied to loudspeakers, the coil can need to dissipate large amounts of heat. In view of the overall construction of the speaker and the coil, heat is not able to be dissipated easily and speaker coil temperatures can rise.
Coils for modern high performance and high wattage speakers - normally bass driver units - are often wound on aluminium formers and high temperature epoxy adhesives are used. These can withstand temperatures of up to 300° or so.
It should be remembered that as the temperature rises so the resistance of the coil increases - typically by around 0.4% per degree Celsius. This can make a significant difference to the performance.
In multi-speaker systems with different speakers used for different frequency bands, the heat dissipation can cause the tonal balance to alter. Not only will there be a different amount of sound content in different frequencies, but the different speakers used for the different bands will respond differently.
To remove the heat from the speaker coils a number of measures can be introduced: a large magnet area helps and some coils are blacked to increase the rate of heat loss. In some very high power units the speaker coil may even have heat fins to help.
Even so it has been know for some speakers to have the coil wire melted by over driving them for extended periods.
Speaker coil repair
There is a variety of issues that can occur with speaker voice coils, and sometimes it is possible to repair them.
Sometimes the coil repair can be a little exacting, but in many cases you have nothing to lose. There are a few techniques that can be employed to repair the loudspeaker system by replacement of a unit, or even the speaker coil.
. Read more about Loudspeaker repairs and speaker coil repairs.
Loudspeaker coils are the main working element of the loudspeaker. They provide the movement and vibration that excites the cone and causes the sound waves to emanate from the overall loudspeaker itself or the loudspeaker system. As such the coil is a key element of the overall loudspeaker.