Copper Voice Coil Formers!


2007-10-16 10:52 pm
The standard voice coil former material is aluminium. It's used with an
insulated voice coil winding and in the form of a cylinder with a gap (so
that it doesn't form a complete 'turn') meaning electrical conductivity
isn't a problem.

The only useful difference I can see is that copper is a better thermal
conductor than aluminium.

Shorting rings are monunted to the non-moving parts of a driver in order to lower flux-modulation of the magnetic assembly.
An eddy-current brake built into the voice coil would lower Qms but not any nonlinear distortion.

Regarding thermal behaviour the inside-outside type of voice-coil seems to be the best. This way both layers of a voice coil are in close proximity of a metal part of the magnetic assembly and therefore heat transfer is improved. The voice-coil former should be made form a material that can withstand high temperatures but which is by itself a bad thermal conductor. This way the heat is kept form heating the cone and especially the joint between cone and voice-coil.


I just made a magnetic drag for a windmill testing rig. It is force proportional to velocity (rotation in this case), with a neo magnet mounted on a spring loaded disk with indicator pointer, and a copper disc that rotates near the magnet.

Putting sheet copper into the gap and moving it at audio rates will certainly cause drag. Heavy duty damping.

Whether or not that is a good thing, the speaker guys will have to say.

The voice coil former resistivity matters even with a gap. Compare Qm of drivers with fiber glass, kapton, titanium and aluminum for an indication.

This is true. Conductive materials lower Qm. You will measure a difference between aluminum formers and Kapton or paper.

We generally prefer to keep Qm high as it gives the best efficiency for a given system Q. That is, lets say we want a closed box system with a Q of 0.7. We place an experimental woofer in a cabinet and find the Q is higher than desired, but we have the option of redesigning the woofer. If we add magnet we can get to the desired shape (lower Q) and at the same time our sensitivity will increase.

On the other hand if we achieve our Q with increased drag, then we will not see sensitivity increase.

As a practical matter, as long as Qm is 5 times Qe or greater, the potential sensitivty gain will be minimal.

The eddy current damping is huge. Have you ever tried dropping a Nd magnet down the bore of a copper pipe? It falls like it is going through molasses and takes several seconds to fall a few inches in free fall. Aluminum is similar to copper with regards to eddy current damping, which is why they cut a notch in it so that the circumferential eddy current (major mode) does not make a complete circuit. Otherwise, the speaker will not work because the eddy current brake is proportional to velocity. The sharper or faster the transient, the harder it brakes to the point of not moving. The braking is velocity/acceleration dependent.

Best way is to make an longitudinally segmented former by taking a tube and cutting longitudinal slots in an alternating fashion.

See for example the free fall in magnetic molasses:

Effect of slots/slits in tube:
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2007-04-11 11:28 pm
Yes, but that's more expensive.

There're other treatments for increasing the power rating, e.g. an thicker top plate. Very good is a defined ventilation of the gap. But to design this, you don't have to think like an audiophile, because they want very open baskets where all the good cooling air is wasted.
I'd say it does increase a voice coil's time constant but it would not necessarily improve the power rating. Modern techniques like inside-outside coils would be better.



Most of the thermal mass is in the winding, so adding a metal former doesn't change the time constant much. It will effect conductivity of heat from coil to magnet structure, so it can actually lower steady state temperature.

Another big issue with coils is that the windings that hang outside of the top plate (the 2 sections of overhung) run much hotter. A burned coil will often have the outer thirds badly charred while the central third is good (It is conducting heat to the adjacent metal bits). A conductive former can help even out the temperature of the respective sections, at least to some degree.

David S.