why use fixed magnets?

[killerfishes]

I never could under stand why speakers use a perminent magnet when you could use two coils. It could shrink the basket in size a lot, The only problem i see is taperd volume responce and higher power needs.

[joe carrow]

It's been done- this type of driver is called a "field coil", and Supravox makes some drivers of this type that are highly regarded.

[Variac]

If I understand you correctly, I think you are referring to what is called a field coil driver. Instead of a permanant magnet, they have a large electro magnet. And that's the problem- in order to get decent efficiency out of a speaker the field magnet has to be very powerful.
A coil the size of the existing voicecoil isn't enough. The only way that speakers work well is having that wimpy voice coil reacting to a muy macho field magnet or field coil.

So, since the field coil needs to be powerful, it is made of a LOT of wire and weighs pounds.Generally it is bigger than an equivalent magnet. For this reason Field coil drivers tend to cost at least $600 AFAIK.

[Variac]

Whoops, Joe posted while I was composing.... Another brand is Fertin, another French company.

[John Sheerin]

Actually a field coil can be more compact and lighter than a ceramic magnet, but you have to wind it, have a power supply for it, and dissipate the heat from it. For general production concerns, a permanent magnet is much easier.

[ocool_15]

would this be more useful for car spl comps I mean 12VDC and at low resistance could be a strong magnet and they only have to go for a few seconds anyhow? I guess wouldnt transfer to the "showroom" as easy but they can spend mucho $$ on there equipment anyhow. I was always curious about the same thing.

[BassAwdyO]

Why use a voice coil? We could simply feed the feild magnet the signal and have a fixed magnet on the cone(neo of course). And for that matter, why use copper. HTSuperconductor would be MUCH more efficient and space saving, but that compressor would be loud!

[planet10]

I understand that the Fertin at least (and i think the Supravox as well) have a field coil + permanent magnet... a field coil is more expensive to manufacture and the power supply can add considerably to the cost.

dave

[Mr Evil]

Originally posted by BassAwdyO
Why use a voice coil? We could simply feed the feild magnet the signal and have a fixed magnet on the cone(neo of course)...

Interesting point. I suppose the reason it is as it is, is because a voice coil can be lighter than a permanent magnet. This reason is obviated by using a coil for both purposes. Since it is possible to achieve the same effect either from a large voice coil and small field coil, or small voice coil and large field coil, I can see no reason to favour one way around over the other. But expanding on this, wouldn't it also be possible to use a very small, light fixed magnet on the cone and a huge, fixed voice coil? It would be a difficult load for an amp though.

[kelticwizard]

Of course, you could probably make them both-the little movable one and the big, stationary one-voice coils if you want to, as Mr. Evil pointed out.

However, my original reading is that killerfishes meant feed the signal to the big stationary magnet.

An interesting point an engineer can help us out with here:

How much magnetic force per gram, (or ounce), does
A) a copper or aluminum voice coil winding produce, whether on a metallic or nonmetallic voice coil former, (or bobbin),

B) How much magnetic force per gram, (or ounce), does ferrite, neo, alnico, or any other usable magnetic material produce?

Perhaps the answer lies there.