Does Voice Coil Affect Magnetic circuit?

[kelticwizard]

I have read of the magnetic circuit of a loudspeaker, that is the magnet and pole piece. There is a certain amount of gauss which can be generated, determined by the magnet size and type, and the qualities of the pole piece.

Two questions.

A) Does the length of the voice coil have anything to do with the magnet and the requirements for it's return circuit? Or is that something determined solely by the magnet and it's return circuit, which is the pole piece?

B) What is the normal limits for Bl value for a speaker with a four inch diameter voice coil? How about a six inch diameter voice coil?

The highest Bl you normally see for a four inch voice coil PA speaker is about 30 Tm, but I don't know if that represents a practical limit or is limited by the fact that the designer does not want too low a Qes value. What would be the limits if I didn't care about Qes value?

[454Casull]

A) No.

B) I'd model it in FEMM for you, but I don't have FEMM and I probably don't remember how to use it. Sorry.

[Svante]

Well, the highest value of B is a bit more than 1 tesla. To get the maximim value of Bl, you will need to multiply B with the length of wire that is exposed to B. If Bl=30, then the length of the wire is a bit less than 30 metres.

Maybe that did not help...

[AndrewT]

Hi,
Alnico magnets get to around 1.8T to 2T.
If the designer is prepared to throw resources at the magnetic circuit and the magnet then around 2.3T can be squeezed from a permanent magnet.

I suspect more is available if the rare earth magnets were used.

1.1T is generally the cheap limit set for a ceramic magnet, but again I suspect a bit more is available.

[Svante]

Originally posted by AndrewT
Hi,
Alnico magnets get to around 1.8T to 2T.
If the designer is prepared to throw resources at the magnetic circuit and the magnet then around 2.3T can be squeezed from a permanent magnet.

I suspect more is available if the rare earth magnets were used.

1.1T is generally the cheap limit set for a ceramic magnet, but again I suspect a bit more is available.

Hmm, I'm no expert on this, but I thought the limiting factor was the iron in the magnetic circuit. That it does not help if you have a magnet capable if high B:s, since the flux density is largest in the magnetic gap (where there is only iron).

[kelticwizard]

Okay, so TM is the flux density?

Let me ask an elementary question then.

Suppose the magnet and pole piece are the same for two speakers. There is only difference.

Speaker A has a magnetic gap height of 3 mm.

Speaker B has a magnetic gap height of 6 mm.

Does this mean that speaker A has a Tm twice that of speaker B?

[454Casull]

Tm is Tesla * meters, which is supposed to be flux integrated over the conductors or something. Tesla (T) is flux density.

[kelticwizard]

454Casull:

Oops, that's right. Now that I think about it, Tm is affected by the voice coil.

So it is T I am concerned about.

So does Speaker A have a twice the T of Speaker B because it's magnetic gap is half the height?

[Svante]

Originally posted by kelticwizard
454Casull:

Oops, that's right. Now that I think about it, Tm is affected by the voice coil.

So it is T I am concerned about.

So does Speaker A have a twice the T of Speaker B because it's magnetic gap is half the height?

No. Or yes. It would if the magnet would have been very weak. However, the pole pieces near the gap are (AFAIU) saturated, and this happens at a bit more than 1 tesla. So, taking a driver with a 3 mm gap height, and increasing it to 6 mm is likely to decrease the B to something between 50 and 100% depending on the degree of saturation. On the other hand, taking a driver with a 6 mm gap and decreasing the gap to 3 mm will probably not affect the B at all. It will definitely not double. This is also due to the saturation.

(B is magnetic flux density, which is measured in tesla, T)

[AndrewT]

Originally posted by Svante


Hmm, I'm no expert on this, but I thought the limiting factor was the iron in the magnetic circuit. That it does not help if you have a magnet capable if high B:s, since the flux density is largest in the magnetic gap (where there is only iron).

Hi Svante,
I'm even less of an expert, but your assumption that the iron is the limiting factor is right but at MUCH higher flux levels. The limit will be approached most closely at the pole piece where the flux is guided into the gap.
My Tannoys are around 1.4T for the bass VC and higher for the treble VC.
The Lowthers are all around 2T. But here they have very short VC to keep the weight (mms) down.