Building a speaker, have a few questions about the voice coil and magnets

[luigidorf]

Hi there,
I'm building a speaker for my intro to EE class. We were given ten small washer magnets, some washers, a nut and a bolt. Enameled wire is also provided. My group prototyped a speaker with the following configuration:

Voice coil wrapped around a paper tube which fits around the bolt (but does not touch it). The bolt is fixed. Two stacks of five magnets each are on either side of the bolt, magnetizing the bolt so there is (in theory) a magnetic field being produced from inside the voice coil as well as outside.

Our prototype sounds pretty good, but I'm apprehensive because it seems like the magnets are being wasted because there's only a small part of the actual magnets that are close to the voice coil. I am considering changing the design to the following:

All ten magnets stacked up, with a larger paper tube around them and the voice coil around that.

My thinking is that it could be better because the voice coil is constantly very close to the actual magnets (not a magnetized bolt). It would also allow for a larger voice coil. The downside I guess is that the voice coil would be heavier, and would only have a magnetic field generated from the inside. Are there any other factors I should consider? Does one voice coil arrangement work better than the other based on intuition or experience?

 

[vk6kro]

One requirement of the magnetic field in a speaker is that it should be constant to get low distortion.

You want the movement of the coil to depend on the current in it and not be influenced by an uneven magnetic field.

Close to a magnet, the field varies enormously depending on the position.

So, it sounds like your experiment has been arranged to give a uniform magnetic field, even at the expense of some field strength.

 

[luigidorf]

One requirement of the magnetic field in a speaker is that it should be constant to get low distortion.

You want the movement of the coil to depend on the current in it and not be influenced by an uneven magnetic field.

Close to a magnet, the field varies enormously depending on the position.

So, it sounds like your experiment has been arranged to give a uniform magnetic field, even at the expense of some field strength.

That's good info. However, I think I didn't explain the setup well enough because the voice coil only moves on one axis, and the magnets are essentially uniform along that axis for both set ups. Hope that makes sense.

 

[vk6kro]

If the field is uniform, then there will be no problem.

If you have a Hall sensor, you could check how uniform the field is when you stack magnets like that.

I suspect the field won't be very uniform at all, but the best way is to try it and see.

Is the coil attached to some sort of paper cone?

 

[Enthalpy]

Speakers have an iron piece to re-orient the induction from axial to radial and to concentrate it, since iron saturates at 2T and magnets produce only 1.1T as a maximum - or 0.6T if used optimally.