Magnetic Repulsion for a Loudspeaker Motor

[SuperTzar]

Hello, I have found an interesting alternative to the common loudspeaker motor in the form of a magnetic repulsion circuit. The picture below shows the basic idea. A coil is suspended between two magnets (or pairs of magnets) that are a certain distance apart. The magnets are mounted so that their poles oppose one another. The picture came from a Japanese article that was impossible to translate, so there was little I could understand from it.

What I'm trying to understand is how would the magnetic field of the voice coil interact with the field generated by the magnets? Say a positive voltage is applied to the voice coil, which way will it move?

If anyone can help me better understand this concept, I would greatly appreciate it!

 

[sophiecentaur]

Interesting idea to use the field between two N-N poles but it seems to go against the two main requirements of a loudspeaker motor. It looks initially as though the magnetic field could well not be as high as in the conventional layout. But I have only ever seen idealised diagrams of the field between poles in a conventional LS and the field may well not be as uniform over the gap as the diagrams suggest. You can see in the diagram above that they have at least addressed the linearity problem - that could be the whole point of the design, of course.
One other problem I can see is that the voice coil would need to be mechanically connected to a remote cone or diaphragm (?). That would add mass to the moving elements. Perhaps, with small modern magnets, the room taken up would allow the cone to be down around the coil. For a bass driver that could work I guess.
Can you find a translation of the page? It may have some references on it.

 

[sophiecentaur]

how would the magnetic field of the voice coil interact with the field generated by the magnets

There is a simple answer to this. The field lines are at right angles to the direction of the current in the coil (ignoring the helix angle) in just the same way as the field lines between the two poles of a conventional loudspeaker magnet. The force on the coil will be proportional to the number of turns times the current flowing times the B field.
If you look at the field lines on that diagram you can see they are comparatively widely spaced out compared with the lines in close to parts of the magnet. That implies that you could use that close-in field around a similar 'strength' permanent magnet but in a N-S arrangement (the conventional LS system and get much higher deflection force.
One clear advantage could be that the voice coil could probably handle much greater current because it is not in the constricted space of a conventional LS gap which allows voice coils to overheat.

 

[SuperTzar]

Can you find a translation of the page? It may have some references on it.

http://tomozon.sakura.ne.jp/speaker/sf_speaker/index.html

There's the link to the page, the section on the repulsion circuit is a little ways down. Google doesn't do a great job of translating the page, but it seems to say that having two magnets opposed like that indeed generate a more uniform field at the cost of weaker field strength.

I wish I could translate the labels on the diagrams, but there's probably an app for that.

There is a simple answer to this. The field lines are at right angles to the direction of the current in the coil (ignoring the helix angle) in just the same way as the field lines between the two poles of a conventional loudspeaker magnet. The force on the coil will be proportional to the number of turns times the current flowing times the B field.

So the repulsion circuit does not restore the voice coil to its resting position? As I understand it, in a conventional loudspeaker, when the voice coil is energized by a +/- voltage it is pushed in or out due to the magnetic field. However, when the signal returns to 0 volts, the voice coil is no longer magnetically charged and the mechanical suspension returns it to its resting position. Some argue that the mechanical suspension alone cannot restore the voice coil to its resting position quickly enough, before it receives the opposing voltage to drive it in the other direction.

 

[sophiecentaur]

So the repulsion circuit. . . . . .

I think this name is a total red herring - except in as far as squeezing two poles together can produce a high field in between. (See the diagram). There is no inherent 'repulsion' of the coil involved.
If you look (Google search) at a normal loudspeaker magnet system, the field lines go from a N pole (say) on the outside and a S pole on the inside pillar. The coil knows nothing about how the field lines were produced. There will be no force on it (copper wire) until a current passes through it. The force direction and strength depends on the sign and value of the coil current. The thing that returns the coil to its central position is the suspension of the coil / cone. It has to be constrained or it will pop out of the end and destroy the cone.

 

[SuperTzar]

The coil knows nothing about how the field lines were produced.

Thank you! That clears it up immensely.

I'm pretty familiar with the magnetic circuit in a traditional loudspeaker, I just thought this one might somehow be different.

 

[sophiecentaur]

I'm pretty familiar with the magnetic circuit in a traditional loudspeaker, I just thought this one might somehow be different.

I looked again, after some years of course, at diagrams of conventional loudspeakers and the construction has just a small region of narrow gap. The field across this gap will be the highest so only a small section of the coil will actually be getting maximum force. As the coil moves up and down, the force could change because there's varying amounts of it in the fringe field. See Fig 4 in this link. Not surprisingly, the problem has already been looked into!
If it really were a major issue, it would surely be possible to tailor the gap in a conventional LS, to give a bigger linear region - especially with magnets being so much stronger these days. (Look for the sales jargon when the manufacturers catch on to the idea.)

 

[SuperTzar]

If it really were a major issue, it would surely be possible to tailor the gap in a conventional LS, to give a bigger linear region - especially with magnets being so much stronger these days.

Funny you should mention that because I found an interesting paper on the design of a loudspeaker motor designed to have a linear magnetic field in a very deep gap: https://www.parts-express.com/pedocs/tech-docs/Aurasound-NRT-Whitepaper.pdf

Page 7 of that document shows an example of the design. Looks like the magnet itself forms the outside of the gap, and the steel on the bottom and inside support the magnetic circuit. Looks pretty ingenious to me, the voice coil should be immersed in an even field throughout the height of the gap.

 

[sophiecentaur]

Funny you should mention that because I found an interesting paper on the design of a loudspeaker motor designed to have a linear magnetic field in a very deep gap: https://www.parts-express.com/pedocs/tech-docs/Aurasound-NRT-Whitepaper.pdf

Page 7 of that document shows an example of the design. Looks like the magnet itself forms the outside of the gap, and the steel on the bottom and inside support the magnetic circuit. Looks pretty ingenious to me, the voice coil should be immersed in an even field throughout the height of the gap.

Yes. Interesting paper but one needs to be careful taking all the claims on face value because it's a sales pitch. There need to be some measurement figures and some other supporting evidence before we can believe in what looks on the face of it to be as good an idea as it appears to be. But there must be many possibilities for using these fantastic modern magnets.