Magnetic field coil ferrous material for loudspeaker

• B
• arivel

arivel

Hi everyone.
there are materials called soft magnetic and they are halfway between a permanent magnet and a ferromagnetic material.
I would like to try to make a field coil loudspeaker where the maximum amount of flux-density is very important but also the amount of current it takes to create the magnetic field.
there are materials like FeCo alloy (soft magnetic) that comfortably exceed 2T but I can't figure out how much current it takes.
if it takes much less current than a material such as the economic FeSi alloy, this would be very interesting, then the price must also be considered.
bye thank you

Every loudspeaker I have ever seen had an air gap in the magnetic circuit. It's been a while since I designed a magnetic circuit, but I recall that the majority of the drive current is used to drive the magnetic field through that air gap. So you need to calculate the magnetic circuit for each alloy, and the resulting total cost, to find the best solution. I expect you will find that your efforts are best spent minimizing the air gap.

DaveE and berkeman
Speakers have used permanent magnets since about 1940. Prior to that, electromagnets were sometimes used. The winding was also used as a choke, in the power supply smoothing circuit. I don't think the power consumption of an electromagnet would be accepted today.

jrmichler, dlgoff, berkeman and 1 other person
I would like to try to make a field coil loudspeaker where the maximum amount of flux-density is very important but also the amount of current it takes to create the magnetic field.
It will come down to the space available for a mass of copper, and the cost of that copper. It is not current, but it is power, temperature and insulation, that will limit the magnetic field.

Early "electrodynamic" loudspeakers could use a fine wire field coil as a low-current high-voltage choke, because it had high inductance. There is a trade-off between electromagnet inductance and frequency response. Do you require a maximum stable magnetic field, or a modulated field?

For the same volume of copper, with the same ampere-turns, if you halve the current, you can wind twice the turns, with half the sectional area wire, and the inductance squares, so rises by a factor of four. When you halve the current, you can double the voltage, so the same power is needed for the same mass, or cost of copper, to produce the same field.

That takes you back to the mass of oxygen-free copper you can afford, and the maximum safe temperature of the electrical insulation in the coil.

hutchphd and jrmichler
If you still want to design a field coil speaker, here are some patents with good discussions of the engineering involved:

US 1,448,279 Electrodynamic Receiver, Edwin S. Pridham and Peter L. Jensen, assigned to Magnavox Company, filed April 28, 1920. A quote from that patent: "We have found that the greater the density of magnetic flux in the air gap, the greater the efficiency of the instrument".

US 1,811,367 Loud Speaker, Edwin S. Pridham, assigned to Magnavox Company, filed March 27, 1929. A quote from that patent: "...it is essential that the voice coil be exactly centered in the air gap...".

1,836,076 Dynamic Speaker, Russell T. Kingsford, assigned to Atwater Kent Manufacturing Company, filed August 17, 1929. This patent also emphasizes the importance of centering the voice coil in the air gap: "...is nicely adjusted, as by a gauge, to obtain exactly uniform thickness of air gap...".

Note the importance of closely centering the voice coil in a small air gap.

There are many more patents for field coil speakers, and they contain much useful information on the design of this type of speaker. The above listed patents are not the best ones, they are merely the first three that I found in a five minute search. Further searching should find better ones.

hutchphd
I wrote amount of current because I was referring to a low voltage power supply. whether you feed high voltage and low current or low voltage but high current the energy consumed does not change. I am interested in the latter being reasonably small, which is why I asked for information on soft magnetic materials.
I want the magnetic field maximum and at a constant flux value. having it variable in my opinion is wrong due to the effect of the hysteresis

I want the magnetic field maximum and at a constant flux value.
Then how can you beat a rare earth permanent magnet?

Electrodynamic speakers (that used electromagnets for the field) died out with the advent of better PMs, about 15 years before vacuum tubes were replaced by transistors.

You are really constructing a linear motor. Field current is seen as a waste of energy, so DC motors now use permanent magnets. To be brushless, those PMs have moved to the rotor, so DC motors can now switch the current driving multiple stator windings, rather than employing commutators on the rotor.

A loudspeaker needs to have a low suspended mass, so it is the voice coil moves, while the PM and magnetic circuit, remain fixed.

jrmichler and berkeman
halfway between a permanent magnet and a ferromagnetic material.
What does that mean? Permanenet magnets are ferromagnets.

Then how can you beat a rare earth permanent magnet?
That is an excellent question.

Maybe we should start by "what are you trying to do that can't be done with a commercial speaker"?

berkeman
the following are some of the reasons that come to mind.
the flux of a permanent magnet decreases over time, then we need to see how and the decreasing curve.
to my knowledge , most loudspeakers have a magnet with a flux density that very rarely exceeds 2T .
in a field coil loudspeaker the flux can be adjusted at will in order to obtain the desired Thiele & Small parameters

What does that mean? Permanenet magnets are ferromagnets.
a permanent magnet is no longer ferroelectric. she was before she was.

... the flux of a permanent magnet decreases over time, then we need to see how and the decreasing curve.
The loudspeaker force is the product of magnetic field and audio drive current.

You must change the volume of the audio drive, to compensate for many changes.
2. The listening space (room), and the people and things that move in that space.
3. The properties of the air at the time.

How will you regulate the electrodynamic field while:
1. The copper coil temperature changes during the operating cycle ?
2. The resistivity of the electromagnet changes as it ages ?

a permanent magnet is no longer ferroelectric. she was before she was.
Ferroelectric and ferromagnetic are two different things. Originally you said "ferromagnetic" and I can assure you that permanent magnets are ferromagnetic. I can also assure you that we won't make any progress by denying physical truths.

You seem to think ferromagnets "wear out" over time. While nothing lasts forever, I think you are solving a non-problem. I have 20-yar old speakers. Work fine. I have 30 year old refrigerator magnets. Work fine. I have a friend who restores century-old electric motors: the problem isn't the magnets, its the wiring.

A mix of being cagey about what you are trying to do and of unconventional physics will make it very hard to help you.

berkeman
translation error I meant ferromagnetic and I am still convinced of what I wrote.
A magnet was ferromagnetic before it became ferromagnetic. There is no permeability in a magnet. However, this is not the forum for discussing how a loudspeaker should be built.

berkeman
translation error I meant ferromagnetic and I am still convinced of what I wrote.
A magnet was ferromagnetic before it became ferromagnetic. There is no permeability in a magnet. However, this is not the forum for discussing how a loudspeaker should be built.
Sorry, this all makes no sense. I'm not sure there is value in keeping this thread open anymore...

close it because I reacted ?
so you don't have to dispute other opinions?

No, and perhaps it's still a language translation issue. Let's start here:

A magnet was ferromagnetic before it became ferromagnetic.
What are you trying to say with that?

There is no permeability in a magnet.
That is obviously nonsense. Another translation issue?

I'm not a person used to making controversy but this is a forum or a court?.
I asked a very specific question but instead of getting an answer I got a trial of intentions.
Thanks anyway to those who took the time to answer

No big deal. It just seems like you have several misconceptions and we have been trying to help you correct those, but we are not succeeding so far. If you are trying to design and build a better loudspeaker, that is fine, but you will likely need to learn more about magnetics and EE in order to be able to try that.

hutchphd
it's certainly not in a physics forum that I learn how to build a loudspeaker.

weirdoguy and Motore
it's certainly not in a physics forum that I learn how to build a loudspeaker.
It is the purpose of Physics Forum to help teach you the physics that might enable one to build a better loudspeaker. If you say things that seem incorrect, either you are mistaken or communication is an issue. It is necessary to repair this in order to proceed!

A magnet was ferromagnetic before it became ferromagnetic.
Reminds me of "I was for it before I was against it."

At best you are not stating what you are intending to accomplish, and your physics ideas are, if not completely wrong, at least garbled. Makes it hard to help.

Why can't you just call Crutchfield and buy what you want?

Last edited:
Crutchfield....(I knew the founder in Charlottesville!)

Fixed.

But really, why not? Before going down this path, one that includes dodgy (or at least misstated) physics, why not call them up and say "I want a speaker than does X, Y, and Z". Why complicate matters with ferromagnets that aren't really ferromagnets and the like?

I found the material I was looking for , it is called hiperco 50 , but there are problems .
it is expensive and difficult to machine.
so i have another question for you .
in the attachment you can see , in section , how the magnetic motor of a field coil loudspeaker is made .
you see a central body, an empty red space where the copper wire winding and a double outer ring are placed.
the question is the following:
can I build the central body with hiperco to have maximum magnetic saturation and the outer ring in another material but with very high magnetic permeability? . how does the magnetic flux intensity change? .
then I don't know if in the long term it is possible to create oxidation on the contact surface between two different metals.

Attachments

• field coil.png
13.4 KB · Views: 17
I see no point in having such thick magnetic material for your field coil, when the space could be taken by copper magnet wire.
The cross-section of the magnetic circuit should be constant along the magnetic path, so that if saturation occurs, it occurs everywhere at the same time.

what do you mean by the following sentence? :
"I see no point in having such thick magnetic material for your field coil, when the space could be taken by copper magnet wire."
would you say that you can use less metal and more wire wrapped in copper?
Do you have experience building this type of speaker?

would you say that you can use less metal and more wire wrapped in copper?
Do you have experience building this type of speaker?
I have experience with that type of electromagnet, but not with ones as small as that.

Maybe prototype by replacing only the magnet on a standard speaker. Will you use a commercial product for the voice coil and for the cone and suspension?

You should start by specifying the voice coil dimensions, the radii and the axial length of the winding, as everything will grow from that by geometry.

Maintain the same section of magnetic path throughout the magnetic circuit, taking into account the circumference at different radii. Avoid sharp internal corners.

specifying the dimensions of the voice coil, the radii and the axial length of the winding
first i need to get the answers i need .
my idea is to use a full cylinder with hiperco 50 only for the central core wrapped in copper coils.
this, because hiperco is expensive and difficult to work mechanically, with it I can get the maximum magnetic saturation with less copper coils and less electric current. then instead use another material for the outer O-ring. I had thought of mu-metal because it is easily machined on the lathe, costs less and has a very high magnetic permeability, then I discovered that its magnetic saturation is very low. So my question is the following: which material can I use for the outer ring ?
maybe steel with low carbon content?. if I do this, however, I get a lower general magnetic saturation. Maybe it's better to use a material that has a lower magnetic saturation but greater permeability than hiperco? .
I await the correct answers from you.
Will you be using a commercial product for the voice coil and cone and suspension?
I remove the permanent magnet from a used loudspeaker or use spare parts.
these are the technical specifications of hiperco
https://f.hubspotusercontent20.net/...ources/Datasheets/Hiperco_50_Alloy_(E200).pdf

I have another question to ask.
I guess there is a relationship between air gap and magnetic saturation drop. how is it calculated?

HI.

I realize that there are many questions to answer and therefore I don't expect to get all the answers but I would like to get the answer to just one question, I don't think I'm asking too much.
what happens if I put two metals with different magnetic saturation but different or equal magnetic permeability in contact?

the metal with higher magnetic saturation is used to generate the magnetic field through the copper coil while the second metal with lower magnetic saturation is used to channel and close the flux of the magnetic field in a toroidal ring so as not to disperse it in the surrounding space.

I would like to know how the characteristics of the magnetic field that passes through two different metals change.

Hello and thanks

I realize that there are many questions to answer and therefore I don't expect to get all the answers but I would like to get the answer to just one question, I don't think I'm asking too much.
what happens if I put two metals with different magnetic saturation but different or equal magnetic permeability in contact?

the metal with higher magnetic saturation is used to generate the magnetic field through the copper coil while the second metal with lower magnetic saturation is used to channel and close the flux of the magnetic field in a toroidal ring so as not to disperse it in the surrounding space.

I would like to know how the characteristics of the magnetic field that passes through two different metals change.
Can you use the "Attach files" link below the Edit window to upload a diagram of this arrangement? How can you have a voicecoil on a toroid? Thanks.

if you say this it means that I used the wrong word.
refer to the image of the number 24 .
there it is shown in section but you have to imagine it in three dimensions