Clarification Required on How Speaker Works

[thirumalai_pm]

Hi,

I am new to Eletronics. This is very simple question, but sorry I am trying to understand the basic concept.

I am reading an article in Howstuffworks "How Speakers Work",

where I got to know that the sound in speaker is generate from the diaphragm by making electromagnet on the

permanent magnet and altering the orientation of the poles of electromagnet which comes from stereo system.

I got one query is : We can make the electromagnet by using a coated coil. So Instead of using permanent magnet I wish to create electromagnet using electric current (not from stereo) so it will create magnet force (like permanent magnet) with north, south orientation of pole, then I can connect stereo wire to voice coil will change the pole as per stereo current. So it will disturb the diaphragm to make the voice.

So can concept work well in speaker, if so we can avoid using the permanent magnet. Can you also please let me know why all speakers are using permanent magnet if we are able to create electromagnet.

Many Thanks, Thirumalai

 

[dknguyen]

So can concept work well in speaker, if so we can avoid using the permanent magnet. Can you also please let me know why all speakers are using permanent magnet if we are able to create electromagnet.

Many Thanks, Thirumalai

That's a good question. I don't know the answer myself but I *think* it might have something to do with linearity from what I know about motors.

All motors use at least two sets of magnets and one set is always an electromagnet (the coils in the motor). The other set is can be a permanent magnet or an electromagnet (called the field winding). I know motors that use permanent magnets have a linear input-output response because the magnetic flux of the coil (which you control to adjust the motor) does not affect the flux of the DC magnet. But motors that use a field winding isntead of a permanent magnet have a non-linear input/output response since the magnetic flux of the coil and field windings can induce currents in each other that changes the magnetic flux of both- even when you are keeping the power input to the field winding the same.

Obviously you want the speaker to play the same signal you send into it without distortion and if this was the reason it would make a lot of sense because all speakers are built differently and would have a different input output response. That would mean a sound signal would have to be processed to play properly on a certain speaker, and you would have to adjust the signal again to play it on another type of speaker.

While we're at it, maybe we can also answer the question as to why the electromagnet is designed to be the one that moves and the permanent magnet that remains stationary. It always seemed to me that it would make more sense to have the permanent magnet move since it doesn't need a wire connected to it.

 

[Nigel Goodwin]

So can concept work well in speaker, if so we can avoid using the permanent magnet. Can you also please let me know why all speakers are using permanent magnet if we are able to create electromagnet.

A LONG, LONG time ago, electromagnet speakers were commonplace, it was the only way to get enough strength for large high power speakers - but 'modern' improvements in permanent magnets means they aren't really of any value any more.

Back in the 70's I used to use a pair of BTH 18inch speakers with electromagnets (along with a single wooden treble horn) - they came out of an old cinema (1930's ?). The original amp was a 6 foot 19 inch rack unit, with one HUGE knob for volume - it output about 5W or so

We happily fed the speakers from a 125W RMS TUAC power amp, and never had any problems with them.

So basically, it's cheaper and easier to use permanent magnets - and more efficient of course.

 

[audioguru]

You want the cone of a speaker to be light weight so it can accellerate and stop quickly and use a fairly low amount of power driving itself so that the power can be used to move the air. A permanent magnet is heavy but a voice-coil is light weight so the voice coil moves with the cone and the heavy magnet structure is stationary.

 

[transistor495]

Why we should waste huge power on an electromagnet when the permanent magnets are very effective, cheap and they keep their strength(shielded ones) for many decades!

Do you think it's worthy operating an electromagnet inside a stereo that works on battery too!

Nowadays neodymium magnets are ultra strong and are very very effective. Think about the huge magnetic units used in high KVA rated gensets.

 

[antknee]

An electromagnet would work fine. It is just a waste of electricity unless you are doing something cool/interesting or generally novel.

Why do you need to know?

 

[JimB]

A LONG, LONG time ago, electromagnet speakers were commonplace,

Also, many mains powered radios used speaker with an electromagnet to provide the constant magnetic field.
I believe that this was done by passing the HT supply through the coil, which may also have been part of the HT smoothing circuit.

On edit:
Have a look at this circuit and description:
https://www.pasttimesradio.co.uk/articles/pilot%20little%20m.pdf

JimB

PS for those here who are less than 60 years old, "condenser" is the old fashioned word for capacitor!

 

[Nigel Goodwin]

Also, many mains powered radios used speaker with an electromagnet to provide the constant magnetic field.
I believe that this was done by passing the HT supply through the coil, which may also have been part of the HT smoothing circuit.

Very true, they used it as the mains smoothing choke - I'd forgotten about that

For those too young to remember, back in the old valve days electrolytics weren't very 'big', 8uF or 16uF were about as big as they got (at 300V or whatever HT was), and half-wave rectifiers were used. So in order to reduce the supply ripple to reasonable levels a 'PI' filter was used, with the cross piece been a hefty choke.

 

[dknguyen]

So did it used to be called condensitance rather than capacitance?

 

[JimB]

So did it used to be called condensitance rather than capacitance?

No, it was called condensance.







Sorry, just pulling your wires!

It always was capacitance, condensers had capacitance.

JimB

 

[john1]

They were measured in Micheal Farads

 

[dknguyen]

Mike-kro Farads.

 

[thirumalai_pm]

Thanks a lot everyone. As I am the beginner I am getting the doubts like that. By clearing my doubts will always gives me more strength to my knowledge.

I thought making electromagnet is more powerful then permanent magnet, so we can get more sound by disturbing the cone much. But I got to know that is not the real true and it will waste lots of electricity will be a correct point. Instead we can use the permanent magnet; will be onetime expense.

Many Thanks, Thirumalai M