Speakers with magnetic coupling done differently?

[Redacted]

I should do my research, but I'm a retired, and now very lazy, engineer...

So, in a speaker, a coil has a varying current passed through it that reacts with a big magnet to move a cone - ancient tech.

The magnetic field from the magnet could be a coil producing a field (c.f. alternators in cars).

What if instead of a steady field in the coil, you could modulate the current in that coil using feedback from the main coil that is attached to the cone?
What if the second coil produced an anti-phase signal, also modulated to potentially reduce distortion? (because of the difficulty of making cones track a signal accurately). A bit like negative feedback in an amp, but correcting the speaker.

Just thinking out loud, so don't do a 'pile in' and crush me just yet!
[Electro-magnetics is not my area of knowledge, even though I have 10 patents in other areas]

I was wondering if there are commercial audio systems along these lines?

 

[voodooless]

Motional Feedback - Wikipedia

en.wikipedia.org

This uses a force pickup element to measure cone movement.

And then we have this:

Rythmik Audio • Direct Servo subwoofer products

They use a second voice coil to measure cone movement.

There are many more variants out there, though.

 

[sam_adams]

The magnetic field from the magnet could be a coil producing a field . . . What if instead of a steady field in the coil, you could modulate the current in that coil . . .

It's been done:

EM magnet - Focal | Focal

The magnetic flux in the air gap reaches 1.75 Tesla (0.9 for the woofer of the previous large Utopia Be).

www.focal.com

 

[Dialectic]

It's been done:

EM magnet - Focal | Focal

The magnetic flux in the air gap reaches 1.75 Tesla (0.9 for the woofer of the previous large Utopia Be).

www.focal.com

The Focal electromagnet woofers and Western Electric field coil drivers sprang also to my mind when I read the top post, but I think the OP is referring to something more akin to self-regulating "motional feedback," as Philips called their sensor-based version of this technology.

In contrast to the feedback systems in servo-controlled woofers like those produced by Rythmik and earlier designs produced by Infinity/Genesis, the electromagnet in the Focal woofers serves only to boost efficiency. (I've heard some big Focals with EM woofers, and, although I doubt that they measure well by our standards here on ASR, the sound quality is, if not accurate, certainly imposing.)

Western Electric manufactured field coil drivers because of the unavailability in the 1920s and 1930s of suitable permanent magnets. Like many other curios, field coil drivers now enjoy a somewhat inexplicable mystique among audiophiles in Japan, South Korea, and, increasingly, the United States.

All of this is different from what the OP described, but servo control/motional feedback is, to my knowledge, the extant technology closest to what the OP describes.

 

[Redacted]

Wow - all good stuff! Thanks

Any ideas why it's not used more?

 

[Dialectic]

I formerly owned Genesis loudspeakers, and I've heard good things about Rythmik subwoofers and considered buying 2-4 of them for my big system.

But I don't think any of these technologies have yielded massive benefits to fidelity.

 

[voodooless]

Any ideas why it's not used more?

It adds quite a bit of complexity to a system. You’re also limiting the box types that you can use. Typically these things are limited to closed boxes.

KEF also uses something similar in their latest soundbar.

 

[Redacted]

Clearly more complexity and cost sure, but if a system could show measurable technical benefits it would have a lot more takers and developers. But I'll look into the devices mentioned above.

It just struck me that speakers are really the main frontier still to have major breakthroughs, the rest of the system components are probably as good as anyone needs these days.... [oops, controversy right there!]

 

[voodooless]

Clearly more complexity and cost sure, but if a system could show measurable technical benefits it would have a lot more taker and developers.

Well, maybe. But it really needs a bunch of prerequisites to work:

- A closed box system, at least I haven't seen any other setups
- It needs to be an active system, and those are in the minority anyway. That's why this technology pops up mainly in subwoofers, where it brings the most objective advantage anyway.
- It needs specialized drivers, either with additional coils, an accelerometer, or some other means of measuring excursion (or a proxy). That is always a major investment if you want to do it at scale.

As mentioned earlier, KEF dared to bring this kind of technology to soundbars: https://international.kef.com/pages/veco-technology

And then we have the other issue: low-frequency distortion isn't all that audible in the first place. So the audible gains might not be as big as it's perceived to be.

 

[DanielT]

Philips is mentioned in the Wikipedia link about MFB in #2. In the Netherlands it seems, for natural reasons ( Philips -Netherlands), still to be popular or at least to exist. See this thread:

Motional Feedback (MFB)?

It seems to be a smart system, right? Common in constructions nowadays? In the art of audio sound reproduction it is well-known that the dynamic loudspeaker is more nonlinear and generates more distortion than all the other system components combined. This is particularly true at low...

www.audiosciencereview.com

If you want to test it yourself:

Motional Feedback Audio.

All things servo. StarBass drivers & accelerometers, loop electronics and cash & carry Little/One and Grown/Up sales via servolabs.nl

piratelogic.nl

For subwoofers, at least sealed, why MFB when it is now so easy to get a lower bass response yourself via EQ / Linkwitz Transform? Plus EQ is still needed to tame room modes.

 

[voodooless]

For subwoofers, at least sealed, why MFB when it is now so easy to get a lower bass response yourself via EQ / Linkwitz Transform? Plus EQ is still needed to tame room modes.

I think the advantage of non-subwoofer systems might actually be much higher. Smaller, high-excursion drivers would benefit a lot from a feedback system. And particularly those systems can exhibit audible distortions when pushed a bit.

 

[Devnull]

Once you get to midrange and higher frequencies it's probably cheaper but not necessarily easier to use a horn for reducing distortion. Diaphragm motion is much lower in a horn than a bare speaker, more motion means more of a chance for distortion as the excursion increases.