"Breaking Speakers' Hoffman iron law"¹: Dual Force Cancellation Subwoofers Vs Magnetic Negative Springs.

[Alfredo02]

I am continuing with my research on how to achieve a cost effective way to create a DIY subwoofer. After the achieved effectiveness of Sonos and their Dual Force Cancellation Subwoofers, it made very difficult to achieve a cost effective way to DIY subwoofer. However, I have found that subwoofers based on "Magnetic Negative Springs" generate the exact similar effect. The opposing forces to the main woofer are created rather magnetically than electromechanically, as the face to face woofer would do. Certainly two distinct ways to achieve the same result.

I found that the company Brane Audio filed in 2020 the patent "Electroacoustic drivers and loudspeakers containing same"², presently with "Pending" status, with the main claim in the abstract being "magnetic negative springs of the electroacoustic drivers can cancel, or partially cancel, the large pressure forces on a sound panel (of an audio speaker) so that substantial subwoofer notes can be efficiently and cost effectively produced in small/portable speakers."

And this is exactly the principle in what "Dual Force Cancelling Speakers" rely onto. I do not have any interest in understanding if their proposed way, using magnetic forces rather than electromechanical forces, represent an obvious innovation to the trained eye of a field expert. There are governing bodies for that purpose.

But if the interest relies on how to generate a magnetic field to create such opposing forces, it is my belief that creating a concentric coil, in opposed phase, will certainly do it. So real interest would lie on how a speaker driver building expert would come with a solution without requiring complex magnetic/electromechanical build ups.

Since speaker driver building it is not my field of expertise, I only recognise that the principle of opposing forces is well known. I have not been able to find any initial article on the subject, but companies that have created products based on these principles, significantly Bang Olufsen with their Beolab 11 and nowadays Sonos with their two Subwoofer versions, the Gen 3 and the Mini. I'm sure more examples might have come to the market, but these two are distinct examples.

In 2020, the company Brane Audio filed a patent based on R.A.D. (Repel Attract Driver).and Permanent Magnet Crown (P.M.C.). Same opposing forces principles, but achieved creating an array of permanent magnets, opposing forces magnetically rather than electromechanically , as it would have been done with a dual facing woofer. They called it "Magnetic Negative Spring".

The pending patent goes as follows : "In general, in one aspect, the invention features a loudspeaker that includes a sealed enclosure. The loudspeaker further includes a sound panel mechanically connected to the sealed enclosure. The loudspeaker further includes an actuator operable to convert electrical energy into mechanical energy. The actuator is mechanically connected to the sound panel. The loudspeaker further includes a magnetic negative spring (MNS) that is mechanically connected to the sound panel.
[0014] Implementations of the invention can include one or more of the following features:
[0015] The actuator can be a voice coil.
[0016] The voice coil and the MNS can share the same magnetic circuit.
[0017] The actuator can be an electromagnet. [0018] The actuator can be a piezoelectric transducer.
[0019] The loudspeaker can further include a position sensor that senses the position of the sound panel.
[0020] ...." and so on, it continues with the language typical of patents.

But this description of the invention, that looks general, certainly has to be distinct to others, since the governing body has granted a "patent pending status" in numerous countries.

I do not know how to translate this definition based on "Magnetic Negative Springs" to the DIY world, without commercial purposes. Any help will be more than welcome .

Alfredo


Notes:
1.- Brane Audio Ltd. "Corporate Web Portal". May 2024
2.- Patent EP4094451A1 "Electroacoustic drivers and loudspeakers containing same", Pinkerton et Al., Brane Audio Ltd., 2020.

 

[kemmler3D]

I am not totally sure how to interpret this summary... there's a magnet attached to the front baffle that somehow opposes the motion of the driver?

Maybe someone smarter than me can explain how it works.

Anyway, to this point:

But this description of the invention, that looks general, certainly has to be distinct to others, since the governing body has granted a "patent pending status" in numerous countries.

Patent pending just means it's under review, it does not mean it will be granted, so it doesn't mean anything except that they submitted an application.

Also, being granted a patent is evidence that the solution is novel, but not hard proof. Patent examiners are not perfect and often grant patents to things that arguably they shouldn't.

It's also important to note that getting a patent doesn't mean the invention actually works. Mark Levinson (ptooey!) got a patent on putting a little EQ and reverb on the signal as a means of curing the (totally imaginary) problems with digital audio. Not only does it not work but the entire concept is BS.

Beware of reading too much into audio patents if it's not clear how it works.

 

[sigbergaudio]

There's nothing special with the Sonos subwoofer to my knowledge. It's certainly not breaking Hoffmans law. A "normal" dual opposing subwoofer with one driver on each side will achieve the same effect.

What is it in particular with the Sonos that you find to be so effective? And what is it that you are trying to achieve that you find difficult from a DIY perspective?

 

[DVDdoug]

You don't need any "tricks" to make a good subwoofer. But you might want DSP/EQ.

it DOES generally require cone area and box volume to get strong-deep bass.

I recommend getting some speaker design software (WinISD is free) and model some different driver/box designs before deciding what to build.

 

[Chrispy]

I'm not understanding the magnetic thing or even the Hoffmans law reference, but a dual opposed sub does tend to cancel out box vibrations but no particular advantage otherwise. I use one as a tv stand.

 

[Alfredo02]

Sorry guys, but I had assumed than the replication of the opposing force principle of dual confronting speakers via magnetic forces with the so called Magnetic Negative Springs were more common field in the industry. Sorry I did not provide any introduction about the technology, nor mentioned the critical advantages are obtaining those using these kind of designs. You can find more information in the following links:

Brane Audio Technology

Brane Audio Technology

braneaudio.com

Brane X review: a portable smart speaker with incredible bass

This wireless speaker has special audio powers

www.techradar.com

As well you can find numerous articles about the increase of bass effectiveness, reducing vibration as well as cabinet volume that SONOS and others have obtained using these kind of subwoofer designs, their introduced variations of the Thiele/Small parameters and the difficulties designing the Plenum dimensions using simulation programs such as winISD.

Best

Alfredo

 

[kemmler3D]

Sorry guys, but I had assumed than the replication of the opposing force principle of dual confronting speakers via magnetic forces with the so called Magnetic Negative Springs were more common field in the industry. Sorry I did not provide any introduction about the technology, nor mentioned the critical advantages are obtaining those using these kind of designs. You can find more information in the following links:

Brane Audio Technology

Brane Audio Technology

braneaudio.com

Brane X review: a portable smart speaker with incredible bass

This wireless speaker has special audio powers

www.techradar.com

As well you can find numerous articles about the increase of bass effectiveness, reducing vibration as well as cabinet volume that SONOS and others have obtained using these kind of subwoofer designs, their introduced variations of the Thiele/Small parameters and the difficulties designing the Plenum dimensions using simulation programs such as winISD.

Best

Alfredo

Interesting but I still don't understand how it works. The limiting factor for a small speaker producing loud, low bass is not necessarily the internal pressure in the speaker cabinet, but total displacement. No matter what's happening inside the cabinet, you need to move a certain amount of air outside the cabinet to produce a certain amount of bass. It doesn't matter how you move the air, but it must move.

It seems they may have found a way to make bass drivers more power-efficient, which is actually very cool and especially useful in the portable / AIO world. I can tell you the portable speaker I helped design had decent bass, but it was a large ported cabinet with a ton of budget devoted to batteries. Either way, I don't think it means we can get 20hz @ 100dB SPL from a small driver like the one in the Brane X.

 

[Duke]

I have no idea what the term "magnetic negative spring" means.

I speculate that the "sound panel" is the woofer's cone and the "actuator" is the voice coil and the "magnetic negative spring" could be a second voice coil, perhaps driven by a second amplifier, and perhaps the "position sensor" and its associated circuitry steers which amplifier is active to what extent, to minimize energy wasted in pushing signal through voice coil wire that is outside of the magnetic gap on long excursions. Perhaps the amplifier steering algorithm takes the non-linearity of the air spring inside the sealed enclosure into account. I think this could increase electrical efficiency relative to a conventional overhung voice coil for the same linear X-max, and could mimic the performance of a larger enclosure in some ways.

Or I could be completely wrong... and come to think of it, you'd probably want three voice coils and three amplifiers with my speculated approach, so I'm probably reading things that aren't there into what little description we have.

@Alfredo02, could you post a bit more of the patent application? I think there are clues in items [14] through [19] that you posted, and I'd be interested in seeing more such clues.

 

[Chrispy]

I have no idea what the term "magnetic negative spring" means.

I speculate that the "sound panel" is the woofer's cone and the "actuator" is the voice coil and the "magnetic negative spring" could be a second voice coil, perhaps driven by a second amplifier, and perhaps the "position sensor" and its associated circuitry steers which amplifier is active to what extent, to minimize energy wasted in pushing signal through voice coil wire that is outside of the magnetic gap on long excursions. Perhaps the amplifier steering algorithm takes the non-linearity of the air spring inside the sealed enclosure into account. I think this could increase electrical efficiency relative to a conventional overhung voice coil for the same linear X-max, and could mimic the performance of a larger enclosure in some ways.

Or I could be completely wrong... and come to think of it, you'd probably want three voice coils and three amplifiers with my speculated approach, so I'm probably reading things that aren't there into what little description we have.

@Alfredo02, could you post a bit more of the patent application? I think there are clues in items [14] through [19] that you posted, and I'd be interested in seeing more such clues.

I get stuck on how the permanent magnet is attached to the box and affects air pressure somehow but overall just get the waft of bs.

 

[NTK]

I have no idea what the term "magnetic negative spring" means.

I speculate that the "sound panel" is the woofer's cone and the "actuator" is the voice coil and the "magnetic negative spring" could be a second voice coil, perhaps driven by a second amplifier, and perhaps the "position sensor" and its associated circuitry steers which amplifier is active to what extent, to minimize energy wasted in pushing signal through voice coil wire that is outside of the magnetic gap on long excursions. Perhaps the amplifier steering algorithm takes the non-linearity of the air spring inside the sealed enclosure into account. I think this could increase electrical efficiency relative to a conventional overhung voice coil for the same linear X-max, and could mimic the performance of a larger enclosure in some ways.

Or I could be completely wrong... and come to think of it, you'd probably want three voice coils and three amplifiers with my speculated approach, so I'm probably reading things that aren't there into what little description we have.

@Alfredo02, could you post a bit more of the patent application? I think there are clues in items [14] through [19] that you posted, and I'd be interested in seeing more such clues.

Here is the link to the patent application (17794567) with their submitted documents. There is a lot of stuff and some are actually quite interesting. Patent applications aren't particularly fun (or easy) to read, at least for me.

Patent Center

PatentCenter UI

patentcenter.uspto.gov

 

[NTK]

Here is their patent application (63022125) of the magnetic negative spring (MNS).

Patent Center

PatentCenter UI

patentcenter.uspto.gov

 

[Duke]

I get stuck on how the permanent magnet is attached to the box and affects air pressure somehow but overall just get the waft of bs.

I think the permanent magnet is the woofer's magnet. I think it's a woofer with an unusual motor (and in particular an unusual multi-segment voice coil) that arguably has some advantages aimed at getting good bass in a small sealed enclosure. Not that I can say I understand it yet (there are multiple documents at the patent office and I'm not sufficiently motivated to try to figure them all out), but I'm a long ways from calling it "BS".

Edited to add: I think the amount of force being applied to the voice coil actually INCREASES as it moves out of its rest position, and the amount of increase has presumably been calculated to offset the non-linear air spring inside a small sealed enclosure, thereby approximating the cone movement that would be enabled by a considerably larger enclosure. I think there is also steering of the amplifier power being applied to the individual voice coil segments so that power isn't wasted heating up coil segments that are not within the magnetic gap.

 

[kemmler3D]

I think this company seems legit, they also have some interesting-looking electrostatic and graphene-based MEMS-type transducers, so I am inclined to think they also have something worth talking about when it comes to the woofer, but to be honest I can't make heads or tails of how it actually works, so far.

 

[Chrispy]

I think the permanent magnet is the woofer's magnet. I think it's a woofer with an unusual motor (and in particular an unusual multi-element voice coil) that arguably has some advantages aimed at getting good bass in a small sealed enclosure. Not that I can say understand it yet (there are multiple documents at the patent office and I'm not sufficiently motivated to try to figure them all out), but I'm a long ways from calling it "BS".

Thanks but the smell is still there.

 

[kemmler3D]

I tried poking around the patent application to figure out how this is supposed to work. I wasn't successful in a 5 minute perusal... patent applications are sort of the opposite of brief and clear. And I'm not an engineer, so it would need to be both if I was really going to grok it.

However, looking at the drawings supplied with the application, I really can't rule out that there is something interesting here. Lots of variations of voice coils and stuff that look novel to my eye.

They also had actual graphs, which is a good sign. Of course, the mere existence of self-reported data doesn't prove anything, the Daniel Hertz patents also purport to have data, and we all know what that's worth.

What seemed weird was that there was a geared motorized (sub?)system in there for whatever reason. Not something I recall seeing in other speakers.

Someone good at reading patent applications should have a look? Maybe we can get a representative from Brane to chime in?

 

[NTK]

Found this earlier patent by Matsushita/Panasonic (US 6,574,346) on their concept of negtive stiffness (similar to Brane's "magnetic negative spring"), which purpose is to increase the equivalent internal volume of the speaker cabinet.

The Matsushita concept is much simpler to understand. They mount 2 concentric magnets, the outside one is stationary and the other attached to the voicecoil bobbin. The poles of the magnets are aligned and they produce a repelling force against each other. When the cone is centered at its resting position, the repelling force acts perpendicularly to the cone axis and therefore does nothing. As the cone moves, the repelling force pushes the cone to move further away. The force works opposite to the pressure load from the air inside the cabinet, partially cancelling each other and the end effect is similar to that of a larger speaker cabinet.

Since this simpler concept was taken by somebody else, Brane had to either come up with their own alternative concept or license it from Matsushita. No idea how well it works in real life.

https://ppubs.uspto.gov/dirsearch-public/print/downloadPdf/6574346

 

[Alfredo02]

The problem l have understanding this patent it is that in the background {010} explains very clearly the problem of all speakers that need force to move the cone to counteract the existing air pressure in a closed enclosure. But a continuation in the first paragraph, under Summary of the Invention {012} goes ahead and sais "the patent is directed .... in particular drivers having a magnetic negative spring (MNS) such as reluctance assist drivers (RAD) and permanent magnet crown (PMC) drivers) and loudspeakers that have and use same"

Hence, it takes MNSs, RADs, and PMC's concepts as given as well known concepts, as common knowledge to anybody. This is a traditional strategy used by patent lawyers, disguising traditional concepts under new technical terms to induce the evaluators to believe into the novelty of the patent invention.

As some of you mentioned, it would be nice if some from Brane Audio will explain clearly the novelty of the invention, since it should be public since it is under patent application. No secrecy laws nor confidentiality contracts apply here, since are overridden by the patent application rules.

 

[Alfredo02]

Or, couldn't it just be that novel Class D amplifiers provide the power needed to ample counteract the air inside a closed cabinet, at a real low cost, and therefore avoiding the need of a large volume enclosure? Two TPA3255s would provide more than 1200watts, being the main cost the power supply ...

 

[NTK]

Or, couldn't it just be that novel Class D amplifiers provide the power needed to ample counteract the air inside a closed cabinet, at a real low cost, and therefore avoiding the need of a large volume enclosure? Two TPA3255s would provide more than 1200watts, being the main cost the power supply ...

I agree. Hoffman's Iron Laws says: Bass extension, efficiency, small enclosure size — pick 2. If we are not concerned about sacrificing efficiency (i.e. we've got big amplifiers), then we don't have much of a problem.

These inventions are to maximize efficiency, really only important for battery powered applications, such as the Brane portable speaker.

 

[alex-z]

Or, couldn't it just be that novel Class D amplifiers provide the power needed to ample counteract the air inside a closed cabinet, at a real low cost, and therefore avoiding the need of a large volume enclosure? Two TPA3255s would provide more than 1200watts, being the main cost the power supply ...

TPA3255 will only do 600 watts with a 2 Ohm load and 10% distortion. If you look at the datasheet you will see it does 495 watts into a 2 Ohm load, at 1% distortion, if using a 53.5V power supply. Driving a 4 Ohm load that number falls to 315 watts. You lose additional power if using a 48V power supply.

If your budget allows, a Hypex NC502MP amp will deliver more power and much better distortion + noise results.