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

[GGroch]

Another difference between Brane's Repel Attraction Driver and conventional drivers that I missed is that the magnetic polarity is reversed. That is, in conventional drivers the voice coil is repelled by the permanent magnet (s) so that the further the excursion the greater the distance/smaller the magnetic force. When no music is playing the coil naturally centers.

Brane's RAD seems to reverse this so that the coil is attracted to (pulled by) 2 permanent magnets, so the further the excusion, the closer to the magnets, the stronger the force.
Brane says this requires electromagnetic "Lorentz Force" to keep the coil centered between the magnets when no music is playing. See attached from their techno press sheet.

 

[Wolf]

I don't know how the flipped magnet polarity would be more or differently plausible since these audio devices are all AC devices. You will get the push and pull regardless. Flipping the magnet just makes the direction change.

I think the inner magnets are attracted to the gaps, and then switched off or on to combat te forces.

 

[GGroch]

I don't know how the flipped magnet polarity would be more or differently plausible since these audio devices are all AC devices. You will get the push and pull regardless.

You are right; I am flummoxed.

 

[Duke]

I'm game if you are. Be gentle.

I'm interested too! I must confess I looked at this and couldn't make sense of it. I thought it sounded a lot like a passive radiator but then what do I know.

The following is my understanding of the Brane “Repel Attract Driver” with “Magnetic Negative Spring”. I do not claim to have a solid technical grasp of the details of the design, but will attempt a ballpark description.

Let's suppose we want to manufacture a portable battery-powered wireless speaker that has good bass. We need to keep the efficiency fairly high for the sake of battery life. To illustrate the tradeoffs involved, let's say we build a prototype that's 86 dB efficient and goes down to 80 Hz in a 5 liter sealed box. Not bad, but not enough bass. Suppose we want the bass to extend one octave deeper (down to 40 Hz). We can either increase the box size to 40 liters (an eight-fold increase), or we can reduce the efficiency down to 77 dB (shortening battery life or requiring much heavier batteries). Or some combination thereof. My point is this: The penalty in box size and/or efficiency for extending the bass significantly deeper is huge. Realistically, we'd probably design for a peak in the bass region and hand it off to our marketing department to make the best of it.

The culprit is the air trapped inside the sealed box. It acts as a spring, resisting the motion of the woofer cone. The smaller the box, the stiffer the spring. And it's a non-linear spring: The futher inwards the woofer cone moves, the harder the air inside the box pushes on it. And the further outwards the woofer cone moves, the harder the now-partial-vacuum inside the box pulls on it. To make matters a little bit worse, in general the farther the voice coil moves from its rest position when doing long excursions, the weaker the force that the motor generates.

Wouldn't it be nice if we could make the force that the woofer's motor generates actually INCREASE as the voice coil moves farther from its rest position? Wouldn't it be even nicer if this increase in force exactly offsets the air-spring behavior of the enclosure? IF we could do this, then the internal air volume of the enclosure would no longer be the limiting factor that it once was! There would still be some practical limitations, but it might actually be feasible to get something in the ballpark of 40 Hz from our little 5 liter sealed box without having to massively trade off efficiency to get it.

This is what the Brane woofer is intended to accomplish (though I have no idea whether the arbitrary numbers I've been using correspond with Brane's targets). Their voice coil has small magnets attached which pull it AWAY from its center position once the voice coil starts moving. The farther the voice coil is displaced from its neutral rest position, the stronger the additional pull. In fact, the amount of this pull is calculated to offset the air-spring effect of the air inside the box. So in effect we have a magnetic “spring” that behaves inversely to the air-spring of the enclosure... a “magnetic negative spring”.

(My understanding is that much of the invention is devoted to properly modulating the force that the small magnets apply to correctly offset the air-spring, and to keeping the voice coil properly centered while subject to these multiple magnetic forces. I am under the impression that a position-sensing device and multiple electromagnets are involved. I'm not going to explore those details here, as I don't fully understand them, but my assumption is that the devils are mostly in those details.)

In my opinion this seems to be an extraordinary development in subwoofer driver design, at least for its particular size-and-weight-constrained application. I'm not in a position to evaluate the claims of their marketing department, but the concept and the technology make sense to me to the extent that I understand them.

 

[GGroch]

Duke, excellent, that sounds right or close to it.

Most of the professional reviews do not attempt to explain how Brane X works. A TechHive article does get into it:
"Squeezing an 8-inch driver, plus a high-wattage amp (necessary to motivate a cramped driver), in a small enclosure is normally asking for trouble. It would create excess internal cabinet pressure and heat that could blow that speaker up, as Brane’s CEO Joe Pinkerton explained.
To sidestep those spoilers, the Brane X’s subwoofer relies on a radically designed voice coil that puts the moves on the backside of Brane’s semi-rigid carbon-fiber speaker membrane. The R.A.D.’s copper winding core is half the normal size and is supplanted with four tiny magnets affixed to a moving bearing. The designers call it a magnetic negative spring. Said bearing maintains precise centering (and sonic refinement) behind the speaker membrane with help from a sophisticated position-tracking/shifting mechanism (you can kinda see it hiding under the white cap in my rear-view image).

In the spirit of noise-cancelling headphone tech, which utilizes countervailing forces to make unwanted stuff disappear, the R.A.D. tech interacts with the core’s stationary magnet to effectively cancel compression force (aka back pressure). In the process, it keeps the speaker running cool–and so efficiently that it performs with just one-tenth the power demanded by a same size old-school woofer, according to Brane.

The tweaking starts the moment you turn on the Brane box and hear a little pumping noise. That’s the onboard air pump equalizing the internal pressure and placing the subwoofer’s driving mechanism in its optimal operating position relative to the membrane at the front. Lots more electro-magnetic magic is happening here; but frankly, that stuff is beyond my pay grade to explain." So, an air pump is involved as well to equalize the presure within and without the cabinet.

Photos attached.

 

[Duke]

Duke, excellent, that sounds right or close to it.

Thank you, if you see room for correction or improvement please don't hold back!

The tweaking starts the moment you turn on the Brane box and hear a little pumping noise. That’s the onboard air pump equalizing the internal pressure and placing the subwoofer’s driving mechanism in its optimal operating position relative to the membrane at the front. Lots more electro-magnetic magic is happening here; but frankly, that stuff is beyond my pay grade to explain." So, an air pump is involved as well to equalize the presure within and without the cabinet.

Very interesting! I was wondering whether changes in atmospheric pressure would have enough of an effect on the natural rest position of the voice coil to matter, and apparently the answer is 'yes'.

Seems to me one of the potential issues from a manufacturing standpoint is the reliability/longevity of all these many pieces of the puzzle, BUT the payoff for success (having a highly desirable and truly unique combination of attributes) is arguably huge.

Must admit I'm very glad to not be competing head-to-head with Brane... well, at least not until they move into home audio.

 

[hex168]

I think, back in the day, Cerwin-Vega addressed the cabinet back pressure problem by filling the cabinet with a more compressible gas (nitrous oxide, perhaps?) inside a plastic bag. I have no idea how long the gas stayed in the bag, but it might have had more longevity than the Brane design. Which, I have to say, is really clever.

 

[Duke]

I think, back in the day, Cerwin-Vega addressed the cabinet back pressure problem by filling the cabinet with a more compressible gas (nitrous oxide, perhaps?) inside a plastic bag. I have no idea how long the gas stayed in the bag, but it might have had more longevity than the Brane design. Which, I have to say, is really clever.

I remember sausage-like extra-compressible bags being offered DIYers back in the late 80's, does that sound like what Cerwin-Vega was using? Anthony Gallo used a material inside his enclosures that had similar properties, I think he has (or had) a patent on it. I don't think these approaches offer as much potential as the Brane woofer.

TC Sounds (sadly no longer in business) made subwoofer drivers which had tighter voice coil windings at the ends of the voice coil than in the middle, but I think they were just trying to equalize BL across the linear excursion range, rather than actually increase BL to compensate for the air spring in an unusually small enclosure. They were superb woofers imo, but very expensive so I only used them in a few custom builds.

 

[Alfredo02]

Now it make sense to me the use of the external air pump, listening to the reviews:

- When the speaker is unpowered, the driver has a rear offset due to the magnetic neutral suspension (MNS) forces of the R.A.D. system. The internal air pump generates pressure to center the driver from its parked position, overcoming these forces2

-Additionally, the air pump operates when powering the speaker off, ensuring that the driver returns to its parked position safely2.

-This mechanism allows the Brane X to mimic an infinite baffle in a compact enclosure, enabling it to produce deep bass notes equivalent to much larger speakers3

Therefore the air pump is essential for the speaker to achieve its high-efficiency and low-distortion performance. I had failed to learn about it's existence and iis significance. Now is clear. And in no reviews there is any complaint about any hissing, hence the pump has to be highly silent

Alfredo

 

[hex168]

I remember sausage-like extra-compressible bags being offered DIYers back in the late 80's, does that sound like what Cerwin-Vega was using? Anthony Gallo used a material inside his enclosures that had similar properties, I think he has (or had) a patent on it. I don't think these approaches offer as much potential as the Brane woofer.

TC Sounds (sadly no longer in business) made subwoofer drivers which had tighter voice coil windings at the ends of the voice coil than in the middle, but I think they were just trying to equalize BL across the linear excursion range, rather than actually increase BL to compensate for the air spring in an unusually small enclosure. They were superb woofers imo, but very expensive so I only used them in a few custom builds.

Yes, that sounds like what CV was using, although I never opened a cabinet to look at one. I missed that they were ever available to the DIY market. I always wondered about winding a voice coil to improve BL linearity but never thought of this application at all.

 

[hex168]

Now it make sense to me the use of the external air pump, listening to the reviews:

- When the speaker is unpowered, the driver has a rear offset due to the magnetic neutral suspension (MNS) forces of the R.A.D. system. The internal air pump generates pressure to center the driver from its parked position, overcoming these forces2

-Additionally, the air pump operates when powering the speaker off, ensuring that the driver returns to its parked position safely2.

-This mechanism allows the Brane X to mimic an infinite baffle in a compact enclosure, enabling it to produce deep bass notes equivalent to much larger speakers3

Therefore the air pump is essential for the speaker to achieve its high-efficiency and low-distortion performance. I had failed to learn about it's existence and iis significance. Now is clear. And in no reviews there is any complaint about any hissing, hence the pump has to be highly silent

Alfredo

Didn't someone make the equivalent of an acoustic transistor by using an electromechanical valve to modulate the flow from a compressor tank? Now, if one had a silent pump...

 

[Alfredo02]

Trying to understand the overall situation I have just found that two years ago SONOS has paid 100m$ in cash to acquire Mayth technologies. Surprisingly enough is a new way to move two cones, displacing them in opposite direction, a somewhat similar principle whale been talking here, but with a different technological way to implement the solution, they call it "heartmotion".

And of course, claiming the speakers have been reborn and we are facing the aurora of new age of speaker making... And all that kind of marketing speak.. Reality is that they paid 100 million cash! One dollar after the other.

What they say is that Heartmotion is a completely reimagined design for speaker drivers. Up until now almost all speaker drivers have followed the same cone-like shape with a single flexible membrane, which is effective, but has its limitations. Heartmotion features two membranes on opposite sides and motors that move both of them simultaneously.

Their claims are very similar to others we have heard recently: Lower cabinet volume (up to 5 times smaller cases) and being self balancing, eliminating each cone the vibrations of the other avoiding shaking. By the way, they have thee air pumps to counter pressure...

Enclosed are the patents they bought.

Alfredo

Ps: My believe is that last developments of class D amplifiers, with tremendous amount of power with very high efficiency in limited size space, has opened the eyes to numerous engineers, recognizing efficiency it is not any longer a limitation in Hoffman's law, bringing new kind of speaker designs unthinkable some years ago.

 

[Alfredo02]

If. You allow me, I will create a different thread with the previous post since the theme has evolved and now it doesn't have too much in common with the title of this thread. It is not so much about DIY but about the interest we may have in a couple of new speaker technologies and their potential in the.market.

 

[ryanmh1]

Ps: My believe is that last developments of class D amplifiers, with tremendous amount of power with very high efficiency in limited size space, has opened the eyes to numerous engineers, recognizing efficiency it is not any longer a limitation in Hoffman's law, bringing new kind of speaker designs unthinkable some years ago.

True to a degree, but this has nothing to do with amplifier power (which I don't think you were saying anyway, to be clear). Their goal is to not use power, and to make something as cheaply as possible, given that this is a portable speaker. Essentially what they do is make the speaker believe that it is in a much larger enclosure than it actually is. The low end extension of the Brane X is quite good for the size of the enclosure. Rtings has measured it. It should be impossible to do this in a tiny, "cheap" package. Yet, they did it. Are they also throwing some power at it? Possibly, but it still has to run on batteries.... That's the real innovation here: Efficiency.

I ran a few quick models. If the driver were able to act "ideally" and behave as if it were in free air, for a given input power they pick up about 6dB (probably driver dependant) at 30Hz and 10dB at 20Hz, once the box size shrinks to .25cf. Granted, this is driver dependent, but that's probably close to the right ballpark. Note that the maximum possible SPL does not increase. You can still get that output from the same tiny enclosure by hammering the speaker with more power (and probably put more voicecoil and magnet and money into the speaker than they have to use). By half a cubic foot, this concept ceases to make sense. The same can be achieved with passive radiators. Granted, expensive ones with an expensive driver that no manufacturer will ever put in a portable speaker, but hey... it's possible. But that does give you LOTS more bass. About 6dB at 30Hz if power is no object. That's where the Class D amp comes in. The actual acoustic efficiency advantage of the radiator is about zero over this solution. Note that I suspect there is TONS of DSP involved. The response curve is anything but flat. The response still drops like a brick after 80Hz. Just not nearly as heavy a brick as a radiator, port, or other tiny sealed enclosure. Also note that the described passive radiator solution with heaps of power is little more than a Carver Truesub.

What Brane has actually is new, to a significant degree. The basic idea isn't that new, but that someone was actually able to build it and make it work is new, along with (I assume) some of the innovations that made this work in production. Is it that big a deal? Absolutely. Does it have any applications outside of tiny little speakers? Not really, since you give up the "free" output of a port or a radiator. But it's hugely neat for big bass in tiny little packages.

 

[AcousticTheory]

A magnetically bistable speaker would be a distortion machine. Just build a system with as much moving mass as you can load onto a driver with the softest suspension that will acceptably support that mass, until the driver sensitivity goes as low as you can tolerate, then put it in a small sealed box and see how far Fs comes back up, and make up the difference between that system response curve and where you want to be using EQ.

 

[ryanmh1]

A magnetically bistable speaker would be a distortion machine. Just build a system with as much moving mass as you can load onto a driver with the softest suspension that will acceptably support that mass, until the driver sensitivity goes as low as you can tolerate, then put it in a small sealed box and see how far Fs comes back up, and make up the difference between that system response curve and where you want to be using EQ.

You're not thinking like someone who has to make a $600 speaker that can play full range and hit 25Hz, weigh enough to be portable, and not use outrageous amounts of power. The whole mess has to weight 8 pounds and be .25 cubic feet total, including room for electronics and the other drivers. Could you absolutely destroy this thing with 20 pounds of driver and 500 watts of power? Probably. That's not what they did. Now apply the same space, weight, and power constraints and beat it. You can't, or some other BT speaker manufacturer would have.