Differential Drive vs Tetracoil vs Split Winding

Different manufacturers have technologies that utilize two voice coils in two gaps.

JBL has Differential Drive that uses two coils and two gaps in opposite polarity of eachother.

Eighteen Sound has Tetracoil that uses two inside-out voice coils and two gaps in the same polarity I believe.

B&C has drivers with split winding coils (also inside-out I think) and I’m not sure about the gaps.

The other manufacturers might have similar technology.

All of these topologies are designed to flatten the BL curve and allow for more excursion with lower distortion.

Is there any appreciable difference among these multi-coil designs? Is one of them the best? Are they actually better than traditional overhung designs?

I’m looking for a good 15" woofer for a 3-way design and want the best one money can buy in terms of output and compression. Naturally that leads to a search for a driver with the highest BL, sensitivity, power handling, and the lowest distortion. I’m not sure if I should be looking at run-of-the-mill overhung drivers or one of these fancy things.

Here is the one set of measurements for an Eighteen Sound tetracoil driver:


Pretty good for the most part but not the lowest distortion I’ve seen.

Here is a traditional overhung driver from B&C with way lower distortion:


It’s a larger driver, but that shouldn’t matter as much at this level.

you’re really making me work here but yes in fact i do have the answers you’re looking for.

i will get into the technology and its implications later but for now just want to point out that Dickason is an idiot who tests drivers in free air like a moron. he didn’t mount either of the two woofers into an enclosure nor in fact even into a baffle. which means that the 21" driver is essentially using a larger baffle ( itself ) than the 18" driver, and that’s mainly how it is able to get better low end performance …

a more appropriate way to measure distortion IMO would be to build manufacturer recommended box, but of course AINT NOBODY GOT TIME FO DAT !

https://data-bass.com seems to not be working but at least Ricci over there would mount drivers into a sealed box before measuring them. that is at least less lazy than testing subwoofers free air ! ! !

free air the bigger driver will always win.

Tetracoil is very similar to Differential Drive and while both are marketed as distortion lowering technologies it’s really about reducing weight and increasing power handling more than anything.

as i will explain later …

aside from what i already mentioned Split-Coil used by B&C is very different in both its design and mission - it is actually designed to reduce distortion. unfortunately it does so at the expense of reducing output, which is a big no-no in prosound and it’s a mystery to me why they even use this technology.

it’s almost like B&C is not a real prosound driver, but a wannabe prosound driver marketed to DIYers.

JBL of course doesn’t sell their drivers at all, which means Differential Drive is definitely not a DIY oriented tech.

well only Differential Drive and Tetracoil have two gaps. B&C Split Winding uses a single gap.

I actually have a TC Sounds LMS Ultra 18" subwoofer that is far more advanced than what B&C is using. Instead of using two winding zones with a split in the middle TC Sounds used multiple winding zones of various winding density and was optimized for perfectly flat BL using finite element analysis. The guy who designed it ( some Asian kid from Los Angeles, don’t remember his name ) actually taught me how to use finite element analysis to design subwoofer motors - it is pretty cool, but he didn’t show me how to calculate the BL curve though. I think he said he was a computer science major who just somehow ended up designing subwoofers. In any case he was clearly good with software and math.


no Tetracoil is exactly the same as Differential Drive except it has inside/outside VC windings.

it’s just like a regular driver but instead of VC winding being continuous there is a split in the middle. so like instead of having 25mm deep VC winding it will have say 10mm winding then 5mm gap then another 10mm.

here is a pic of SW VC but it’s not from B&C


the effect of this gap is to flatten the peak of the BL curve resulting in lower distortion at the expense of lower output. a good solution for home high end audio but not so great for prosound. this approach was fairly popular in various forms for home high end subwoofers.

here is an Adire Audio motor for example:

it has a split gap underhung design, but both gaps are in the same polarity resulting in extremely linear BL as well as fairly low induction but at the expense of relatively limited power handling and extremely heavy motor - perfect for home high end audio but piss poor for any proaudio application.

and here are TC Sounds LMS Voice Coils:

as you can see the winding has no gap but rather is variable in density - there is more windings on the edges of the VC than in the center …

the B&C motor is just a primitive implementation of what TC Sounds was doing decades ago. Instead of flattening the entire BL curve by fully optimizing the winding they just flatten the tip of BL curve by introducing a small split in the winding.

well yes there were many such innovations in the high-end DIY subwoofer space back in the day, but not in Prosound both for reasons i explained already and also because you actually want a prosound driver to distort audibly and lose motor force when overdriven

by contrast TC Sounds LMS woofers were known to blow up because the motor was too linear - it just kept going and going until the cone broke. i believe some models were actually equipped with titanium cones to prevent cones from breaking but the real problem wasn’t the cone - it was the motor. the motor was like a train with no brakes. this is why Prosound companies were not interested in that tech.

TC and Kyle ( i think that was his name ) tried to branch out into prosound but they had no clue what makes a good prosound driver. I tried to explain to Kyle everything that was wrong with his drivers but he just got upset.

not really. in fact not at all.

the Split Winding is designed to do what you say.

Differential Drive and TetraCoil are designed for pretty much the opposite. They are designed to produce a high BL curve peak that is perfectly centered and symmetrical thus maximizing output without increasing weight of the driver, which is what ProSound woofers should strive to achieve.

Absolutely they work. a JBL differential drive woofer with inside-of-the-voice-coil neodymium magnet and aluminum basket weighs about 1/3 as much as regular driver of similar performance. If you’re going to pole mount a speaker above the crowd this is a HUGE benefit.

JBL actually drop tests their plastic speakers. as you might imagine no Audiophile speaker would survive such a drop test because they all focus on increasing the weight rather than cutting it.

as for Split Winding design yes it will absolutely reduce distortion, but also keep in mind that it isn’t only BL curve linearity that matters but also suspension linearity as well … Prosound drivers have very stiff suspensions ( to increase durability ) and even with a perfectly linear motor there would be significant distortion from the suspension …

in essence there is no point in making the motor more linear than the suspension, and in fact it is more detrimental than anything because you will just end up breaking the suspension. JBL understands that which is why they focus on SYMMETRICAL BL curve rather than flat one. a symmetrical BL curve matches the shape of Suspension compliance curve which means you can push the driver without breaking it - and that’s what you want in a Prosound driver.

what sort of a speaker is it - is it a 2-way, 3-way, a subwoofer ?

because if it’s a subwoofer then you should just go for the cheapest driver and use as many of them as possible.

if it’s for a 3-way then two good woofers will probably be better than one great woofer.

and if it’s for a 2-way and you must stick to a single point source then you should be primarily worried about things other than power and instead focusing on sound …

to put it another way JBL itself uses both regular and Differential Drive woofers as well as both Ferrite and Neodymium versions of both motor types depending on which makes more sense for any given application.

advantages of both neodymium and differential drive are mainly performance to weight ratio, which is absolutely critical whenever you’re either a mobile DJ or a on a Tour or some sort of flying the speakers above the crowd and so on … but is pretty much a non issue for some subwoofer in a basement home theater …

B&C SW is a bit of an odd duck IMO - maybe what in car audio was called SQL meaning a hybrid between SQ ( Sound Quality ) and SPL

for a given magnet material the B&C SW driver will be significantly heavier than either Differential Drive or TetraCoil … on the plus side it will not be as deep, if that matters to you.

i could say a few more words about Inside/Outside VCs, VC diameter and power handling of Push-Pull motors ( like DD and TetraCoil ) but that would be rather tangential to your questions.

i believe i answered your questions, but if i haven’t - let me know and i will try to explain better.

you’re making a mistake i used to make. you’re focusing too much on using the best components.

a piston used in Formula 1 is probably 1,000 times more expensive that one used in a Toyota Corolla, yet if you were to put Formula 1 pistons into a Corolla at the end of the day it would still be a Corolla …

yes a Formula 1 piston can withstand 20,000 RPM - so what ? that won’t magically turn a Corolla into a sports car …

you need to start with a clear objective - what are you trying to accomplish ? are you trying to win an F1 championship ? or are you trying to get to work. are you trying cover a crowd of 100,000 people at Coachella or are you trying to listen to some relaxing music in a NY / SF apartment at night ?

once you decide what you’re trying to accomplish then you come up with a design that can COMFORTABLY accomplish it using INDUSTRY STANDARD level components, not crazy unicorn stuff.

if your design needs “the best” components to work then your design is wrong. unless you’re trying to win a championship you shouldn’t use “the best” of anything. instead your strategy should be to use “good enough” components for everything.

to me Tetracoil would be “the best” but Differential Drive is “good enough” and B&C SW would also be very good but to me is a solution in search of a problem …

yes the SW increases Xmax and reduces distortion … but for what ? it doesn’t make for a better speaker. it’s heavier and more expensive without any extra output and with greater potential for damage from over-excursion.

i would rather use lightweight, efficient and reliable drivers and simply reduce distortion by running multiple woofers in parallel.

of course your design has to be able to accommodate multiple woofers in parallel …

so the real question isn’t what woofer is best - but what is the right design that will give me the performance level i am after without having to resort to exotic drivers.

even the Compression Drivers i mentioned in another thread are frankly exotics nobody should ever have to use - i just brought them up because they are fun to contemplate.

but a good designer should be able to achieve impressive performance using more pedestrian tools.

it is the overall design / strategy that will determine performance level not so much components used.

a 8 foot tall line array will perform like a 8 foot tall line array and a 6" bookshelf will perform like a 6" bookshelf regardless of whether you use $30 components or $300 components.

is a 8 foot tall line array always better than a 6" bookshelf ? no. if you need nearfield monitors then that is what you need. you have to be clear as to what you’re trying to accomplish and what is the best strategy to do it. getting “the best” components should be the last of your concerns.

but of course it happens to the best of us - i understand how easy it is to get carried away hunting for that perfect component - this is how DIY industry makes money. you just need to be aware of this and make a conscious effort to fight this tendency to ask “which component is best” and instead ask yourself “which strategy / approach / design” is right for my goals and what my goals even should be …

when i buy parts i like to reward companies for good behavior. which is to say i like to buy quality parts. but not because i actually think it will make a difference for the final result - but rather if i am to be proud of the final result i will also have to be proud of my component choice as well.

to be proud of my component choice though isn’t the same as using “the best” components.

if your component was well designed and well chosen and works well in the design it doesn’t have to be “the best” for you to be proud of that choice …

and vice versa if your component was the best but you chose it for the wrong reasons and it does nothing for your design - then you won’t be proud of that choice …

the component needs to be THE BEST CHOICE FOR YOUR DESIGN not “the best” as in most advanced or most amazing or highest performance or whatever …

of course some components are just TOO BORING and we ultimately build stuff to have fun … but that again means that maybe you should use components that are “interesting” rather than “best” …

personally i never like to build the same design twice even if worked great the first time. there is simply no fun in that. whenever i build anything i always want to make it as different as possible from both what i have built before and what others are building.

you can just go to DiyAudio and ask them what is the most interesting driver you would like me to use ? there is always some new driver that everybody is curious about but nobody has tried and everybody is looking for information about it - you can use that driver. maybe it won’t be the best but you will make a meaningful contribution to the community by building with it and sharing your experience.

DIY should be about uniqueness and creativity, not just pure performance. Just like collectors cars are about freezing a moment in history, and not winning drag races.

ask yourself - 20 years from now - what driver will be a collector’s item ? or ask the DIY community.

the overall system performance should be determined by the design and DSP used, not by components. the components are there either for you to be proud of ( if you’re an introvert ) or for you to brag about ( if you’re an extrovert ). with good research and communication with the community you should be able to hit both.

you might have to switch communities though. for any design philosophy there is probably a community for it out there somewhere. though i was not able to find many fans of arrays among peasants even as they are pretty much the only option right now among real pros. arrays are too expensive and complex for little guys to get right so little guys tend to poo poo them. but go to any real event like Coachella and it will be all arrays all the time because there isn’t even any other option at that level. but talk to low to mid level sound techs and they will all be like “arrays are bad man” …

The design goal is very straightforward. I want to build a point source speaker with maximum out and minimum distortion and thermal compression from a single driver. Basically the woofer should be able to get as loud as possible without blowing up and it should have minimal distortion at lower playback levels. I don’t want it to use two drivers. Just one really good driver.

This is what Eighteen Sound has to offer with their Tetracoil technology. It’s a ferrite motor because they don’t make a neodymium version with the 4" voice coil:


This is what B&C has to offer with split winding:

Lavoce has these:


From what I can tell the B&C SW is the best driver. It has a very high BL, high sensitivity, the highest power handling, and with the split winding it should have low distortion.

these are all subwoofer drivers.

subwoofers can’t be a point source as they aren’t directional - they pressurize the room.

multiple subwoofers distributed throughout the room are known to produce more even response as standing waves are minimized.

also Eighteen Sound makes Neodymium Tetracoil with 4.5" VC in both 18" and 21" size. of course it is also a subwoofer.

the difference between woofer and subwoofer isn’t the diameter but the tuning. if you look for drivers with most impressive motors you will always end up looking at subwoofers regardless of what diameter they are.

non-subwoofers will be optimized for things other than BL or Xmax.

even though some of these subwoofers you linked can be crossed over directly to a compression driver it wouldn’t be a good design to do so IMO unless there is some specific reason why you can’t use any other kind of design.

subwoofers are optimized for power and strength. regular woofers are optimized for sound quality. even though the two aren’t necessarily mutually exclusive normally you would optimize for one or the other. just like normally a car is optimized either for track use or off-road use. although rally cars exist that are basically optimized for off-road track use that is really not normal. normally you have your SUVs ( subwoofers ) and you have your sports cars ( woofers ) and it’s one or the other, though there are always freak shows like Lamborghini SUV but you get my point …

a typical setup for a mobile DJ would be for example a 2-way using a 12" woofer, plus a 18" subwoofer.

even though 18" is not that much bigger than 12" the two speakers would be quite different. motor, voice coil, cone, cabinet and tuning would all be different. the 18" would hit hard and low while the 12" would provide a balanced performance from about 100 hz to about 2 khz.

the moving mass on the 18" would be perhaps five times as much as on the 12" even though the diameter might not be that different. likewise the volume of the sub enclosure would be perhaps five times larger as well and tuning twice as low.

you probably think this is wasteful but not really because the sub would take most of the abuse at the low end where distortion is barely audible and allow the woofer to operate cleanly where it counts the most - midrange.

this isn’t the only possible configuration obviously, but it’s probably the simplest one that i would consider a no compromise solution for an average size room.

crossover points 100 hz and 2 khz.

the next step up from that would be a 3-way using a compression driver, a 8" midrange and a 15" woofer, plus dual 18" subs ( per side ).

again same logic - the subs take the strain off woofers and the woofers take the strain of the mids.

crossover points 80 hz, 250 hz and 1.5 khz or so.

of course you can still do a regular 2-way, but in that case you must look for woofers that are known to have good sounding midrange rather than ones that have the most powerful bass.

generally if a driver is suitable for 2-way use the prosound manufacturer will indicate this somewhere in the PDF under “applications” …

if the driver has “subwoofer” in the name it probably isn’t really meant for 2-way use.

another tell tale sign a driver can be used in 2-way is it will usually have the letter “M” somewhere in the model name, for example this driver:


it has “MB” in the name probably for “mid bass”

it’s 15" but in the description it states:

“The 15MB1000 is a 100mm (4 in) voice coil 380mm (15 in) diameter mid-low frequency transducer which has been created to meet requirements for low bass applications where a significant extension in mid frequency is needed.”

the Xmax is only 6 mm but that is realistically what you should expect from a driver that can be used in a 2-way.

this woofer should handle about 60 hz to 1.2 khz or so, which would be easy to cross to a compression driver on top and a sub on the bottom.

of course this is just a random example of a 15" woofer suitable for 2-way use. not claiming it is best or even good, but at least it’s designed for 2-way use.

you could also use it as a 2-way without a subwoofer with reduced output of course.

What makes a subwoofer unable to play bass frequencies that a woofer would normally play? The frequency response for these subwoofers is flat and extended well past where they would be crossed to the midrange. It seems like they should have no problem handling everything up to 500Hz or so.

check out this system ( drivers used are in video title ):

B&C built it to demonstrate the abilities of their Coaxial Compression Driver which is similar to the one from BMS that i mentioned before …

They are using a Split Winding driver, namely this one:

in the video ( which includes a sound test ) they praise this ( their own ) design for “not needing a subwoofer” so it’s basically what you’re trying to do …

but keep in mind they are literally using the most expensive horn on the market as well as one of the most expensive compression drivers, which is also a coaxial and has its own passive crossover to be able to pull of this “2-way” ( which is really a 3-way when you consider that the compression driver itself is already a 2-way ).

and the woofer they’re using while i would consider it subwoofer-ish with 12mm xmax it’s slightly less sub bass focused than ones you posted.

so yes what you’re trying to do CAN BE DONE as evidenced by the system in the video …

but the reason that system exists, like i said before, is really to make a point about how amazing the coaxial compression driver and the huge horn ( all made by B&C ) are. it is impressive BECAUSE it is not something you would normally do. you would not normally do a 2-way with a driver that has subwoofer level bass capabilities, but with an ultra high end compression driver and and horn you can pull it off.

is it worth it ?

my guess is no.

i haven’t checked the current prices but when i was checking those components out last time ( before the inflation ) it was something like $700 for the coaxial compression driver and something like $400 for the horn. you could buy an entire 3-way 15" PA speaker system complete with a woofer, plywood cabinet, integrated amplifier and DSP for just the price of this Compression Driver and Horn combo.

in other words yes it can be done but it isn’t super practical.

REMEMBER the video was made by people selling these components. their goal is to get you to spend as much money as possible on their most expensive drivers. your goals seems to be the same - to buy the most amazing-est drivers. well if that’s your goal just do a variation of what they built.

however if your goal is to have a good price performance ratio then the designs i suggested will give you more for less as well as be more practical to set up and move ( the subwoofer acting as a stand for the speaker ).

the PA industry settled on a “top box” over a “subwoofer” setup for a reason - because it works well.

outside of DIY audio and Home High End the goal is to achieve best results with the cheapest components. but in DIY audio it is the opposite - the goal is to spend as much money on components as possible for bragging rights.

because the assumption of DIYers is that quality of components used will translate to system performance. this is a LIE that is taught to DIYers by component manufacturers. in reality it is the system overall design that will determine performance, not the components.

designs i suggested will give you more for less ( we can discuss why if you want ) however the design show in the video ( basically crossing a tweeter to a subwoofer ) can also be done, if you want to spend more and get less.

the woofer these guys used lists 40hz-1,500hz response in the specs. my guess is their actual crossover is about half that at around 700 hz, and it’s more a function of the horn than the compression driver.

there are other drivers ( like 4" RCF ) that can hit 700 hz but there aren’t many horns that can. the horn is the limiting factor how low the compression driver can go here, which is why it’s such an expensive horn - because it opens new possibilities and you will have to pay for that.

so it CAN be done but i would be nervous doing it. those guys had the luxury to try out a few setups to see if they worked or not. you would be taking a bit of a risk - a plunge into the ukknown - unless you copied a design that is known to work, like the one in the video.

keep in mind manufacturer posted speaker measurements are SMOOTHED which means real breakup is much more severe than what you see on those graphs.

however if you have access to 3rd party measurements of some of these drivers ( like the ones you posted from Vance Dickason etc ) then maybe you have enough enough data.

i actually have JBL EON 1500 speakers that take a 15" woofer all the way to 2.7 khz and they work great. the woofer in my JBL EON 1500 is really like a huge midrange though … but of course it had to be taken to 2.7 khz because the Compression Driver is just 1" VC and couldn’t go any lower … with a better / bigger CD you can go MUCH lower on XO and thus you would be able to use a much more subwoofer-y driver for the 15" …

you definitely can do it …

personally i wouldn’t though. the price would be very high, the performance would be only good, and ergonomics would suck as you would have to lift a very heavy speaker to ear level somehow.

IMO better to build a huge, heavy subwoofer and put a lighter “top box” on it, not unlike that JBL EON, but better quality obviously. and by “better quality” top box i don’t mean one that can go lower, because you would be using a 100 hz crossover anyway.

once you remove the need for the woofer to do subwoofer duty you can use shorter xmax which in turn means lighter VC with greater portion of it in the gap, which means lower mass, lower inductance, higher efficiency and so on …

there are a lot of benefits to the driver design when you don’t need to chase subwoofer performance …

division of labor. it is better to have a proper dedicated sub and a proper dedicated mid-woofer than try to do it all with one driver. you can do it, but you will spend more and get less IMO.

personally i would do a 3-way plus Sub. here is an example set of components i would use:

2.5 khz - 20 khz:


500 hz - 2.5 khz:


100 hz - 500 hz:


25 hz - 100 hz:


i would use four subs in total - one in every room corner or so.

there is a strange notion among DIYers that a multi-way system is harder to design. i think it is the exact opposite. with a 3-way plus sub every driver is already optimized for its duty - there is almost no work left for you as a designer. on other hand with a full-range 2-way like what you want it’s on you to find the unicorn drivers and horns that can pull it off and then to make sure that the measurements you’re going off of weren’t fudged by the manufacturer in some way.

for example you may see compression driver measurements showing it can hit 300 hz - yes, but ON WHAT HORN ? the Celestion Axiperiodic can hit 300 hz - good luck finding the horn they used to get that measurements. It exists - but you have to order it from Europe and it must be hand built to your order.

by contrast the 3-way + sub design you will have multiple affordable horn options for the CD. you will be able to easily build a simple waveguide for the 8" midrange yourself. and the four 21" subs will reach way lower than even the highest end 15" you can find.

remember there is no real low limit to audibility. 20 hz is a made up number. we have a real limit to audibility at the top end which is about 20 khz for children, 16 khz for adults and 12 khz for seniors or so. but at the low end there is no real threshold - the louder the bass the lower you can hear it and when you can’t hear it anymore you can still feel it ALL THE WAY DOWN TO ZERO HZ.

so while there is zero benefit to having tweeters that can go past 20 khz the opposite is true for subs. you can never have too much subwoofer capability.