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Your idea about the "perfect" pro audio subwoofer driver

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I will state my idea of this "perfect" pro audio driver for the most powerful subwoofer one could make in a given volume.

The enclosure volume is maximum 1000 liters, weight is 175 kg max, frequency response should be -6dB at 25hz and +3 dB at 120Hz from the 40-50 hz output.

This driver might already exists, but it is not readily available, it is the 24" driver from the new Funktion one F124. It is dual 8 ohms 6" coils, BL is 50 Tm but that is the only thing I really know about it, I didn't even get the chance to hear it.

But now, going back to my idea of it, I know that any driver will in fact be just the engine part of a sub and it has to be matched to the enclosure for really good performance. Also , some of the qualities I will state about that driver might be negated or might be improved by the specifics of the enclosure design, but I will still play this game.

The best driver IMO is always big. This means heavy most of the time.

A 21-24 inch driver, with dual 8" diameter differential drive 1 ohm coils each. Length of each should be 70 mm , copper clad alluminum or just Al, flat wound ribbon wire, with inside and outside turns. The coil former made from a better conductive material than fiberglass and also a better thermal inertia, I am still searching for one with similar hardness/density with those properties.

magnetic gap should be 30 mm, but with the proper extension on the pole pieces to decrease magnetic losses and to keep the magnetic lines perpendicular over the coils for the whole length. overhang should be 30 mm a side plus 10 mm the other, giving it a perfectly flat BL curve for the first 10-12 mm for maximum impulse response accuracy on the higher end of the spectrum, but this will also give a lot of excursion capability, good low end response and also cooling.

This means it will need a smart magnetic circuit design, and the way the guys at Digital Design are doing it looks like a winning strategy IMO but it could be improved. Using radial magnets on the inside and outside of the coils, fixed in place by iron bars in anodised alluminum casings on the faces opposed to the coil (inside and outside of the motor toroid with the coils in the center), but with iron pole pieces shaped for conducting all the field perpendicular to the coils,  The bar design with the visible coils are making the cooling easier if smart port positioning is used, and from the inside, there are a number of ways one could use the diaphragm displacement to cool the coils. 

From my calculations, about 6-8 kg of neo and  12-16 kg of iron and 2.5 kg of anodised Al would be needed to make it close to the M-Force motor strength but with way lower Le/Re. It will have much lower thermal inertia and it will still be harder to cool down, but the lower moving mass would make it more efficient.

Diaphragm made from a laminated carbon fiber/aramid honeycomb/ carbon fiber sandwich, low mass , huge strength, W shape with the dust dome and cone in a single piece.  The centering of the driver in the magnetic gap could be made with a spider separation ring that could be screwed into the chassis directly with all the parts already glued, like A&D Audio from China is already doing. Spider materials and shape should be similar to that of the new NTLW5000 drivers but probably at 12-15" diameter for enough excursion, dual layer separated by a ring with enough distance to ensure the very long coil stability.

The total weight will be somewhere between 30-35 kg if the whole chassis is made of aluminum, encasing the whole motor like Faital pro are doing with their drivers.

I would make a single driver enclosure with this one and it would finally satisfy my stupid wishes.

With smart cooling it could take 2.5 kw of real power indefinitely with 6dB pink noise in band inside the enclosure with all setting needed. 

It might still not be exactly where M-FORCE+M-DRIVE are, by using a 30" diaphragm, but not too far either.

What's your opinion?

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Gonna scribble down a few notes, I like your effort here and enthusiasm. Here is what I can comment on really quick...

I think the first thing here is to throw size out the window and just set up ratio's that make sense for scale. You can always scale up a driver or use 2 smaller ones that are equivalent to a single larger one. In that sense size does not matter too much. For practical purposes (lifting up the subwoofer) a 21 seems to be about the limit (give or take). At the end of a day, if you make a 24" can it really beat two top tier 18's? I think its important to keep that in mind because otherwise, why scale vertical when you scale horizontal -- to use a software term. 

6-8kg ... my god! Are you making a subwoofer or a Electric car motor?

You reference Digital Designs? Are you talking about the car audio company or the Differential Drive motor?

With respect to JBL:DD, it does have high distortion when being over driven due to braking -- not sure its win win. I tend to like BL curves that gradually fall off and a suspension that can stand abuse. It will still distortion but might be less harsh. I'm also an advocate for linear BL if you can get it.

I agree whole that pro drivers suck in terms of xmax. I think Josh's new additions this week show the market is moving in this direction but not fast enough imo.  

honeycomb can be a problem when it breaks, close cell foam is a great core material,.

When you make the former into a conductive material you allow current to be introduced there when pushing it through the gap. When that becomes too high relative to the coil itself it causes distortion. This is why most pro drivers are  fiberglass, titanium or stainless steel. Note: Carbon fiber is conductive, bad choice here again I think. Car audio drivers which tend to less concern themselves with distortion will go with AL for cooling but they also have thicker coils to offset that distortion (but of course, creating more inductance in the motor itself and hence creating other THD) 

I'll comment more later...

 

 

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Wow!  I'm glad someone replied because this very interesting but is definitely not an area I know much about.  :)  My inclination is rather to think about the system as a whole, and I don't expect there to be any single "perfect" pro audio sub drivers.

8 hours ago, Kyle said:

Gonna scribble down a few notes, I like your effort here and enthusiasm. Here is what I can comment on really quick...I think the first thing here is to throw size out the window and just set up ratio's that make sense for scale. You can always scale up a driver or use 2 smaller ones that are equivalent to a single larger one. In that sense size does not matter too much. For practical purposes (lifting up the subwoofer) a 21 seems to be about the limit (give or take). At the end of a day, if you make a 24" can it really beat two top tier 18's? I think its important to keep that in mind because otherwise, why scale vertical when you scale horizontal -- to use a software term.

I think the 24"s I know of do beat a pair of 18s, in their own class at least.  Compare Stereo Integrity's 24s with its 18s.  Or look at Funk Audio.  I guess I haven't seen verification, but the specs on Funk's 24" look outrageous.  Despite this, I think larger drivers have a lot of disadvantages compared to smaller ones.  The reason they "win" for big subs is often simply a matter of economy.  Chances are a single 24" will cost less to manufacture than two 18s even with all the expensive upgrades.

At the same time, "other" concerns often become important.  In my own room I couldn't fit any cabinet taller than about 23" in my room, so a 24" driver was out.  The 21" size hit the spot in that regard.  Design flexibility/convenience, perhaps more than other factors, may be why super subs won't get much larger than 21".  At some point, the cabinets they require just get too big for anything other than permanent installation.

On a related note, I am designing a one-off "demo speaker" / semi-PA for myself and will probably opt for 2 x 15" B&C 15NDL76 instead of 1 x 21" B&C 21DS115 to drive a 32 Hz tuned vented cabinet.  Without question, the 21 smokes the 2 x 15s for deep bass   However, given that I have 700 W to throw at the sub(s) in each speaker and the fact that the speakers will still need big boy sub (Othorn/Skhorn class) augmentation in an outdoor setting, it makes more sense to go for the crazy mid-bass efficiency.  And I do expect I'll use it indoors a lot, often with half the ports stuffed to push the tune down to ~25 Hz, when I know I have plenty of spare headroom.  I'm sure my situation is pretty unique though.

8 hours ago, Kyle said:

When you make the former into a conductive material you allow current to be introduced there when pushing it through the gap.

Is this true for all conductive materials?  Or only for those that interact magnetically, which does I think cover a lot of the common conductors.  I don't think copper is very magnetic, but I don't think I've ever heard of a copper former.

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A single DSS Mariana 24S sub with 4,000w beats a pair of Mariana 18S subs with 8,000w though you’d have to have measuring equipment to notice the additional 1-2 Hz deeper (at reference the 24 is flat to 6/7 Hz in the dozen or so rooms I measured) or the 1-2 dB louder it plays at some frequencies.  Enclosure wise the Mariana 24 is 8+ cu.ft. and each Mariana 18S is 3+ cu.ft.  Cost wise, a pair of Mariana 18S subs cost a little more with the SpeakerPower plate amps being the largest cost.  

While a single Mariana 24 plays a little deeper and louder, I usually try to push prospective customers to a pair of 18s as they’ll get more even bass in a room, which I know isn’t necessarily a top priority on the pro audio side.

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@dgage. I wonder if you put the 18" in a 4 cubic foot box if it would make up a little bit of that low end SPL? Also makes it more apples to apples.

I want to see the DDS 24S vs the FA 24.0 :)

 

 

 

 

 

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I see your point that our 18” is in a slightly smaller enclosure than optimal but so is the 24.  So I’d say it is an apples to apples comparison but it might be nice to see an oranges vs oranges comparison but that’s up to Stereo Integrity and Josh to address. :)

 

I’m game for a review.  Tried to get Josh to review last year but he was too busy and I haven’t bothered him this year.

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Josh is uber busy! But I think doing some head to heads (maybe even 3 or 4 subs at a time) would really be neat. 

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Its fun to design drivers, I like to start with a frame, work from there. This is an 18 w/ 11.5" spider to account for a oversize size VC like 5 or 6". Soldiworks!

 

18_c4.png

18_c4b.png

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