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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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I'll add some comments after I get done working on adding another driver to the site. 

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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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Dgage,

 It probably doesn't make you feel any better but it is not just you. There are a number of companies I've declined sending me products. It's a lack of time / will on my part. I just can't keep up with the demand. I've got things in my garage that should've been done years ago and there are other guys like you that have been trying to get me to test their products for years. After we finally got the new website going this year I got more inquiries about testing immediately. I'm going to try and work through all of my current backlog this summer. Once I clean the slate off you'll be one of the first I contact.

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Back to driver design. This is one of my favorite topics. 

I agree with Kyle in a lot of aspects. Pro woofers have the engineering, the quality control, good inductance management and efficiency , but still lack a bit in the xmax department. 15mm is plenty when the cut-off is 30Hz, but it does limit the low frequency output capability with deeper tunings. On the other end of that more Xmax costs money and increases weight. Diminishing returns kick in quick. Application matters too. A 40mm Xmax driver is probably not the best choice if the application is a 35Hz tuned bass bin for pro-audio use and the amplification will be about 1000w per driver.

If the design work is right multiple smaller drivers can often work as well if not better than a single big driver. Multiple smaller coils to share the heat for one and they may have a larger selection of standard /more cost effective components to choose from. It's also easier to make a higher damped more efficient driver with less cone area and more modest components. It's way cheaper & easier to make a 10" driver with good inductance and a 0.250 Qts than it is to make a 21". On the other hand small drivers can run into real limitations for excursion and voice coil size. 10" and sometimes even 12" drivers simply don't have the room available for equivalent voice coils and suspension components, or even the motor diameter that larger sizes can use.

I've thought about designing some subs and having some drivers built. I can't find any 10 or 12" drivers that are like what I'd want to see on the market for example.

As far as future materials I'm wondering how long before we start seeing graphene incorporated into speaker components. I'd expect to see it in domes and cones soon if it isn't already out there.

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Just now, Ricci said:

Back to driver design. This is one of my favorite topics. 

I agree with Kyle in a lot of aspects. Pro woofers have the engineering, the quality control, good inductance management and efficiency , but still lack a bit in the xmax department. 15mm is plenty when the cut-off is 30Hz, but it does limit the low frequency output capability with deeper tunings. On the other end of that more Xmax costs money and increases weight. Diminishing returns kick in quick. Application matters too. A 40mm Xmax driver is probably not the best choice if the application is a 35Hz tuned bass bin for pro-audio use and the amplification will be about 1000w per driver.

If the design work is right multiple smaller drivers can often work as well if not better than a single big driver. Multiple smaller coils to share the heat for one and they may have a larger selection of standard /more cost effective components to choose from. It's also easier to make a higher damped more efficient driver with less cone area and more modest components. It's way cheaper & easier to make a 10" driver with good inductance and a 0.250 Qts than it is to make a 21". On the other hand small drivers can run into real limitations for excursion and voice coil size. 10" and sometimes even 12" drivers simply don't have the room available for equivalent voice coils and suspension components, or even the motor diameter that larger sizes can use.

I've thought about designing some subs and having some drivers built. I can't find any 10 or 12" drivers that are like what I'd want to see on the market for example. We all like to speculate about ultimate drivers using all the cutting edge tech, but I see a lot of improvement that could be made in lower cost drivers through a few simple changes in most cases.

As far as future materials I'm wondering how long before we start seeing graphene incorporated into speaker components. I'd expect to see it in domes and cones soon if it isn't already out there.

 

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They make some noice look products :)

 

This horn probably sounds really good :)

2.jpg

 

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Looks like this Ora company is doing a lot of work with graphene for cones and domes. I think we are far more likely to see small multimedia transducers using this before we start seeing 18" graphene composite cones. The name of the game with this stuff is a very thin layer to beef up a thicker carrier material or perhaps a multi layer foam core sandwich.

I do own a fishing pole with some graphene in the blank.

Ora.png

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I would guess that graphene will be of greater benefit for high frequency drivers.  We can hope that graphene based composites will allow for large format compression drivers that don't break up until well above 20 kHz.  For subwoofer drivers, break-up is not usually a problem.  The issue is trade-off between loss through the cone material and unwanted mass, but sub drivers also usually have heavy coils making cone mass relatively less important.  I guess it may still make a marginal difference in "no expense spared" designs.

===

On the subject of drivers for pro use and for deep bass (tunes less than ~35-40 Hz), I have to wonder:  Is there a point of major diminishing returns, where better drivers don't help much more?  Certainly for use in sealed boxes in home theaters, more linear excursion is always welcome if accompanied by sufficient motor force and power handling capability, but for pro use, larger resonant cabinets are almost always a must.  Output within these cabinets is often constrained by other factors like port flow area, unless the cabinet is enormous.

Part of this discussion involves the somewhat philosophical question of how to define "useful headroom".  In a vented cabinet, substantial compression is likely to set in well before audible distortion/chuffing sets in, yet this compression is not necessarily innocuous.  It alters the "large signal level" frequency response.  So supposing the subs were setup using "-20 dBFS" (relative to amp clipping) test signals before the show, but then the subs get run closer to "-6 dBFS" where the amps and drivers might still have "headroom" but the ports may already be down 1 dB+  relative to the mid bass.  It's probably similar to (but not the same as) the frequency balance changes that occur with small vs. large signal effects in drivers with poor inductance control.  It's likely to have a substantial effect on perceived transient response.

In my "semi-PA" design I mentioned in an earlier post, the port velocity hits 25-30 m/s @ 53V where the pair of 15" 15NDL76 drivers (as simulated) are just reaching their Xmax/Xvar.  With a 21" 21DS115, I have almost twice as much Xmax and could conceivably put twice the voltage to it, but even at 25-30 m/s, the port is probably compressing a lot.  I doubt I'd see any of that "+6 dB" in deep bass output without making the port area and cabinet twice as big, which is not acceptable for my intended use.  (I want to be able to move it by myself.)

I imagine the situation is similar with the Skhorn.  A pair of IPAL-21s can easily overload the ports, so what is a more capable driver going to offer without making the cabinet even larger?  I imagine anything larger than a Skhorn is going to be a much bigger pain to transport and setup.  Even the IPAL-21 (vs. one of the much cheaper cousins) may be "overkill" in the Skhorn for a lot of people in that it may not be contributing much additional "useful" output at all.

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SME, most subs have undersized ports, the box becomes so large (as you know) to get a proper port, most have to by DIY and very large.  What we need is to fill the room with argon gas, then the tuning would be awesome!

 

 

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Good points made SME.

Vented subs are tough. They are arguably the most common type of sub because they can be reasonably compact, easy to construct and design, can have good response and offer large output advantages around the vent tuning. With a vented design we usually want to maximize vent area. This involves either a higher tuning, larger cabinet, or longer vents with worse pipe resonances. The pipe resonances are a major issue IMHO as is trying to stuff extremely long vents into a cab with limited dimensions. Back in the day with lower capability drivers this wasn't as much of an issue but the physics of port operation hasn't changed while the drivers and amps keep getting more powerful. Basically modern designs are pushing the ports further into compression than ever before. At the same time the demand is for everything to get smaller. Designing a high output vented sub is an exercise in knowing there are major compromises with the vent and making peace with it.

Compression and noise doesn't just affect bass reflex subs either. Horns and TL variants are not immune if they have small areas in the path length. It doesn't matter what type of alignment it is if there is a lot of air movement being forced through a relatively small area. Even if it is buried in the cabinet so that you don't hear chuffing it can still compress and shift the output. Akabak and ABEC3 are capable of looking at the air speeds at various points in the system. It’s a good exercise for those who think that because it isn’t technically a “port” or bass reflex it isn’t a consideration. In general horns and TL's do fair better due to much larger areas though. These types of designs can also have ringing and group delay issues just as bad or worse than vented designs.

There is a trend in the vented sub measurements that shows that compression sets in earlier than many think. It gets progressively worse from there. Another takeaway is that they do not seem to brick wall absolutely but the compression keeps growing. Also the driver starts to unload as the vent compresses and the driver will start to exhibit higher excursion near tuning which somewhat offsets the vent compression. So if the driver is already able to overload the port badly what good is an even more powerful driver, more excursion, more power handling, more amp power? In theory more maximum output potential, less compression, less response shifting, lower distortion. In effect the envelope of acceptably linear operation is pushed a little higher. This has measurable effects even when the system is not being pushed beyond where the lesser driver is comfortable. At some point it is debatable whether these improvements are audible or worth it. Despite the fact that the vent might severely limit the extra performance near the tuning there should still be some performance gains even if they are miniscule at the vent tune itself. There is a lot going on above vent tuning which includes the area of maximum driver excursion and most of the audible bass bandwidth. The improvement may not be as large as what would be seen in a sealed system but it can still show an improvement overall. You may see little improvement near vent tune though if the vent cannot cope with the demands.

The Skhorn has what I would consider as enough vent area at the 30Hz tuning to keep compression and noise minimal. Enough meaning barely adequate LOL. Blocking vents for a 25Hz tuning isn't too bad of an impact but it does worsen the vent compression and noise a bit. Blocking for the 17Hz tune puts it into a state of being under-ported like most subs. Vents can work great but keeping airspeed and compression low is always difficult. Almost every sub tuned below 25Hz is under-ported. All of the home audio style subs are. Some of them woefully so once you look at the specifics of the design. The rare exceptions are some of the higher tuned pro subs and car audio subs but it's a lot easier to get massive area when the tuning is >30Hz and the cab is big. Using drivers with higher capability and lower tunings only makes it worse if you want to use the extra capability. The MAUL has a 730cm 12” pipe that’s 41” long for a port and HR sims a max airspeed of 120m/s when bursted with a bridged K20. It should be at about 30m/s with 70 volts. There really wasn’t a “win” to be had there. I wanted a 10Hz tune but the vent area would be severely compromised to an extent I was uncomfortable with. A 41” vent length is already far longer than I wanted it to be and the vent resonance is lower and more severe than I wanted. About 15Hz was as high as I wanted the tune to be to make it worthwhile. So at the end of the day I am not happy with the vent area, length, tune, or resonance. I still love those subs though and vent compression is the last thing I notice when I hear them. Maybe that’s because other systems are even worse or maybe it’s because a 12” pipe will handle quite a bit before it gets out of sorts.

With the Skram most of the extra size over half of a Skhorn is extra vent area and a little bit more air volume. When designing a vented sub you'll often see that with a smaller vent it leaves more air space in the cab which helps with a lower tuning or more vent area and also helps boost the efficiency at the vent tuning. A lot of BR subs are designed that way. I go the other way and go for the largest vents I can get away with in the cab that will still hit the tuning required. This will not result in the most sensitive or efficient small or medium signal response but it gains it back and then some at high output. The big tradeoff is longer vent lengths and the associated pipe resonance.

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On ‎5‎/‎18‎/‎2019 at 2:14 PM, Kyle said:

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

Nice!

Here's something I was fooling around with back in 2013/2014.

ZOD NOVH-4 2.jpg

ZOD NOVH-4 1.jpg

ZOD NOVH-4.jpg

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Puck neo, very efficient. I like the heatsink above the t-toke there. I don't have a lot of puck magnetic simulations but the thing I noticed a lot of those was the behavior of the flux under the magnet. I noticed some of it shorted to the other side of the gap causing a reverse B field. I'm not sure how that plays out in real life but clearly there are plenty of examples.  The shorting rings are nice on that motor for sure, and I see that's a pretty tall gap, will be very efficient.

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10 hours ago, Ricci said:

Good points made SME.

Thanks for your elaboration.   I generally agree, and I'd like to add a bit more.

There is the issue of particular application, which concerns home vs. pro sound as well as content and desired extension.  I suspect under-porting works quite well for home theater subs because: (1) most of the heavy content is well above the tuning frequency, especially with music; (2) for home theater the big hits at or around tune are likely to involve brief effects that are probably heavily processed and "artificial" sounding anyway.  Will vent overload problems be noticed under such circumstances?  Either way, an under-ported low tune sub is a pretty good compromise if the sound is reasonably accurate 99% of the time.

In contrast consider electronic music with heavy deep bass reproduced in a large scale pro environment.  Here, the tune will almost certainly be high enough to substantially interact with the music.  The music itself may demand high level continuous output and may be of a character that makes problems at certain frequencies stand out a lot more than they would with typical movie content.  While a good HT system is essentially running with "small "signals 99% of the time, the pro system is likely to be running with "large" signals most of the time.  Or there may be alternating periods of "large" and "small" signals.  These are much more challenging conditions.

I suspect with the M.A.U.L. the tune is low enough that it doesn't interfere with content much.  This is a situation where under-porting may be entirely OK, but there's no free lunch here because with the lower tune, the vent contributes a lot less output at higher frequencies than it would.  This is not really a "fault" of the M.A.U.L. though with its 4 x 19" RF TS3 drivers.  With the Skhorn, the design is a good compromise, which I imagine works well with "most" content.  OTOH, If one wishes to flawlessly play electronic music with high level continuous tones centered at 30 Hz, then "extra" cabinets  might be called for.  Orbone can build twice as many Skrams instead!

11 hours ago, Ricci said:

So if the driver is already able to overload the port badly what good is an even more powerful driver, more excursion, more power handling, more amp power? In theory more maximum output potential, less compression, less response shifting, lower distortion. In effect the envelope of acceptably linear operation is pushed a little higher. This has measurable effects even when the system is not being pushed beyond where the lesser driver is comfortable. At some point it is debatable whether these improvements are audible or worth it. Despite the fact that the vent might severely limit the extra performance near the tuning there should still be some performance gains even if they are miniscule at the vent tune itself. There is a lot going on above vent tuning which includes the area of maximum driver excursion and most of the audible bass bandwidth. The improvement may not be as large as what would be seen in a sealed system but it can still show an improvement overall. You may see little improvement near vent tune though if the vent cannot cope with the demands.

As you point out, more excursion, power, and power handling, are pluses, *all else the same*.  But in reality, greater excursion and/or power handling almost always come with a price, both literally and in terms of design compromise.  More excursion and power handling almost always add mass, which impacts high frequency efficiency.  And of course, spending a lot of extra money to expand the linear envelope by say 1 dB may or may not be worth it if that same money could instead be put into building more cabs.  :)

It's interesting that the "horn-loading" in the hybrid BP6 designs essentially compensates for the loss in mid bass efficiency that comes with the heavier, high displacement drivers.  In some ways, the hBP6 designs bring out the best in these drivers.

With that perspective in mind, I expect that the "perfect" driver would enable a design that offers higher output density and/or lower extension than is available now.  What that might look like, I cannot say, which makes it hard to figure out what capabilities the driver should have.  And indeed, the "perfect" driver may not be a driver at all.  Getting down to 30-40 Hz doesn't seem too hard, so some device that can take over and make lots of sound below 30-40 Hz might just be "perfect".

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I think the ultimate driver would be designed for a sealed box avoiding these port compression issues.  To generated good SPL it would need a large surface area and excursion capability.  To do this we would have to move beyond single voice coil motors and suspensions systems.  I have had several ideas which have also been in part explored by others.

Ideas for increased excursion:

1. No front surround instead the driver is a piston inside a tube with a piston ring style seal. 

2. Motor is linear motor BLDC type using closed loop position control.  I have attached a paper about a motor designed for vibration damping train carriages, perhaps a smaller version could work.  Linear bearings replace the spider.

3.  In the most advanced case no bearings would be needed as a magnetic bearing would be used.

Ideas for increased surface area:

1. Distributed motor drivers, instead of pushing the cone in one place push it in many places allowing a thinner larger flat diaphragm.  This way one face of the box could be almost all driver.

If we look at something like the Maul it can do 134.6 dB @ 20Hz/1m and has a frontal area equivalent to a 40" driver to equal this with a sealed box where the whole of the front was a driver would require an xmax of 93 mm.  However the moving mass of such a box would necessitate dual opposed operation in which case this would be halved. 

applsci-07-00152.pdf

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19 hours ago, Kyle said:

Puck neo, very efficient. I like the heatsink above the t-toke there. I don't have a lot of puck magnetic simulations but the thing I noticed a lot of those was the behavior of the flux under the magnet. I noticed some of it shorted to the other side of the gap causing a reverse B field. I'm not sure how that plays out in real life but clearly there are plenty of examples.  The shorting rings are nice on that motor for sure, and I see that's a pretty tall gap, will be very efficient.

It seemed ok in FEM but I did notice that this type of neo design does start to generate those stray fields below the gap. I assume they are not of sufficient strength to cause issue. 7 component motor though. Machining of the top aluminum cap would've been expensive and the quote I got on the T-pole / backplate and outer case were $$$!

Looked good on paper but I worried about alignment issues, motor assembly and cost. How to assemble with charged neo is a whole other deal too.

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6 hours ago, kipman725 said:

I think the ultimate driver would be designed for a sealed box avoiding these port compression issues.  To generated good SPL it would need a large surface area and excursion capability.  To do this we would have to move beyond single voice coil motors and suspensions systems.  I have had several ideas which have also been in part explored by others.

Ideas for increased excursion:

1. No front surround instead the driver is a piston inside a tube with a piston ring style seal. 

2. Motor is linear motor BLDC type using closed loop position control.  I have attached a paper about a motor designed for vibration damping train carriages, perhaps a smaller version could work.  Linear bearings replace the spider.

3.  In the most advanced case no bearings would be needed as a magnetic bearing would be used.

Ideas for increased surface area:

1. Distributed motor drivers, instead of pushing the cone in one place push it in many places allowing a thinner larger flat diaphragm.  This way one face of the box could be almost all driver.

If we look at something like the Maul it can do 134.6 dB @ 20Hz/1m and has a frontal area equivalent to a 40" driver to equal this with a sealed box where the whole of the front was a driver would require an xmax of 93 mm.  However the moving mass of such a box would necessitate dual opposed operation in which case this would be halved. 

applsci-07-00152.pdf

2 problems with that approach as well.

Power handling. There's a practical limit to how much power you can supply to a single cab and how much power a single coil can absorb.

Efficiency. There is room to make transducers more efficient but even if we get the reference efficiency way up we still run into practical limitations on the amount of output that can be achieved from the amplifier limitations that will exist.

The MAUL is a pretty good study of this even though it isn't sealed. It was amp limited with a whole K20 bridged into it at 4 ohms nominal. As a practical matter supplying more power to the cab isn't an option. Even using the 40" M-force unit, which is the most efficient driver I'm aware of and assuming unlimited xmax and power handling, which it most definitely is not, cannot get there in a sealed design the same size as the MAUL. There isn't enough power available even with a large cab and incredible efficiency.

This type of avenue should be explored further and pursued but there will always be room for vented and other bandwidth limited cab designs due to the need for more output over a limited range with less power.

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