Your idea about the "perfect" pro audio subwoofer driver

[radulescu_paul_mircea]

Good work guys! Attacking the subject from so many angles. 

Regarding a piston in a sealed enclosure. It will never be a good Idea. The efficiency of a piston in an infinite baffle is dependent on the force^2* SD^2/ MMS^2*Re and some constants in the fraction like speed of sound and density. This means that any increase in force squared/mass squared will give an increase in efficiency. But also having bigger radiation surface. Also, there is a relationship between mass and reactance which will change the behavior quite a lot and will make the system non linear. So a hugely powerful piston would be very good only if attached to a huge light weight surface and an amp made for constant current instead of constant voltage to be able to properly work with huge reactance.

But a horn system is a way of making this behavior change. Higher compression ratio with long enough path and big enough mouth will improve efficiency and could make use of a higher force. But at the cost of quality, having higher distortion and nonlinear behavior, like power compression, mechanical noises.

I feel that for pro audio and my own goals , a dual coil dual 21Ipal enclosure, with 4 times the thermal capability and twice the excursion, with an amp capable of delivering enough sustained power for enough time in a 150*100*70 cm enclosure, tuned to do 30-150 hz full and flat, having 3 dB more than Skhorn XL with K20 bridge maximum burst output at 50-100 hz and 6-8 dB more from 50 hz down.

This year I was able to play with Danley BC218 . They were stunning, fully flat and hugely powerful from 26 hz up! More output than my pair of Skhorn XLs, cleaner, deeper, less compressed, huge directivity. They are made with good but normal drivers, 18SW115 so we can see here how the enclosure contributes to the end result as much or more than the driver's capability. But way too big for anything else than a huge stadium or a funny outdoor festival. And if a better driver was being used , the enclosure would be better? If the answer is yes, how much could you increase the drivers performance until you reach the limit of the enclosure?

 

[kipman725]

I actually got as far as phoning up a company that made piezo actuators once when I saw they they were offering actuators that could do mm movements.  My thoughts where that they would be ideal for the distributed motor idea.  As power is not required to hold position against zero presure they could potentially make very large surface area actuators (something like an air motion transformer) and be efficient at bass frequencies.  Unfortunately I found out such actuators where about 8000euro per actuator, which put a stop to the idea of using 100's of them.

It would be interesting to see a loss breakdown of something like an IPAL driver in a sealed box, I presume that most of the losses are 'copper' (yes I know that coil is alu on the B&C) losses in the coil.  The other losses like suspension and air compression/expansion must be small otherwise they would get hot.  This would imply that significant improvements in efficiency without radical change would only be possible via:
1. Lighter moving parts
2. more conductive coils (I come from the wireless power world.... everyone wants more conductive materials, its not going to happen, at least at sub MHz frequencies)
3. Better magnetic materials (sometimes there is slow improvement here, slightly improved neo grades etc...)

I have just thought of one improvement for situations which arn't excursion limited.   Simply double up the motor on both sides of the coil.  Should give twice the power handling, twice the motor force and have slightly less moving mass gain than using 2x the number of drivers.  It should also improve linearity a little, as one motor is going in the other goes out.  Probably would mean the spiders could be less stiff as well as there isn't the pivot point that normally exists.

 

[Kyle]

Those horns look crazy! I respect horns, but at the end of the day, I question the volume SPL trade-off. Not saying they don't help just questioning the amount of gain you get. Josh, what say you?

 

 

 

[SME]

2 hours ago, radulescu_paul_mircea said:

Good work guys! Attacking the subject from so many angles. 

Regarding a piston in a sealed enclosure. It will never be a good Idea. The efficiency of a piston in an infinite baffle is dependent on the force^2* SD^2/ MMS^2*Re and some constants in the fraction like speed of sound and density. This means that any increase in force squared/mass squared will give an increase in efficiency. But also having bigger radiation surface. Also, there is a relationship between mass and reactance which will change the behavior quite a lot and will make the system non linear. So a hugely powerful piston would be very good only if attached to a huge light weight surface and an amp made for constant current instead of constant voltage to be able to properly work with huge reactance.

I beg to differ!  Realize that the I.B. efficiency you describe is only really applicable for HFs, where practically realizable output depends a lot on inductance too.  For sufficiently LFs where an I.B. or sealed system can be approximated as an air pump, Mms becomes unimportant and Cms/Kms plus the air spring become very important because they provide the dominant forces opposing the motor.

What's particularly fascinating is that increasing Sd alone actually harms LF efficiency.  The pressure difference in a particular enclosure is proportional to air displaced, which in turn is proportional to output at a fixed frequency.  However, the motor force required to overcome the air spring from pressure difference (i.e. ignoring suspension Kms) is proportional to the cone area in addition to pressure (motor force = pressure * area).  So a hypothetical cone with very small diameter but enormous excursion can potentially play low in a sealed box much more efficiently than a cone with very large diameter but equal displacement capability.

For an example of a highly efficient small diameter "piston" in a sealed enclosure, consider the rotary woofer which operates using an impeller to drive an air piston.  It would be interesting to see this concept developed and improved further as that might eventually allow for very efficient operation of ULF in relatively small cabinets.

I can also see potential in novel methods of passive radiator design or maybe vented / PR hybrids.  One crazy (not necessarily practical) idea that comes to mind is to use a liquid like mineral oil as a mass element, which might allow lower tunes with much shorter ports while hopefully avoiding the air pipe resonance.  If the problems of vent length and pipe resonance can be gotten around, then drivers with more mass and stronger motors would enable substantial improvements in output density in these types of systems.

[Kyle]

omg i forgot about the fan sub! that thing was a best at 10Hz.

Ya, small motor force or large cone, struggle to counter the air pressure in the box. Bigger motor = better, but requires more voltage to adjust for the back EMF generally seen with huge magnets.

 

 

[kipman725]

In terms of voltage I have used SiC modules rated at 1200V hard switched at 100khz with decent efficiency so it should be posible to make ~1000v rms amplifiers.

[Kyle]

So in terms of this 60 cubic foot Danely sub (a pair at 120 ft cubed)  One could create 30 (~4 cubic foot) with just a simple lab-12c driver tuned to 20Hz (or higher if needed) with an effective sensitivity of nearly ~120dB @ 1 watt for the array. Does that beat these horns?  I think we have a good fight here... The power advantage is squarely on the horn but if you have the power, I think the ported box array is going to win out in a lot of ways, esp. down low.

We need to test this...

 

 

[Ricci]

1 hour ago, Kyle said:

So in terms of this 60 cubic foot Danely sub (a pair at 120 ft cubed)  One could create 30 (~4 cubic foot) with just a simple lab-12c driver tuned to 20Hz (or higher if needed) with an effective sensitivity of nearly ~120dB @ 1 watt for the array. Does that beat these horns?  I think we have a good fight here... The power advantage is squarely on the horn but if you have the power, I think the ported box array is going to win out in a lot of ways, esp. down low.

We need to test this...

 

 

This gets back to the issues with the amount of power available though. The horns might still be more sensitive than a pile of DR's and they will probably be lighter and cheaper to build. 30 lab12's weigh a ton by themselves. Wiring complexity as well. A pile of double 18's would be a good comparison say 8 to 10 of em.

I'm jealous Paul got to hear those Danley's. Those are some outrageously big and powerful subs no doubt about that. More powerful drivers could potentially get even more out of em if they really got pushed hard enough.

[Kyle]

Well, lets ask it this way, will this thing beat 2 Skhorns? Its bigger than 2.

 

[radulescu_paul_mircea]

On 5/27/2019 at 3:36 AM, Kyle said:

Well, lets ask it this way, will this thing beat 2 Skhorns? Its bigger than 2.

 

It does have over 3 dB higher sensitivity at 1.41 V across the band from 28 hz up, 1 vs 1 but it doesn't get a lot more maximum output, at least not between 28-45 hz. The Ipal drivers can take +4dB more power so they compensate.

[kipman725]

BC218 is the first time I have seen a prosound sub sold on how big it is. (to get the directivity)

https://hal.archives-ouvertes.fr/hal-00462380/document

Consider figure 15. of Performances and Design of Ironless Loudspeaker Motor Structures.  Reduce distance e to 0 resulting in maximal magnetic flux but over a very small depth.  However compensate this by repeating the motor structure and using voice coils of depth J.  Put hall effect sensors in the centre of the coils or measure linear displacement of the whole voice coil assembly to work out the current direction in each coil.  Use acceleration feedback to control the coil current to make the whole system linear.  A kind of brush less DC linear motor voice coil that would only be limited by suspension performance.  Also no iron so reduced mass and higher peak flux density possible.

[Ricci]

Good paper Kipman. Hadn't seen that one that I recall. Ironless motors aren't a new idea, but they often turn out to be very expensive and have a lot of stray field. Stray field is wasted field.

There are many good ideas out there for improving moving coil speakers linearity and efficiency. A lot of the time there are easy ways to improve drivers that don't involve exotic new technologies, or materials. Very good, relatively inexpensive devices could be made with standard materials and construction, but usually they aren't or have what I would consider to be an Achilles heel. A big part of it seems to be that driver designers simply do not design woofers with their priorities in the same places that I would. I think a lot of that stems from fixation on certain goals or ideas rather than looking at the whole picture from a birds eye view. Some of it is due to thinking that is to some extent crystalized around old school design philosophy.

In a nutshell what I want to see is a driver with linear xmax well beyond what's needed for the app, which reduces distortion related to suspension and BL variation greatly. This is technically possible quite easily, but it increases Mms and suspension stiffness, which has to be countered with more motor strength. There are limitations on how much power can be absorbed by a driver or put out by an amp, or pulled from a outlet, so efficiency needs to improve too, which requires more powerful motors also. Doing both of those opposing things, cost effectively is the big issue.

Examples of a couple of drivers I would like to see...

A 12" sub with a legit 25mm xmax,  3" voice coil, neo motor, shorting rings, Fs around 25Hz and a Qts in the neighborhood of 0.200. Weight <30lbs. No one makes this type of 12" woofer that I'm aware of. It wouldn't be a cheap $150 woofer but there's no reason it would be an $800 woofer either. The closest thing I've found from any of the major brands is one of the old Peerless XLS 12's and it has half the xmax and not much in the way of power handling. It's also overpriced for what it is. I have designs for these non existent drivers.

As far as a big pro woofer goes...Something like the RF-19 concept is a good start. Basically the in between of it and the 21NTLW5000 would be a good start. For pro audio the RF-19 needs a serious diet. The 1000g mms and super tight suspension squelch the efficiency. We don't need the 34mm coil overhang and 80mm long coil winds. That's twice as much overhang as the highest xmax pro woofers currently on the market. For tunings at 25Hz and above that much excursion just isn't needed. Cut the coil length back to something more appropriate for the app like 60mm long winds which would still leave something like a 24mm overhang with A 12mm gap, which is still quite a bit beyond any of the pro woofers on the market and would remove 40mm of the total coil length and 25% of it's mass. The former would be shortened by 40mm too. The huge, heavy surround could be switched out for a lighter cloth style one. Due to the big reductions in moving mass and less excursion potential the huge 12.5" spiders could be downsized to 10 or 11" spiders with higher compliance and less weight. There are probably some weight savings in the aluminum dust-cap and cone to be had as well. Even the leads are way overkill and probably add a few grams extra. I'm pretty sure you could drop 300 or even 400g off of that driver fairly easily which would do worlds of good for its efficiency. A much lighter and shallower frame and 60mm shorter motor would greatly reduce weight and depth.

[SME]

6 hours ago, Ricci said:

There are many good ideas out there for improving moving coil speakers linearity and efficiency. A lot of the time there are easy ways to improve drivers that don't involve exotic new technologies, or materials. Very good, relatively inexpensive devices could be made with standard materials and construction, but usually they aren't or have what I would consider to be an Achilles heel. A big part of it seems to be that driver designers simply do not design woofers with their priorities in the same places that I would. I think a lot of that stems from fixation on certain goals or ideas rather than looking at the whole picture from a birds eye view. Some of it is due to thinking that is to some extent crystalized around old school design philosophy.

This makes a lot of sense.  Related to this is classification of drivers by application like "home", "car", and "pro" and design toward ideals within those categories.  The trouble is that the distinction is rather artificial.  The best drivers tend to have useful application in all three categories.  Thinking of the RF T3-19, it's a "car audio" driver that you are using for both home (8 sealed cabinets) and pro (2 M.A.U.L.s).  Likewise, many pro-style 21" "super subs" are finding application in vented cabinets for home theater and are probably quite capable in cars too.

6 hours ago, Ricci said:

A 12" sub with a legit 25mm xmax,  3" voice coil, neo motor, shorting rings, Fs around 25Hz and a Qts in the neighborhood of 0.200. Weight <30lbs. No one makes this type of 12" woofer that I'm aware of. It wouldn't be a cheap $150 woofer but there's no reason it would be an $800 woofer either. The closest thing I've found from any of the major brands is one of the old Peerless XLS 12's and it has half the xmax and not much in the way of power handling. It's also overpriced for what it is. I have designs for these non existent drivers.

I noticed in the pro world, no one really makes good subs in 12" size.  At 12", you're basically looking at a mid-woofer/woofer.  You need to go to 15" or (usually) 18" before you get a real pro sub.  And while 12" is a popular size for car/home subs, such offerings typically come with much higher Qts.

6 hours ago, Ricci said:

As far as a big pro woofer goes...Something like the RF-19 concept is a good start. Basically the in between of it and the 21NTLW5000 would be a good start. For pro audio the RF-19 needs a serious diet. The 1000g mms and super tight suspension squelch the efficiency. We don't need the 34mm coil overhang and 80mm long coil winds. That's twice as much overhang as the highest xmax pro woofers currently on the market. For tunings at 25Hz and above that much excursion just isn't needed. Cut the coil length back to something more appropriate for the app like 60mm long winds which would still leave something like a 24mm overhang with A 12mm gap, which is still quite a bit beyond any of the pro woofers on the market and would remove 40mm of the total coil length and 25% of it's mass. The former would be shortened by 40mm too. The huge, heavy surround could be switched out for a lighter cloth style one. Due to the big reductions in moving mass and less excursion potential the huge 12.5" spiders could be downsized to 10 or 11" spiders with higher compliance and less weight. There are probably some weight savings in the aluminum dust-cap and cone to be had as well. Even the leads are way overkill and probably add a few grams extra. I'm pretty sure you could drop 300 or even 400g off of that driver fairly easily which would do worlds of good for its efficiency. A much lighter and shallower frame and 60mm shorter motor would greatly reduce weight and depth.

What you describe sounds a lot like the Funk TSADv2 and UH-21v1.  Specs I have for the latter: Fs 21.6 Hz; Mms 620g; Qts 0.23.  They don't use a cloth surround though.  The obvious downside is they are expensive.

[kipman725]

Alpine do a quite interesting 12" (SWR-12D2)  that claims Qts = 0.44 and a one way xmax of 20 mm:

https://www.alpine.co.uk/fileadmin/user_upload/manuals//car_audio_manuals/SWR_10D2_4_12D2_4/OM_SWR-12D4_EN.pdf

not too expensive but needs a bit more motor to be your dream driver.

 

[Ricci]

37 minutes ago, kipman725 said:

Alpine do a quite interesting 12" (SWR-12D2)  that claims Qts = 0.44 and a one way xmax of 20 mm:

not too expensive but needs a bit more motor to be your dream driver.

 

A bit more motor is an understatement! It's a looooong way from 0.44 down to 0.20. I'd take even lower if I could get it. Alpine makes good woofers and I've used a bunch but like most they haven't discovered the importance of sub efficiency yet. 

The reason I would like a 12" is for a number of reasons. It could be cheaper and easier to make a very efficient woofer with less cone area and use multiples than to do a single super woofer. Plus they can fit in geometries and shapes that the larger drivers can't. Keeping cost down would be the main thing.

[Ricci]

13 hours ago, SME said:

This makes a lot of sense.  Related to this is classification of drivers by application like "home", "car", and "pro" and design toward ideals within those categories.  The trouble is that the distinction is rather artificial.  The best drivers tend to have useful application in all three categories.  Thinking of the RF T3-19, it's a "car audio" driver that you are using for both home (8 sealed cabinets) and pro (2 M.A.U.L.s).  Likewise, many pro-style 21" "super subs" are finding application in vented cabinets for home theater and are probably quite capable in cars too.

I noticed in the pro world, no one really makes good subs in 12" size.  At 12", you're basically looking at a mid-woofer/woofer.  You need to go to 15" or (usually) 18" before you get a real pro sub.  And while 12" is a popular size for car/home subs, such offerings typically come with much higher Qts.

What you describe sounds a lot like the Funk TSADv2 and UH-21v1.  Specs I have for the latter: Fs 21.6 Hz; Mms 620g; Qts 0.23.  They don't use a cloth surround though.  The obvious downside is they are expensive.

Exactly...There's good development and practices in each field but the number of companies putting it all together are limited. Good drivers work regardless.

I like the Funk drivers. Some of them are very good but the prices are so far beyond other competitive products it makes it hard to consider them for systems that may end up requiring 8, 16 or more drivers.

[Kyle]

I'm just so picky about Qes, I wouldn't consider the SWR-12D2 for pretty much anything since I like small boxes. I honestly still like the god damn Lab-12C, but it does fall show in xmax. It would have to have more magnet to maintain its SPL / Qes if xmax was to increase. Eminence, give us a Lab-12 Ultra please! 1.8ww coil, 7.5" magnet We had a prototype TC-9 exactly like that (used a 3" coil of course, not 2.5") and that thing hard with 1000 watts - pro cloth surround. About  1.8" long coil -- not expensive by most accounts.

 

 

 

[Kyle]

I would like to see the MAUL with 4 K20's on it. We would need a 600 volt feed from the power company at least.

 

[kipman725]

16 hours ago, Kyle said:

I would like to see the MAUL with 4 K20's on it. We would need a 600 volt feed from the power company at least.

It would be easier and cheaper to hire a generator if you have a site for testing with road access however you would want something like the X4L instead of the K20 to load the 3 phases equally.  In the UK it would not be too much bother to run 4*K20 off a domestic supply (still involve adding some wiring), houses are typically fused at 60A but its often possible to upgrade to 80 or 100A.  I have run a light industrial building in the UK at its fuse rating (80A 3 phase) for a week and it damaged the fuse holder, I'm not sure if the installation was defective or I was accidentally inducing additional high frequency currents or that this is normal behaviour; however its probably best not to run things at there absolute maximum all the time.   

 

[Ricci]

Might break the cabinet joints on em if pushed too much further. LOTS of pressure developing. 4 K20's is theoretically only 6dB more and that's not factoring in compression of which there surely would be a large amount. So maybe another 3-4dB? Not worth it. If one of the biggest touring amps isn't enough it's time to add more cabinet/subs. 

[SME]

2 hours ago, Ricci said:

Might break the cabinet joints on em if pushed too much further. LOTS of pressure developing. 4 K20's is theoretically only 6dB more and that's not factoring in compression of which there surely would be a large amount. So maybe another 3-4dB? Not worth it. If one of the biggest touring amps isn't enough it's time to add more cabinet/subs. 

I haven't looked much into the mechanical side of things, so I'm curious and rather fascinated that this issue exists.

Are you more concerned about the effects of pressure inside the cabinet or the reactionary forces from the drivers?  I may be missing something, but I doubt the pressure inside the cabinet (alone) is likely to get high enough to threaten anything structurally.  Though I expect that if pressures get high enough, new forms of non-linearity come into play which could increase distortion substantially.

On the other hand, I can see how reactionary forces from the drivers could be a real issue.

[Kyle]

1 hour ago, SME said:

I haven't looked much into the mechanical side of things, so I'm curious and rather fascinated that this issue exists.

Are you more concerned about the effects of pressure inside the cabinet or the reactionary forces from the drivers?  I may be missing something, but I doubt the pressure inside the cabinet (alone) is likely to get high enough to threaten anything structurally.  Though I expect that if pressures get high enough, new forms of non-linearity come into play which could increase distortion substantially.

On the other hand, I can see how reactionary forces from the drivers could be a real issue.

Normally when subs break things is a resonance dance.

 

 

[tktran303]

Hello,

Long time lurker, first time poster.  After many years perusing this website I was pleasantly surprised to see the V2 update.

In it all the systems are placed into tables which can all be organised by Max SPL, sensitivity and cabinet volume etc.

When I arrange all the systems in terms of cubic feet, to me there’s seems a sweet spot, or a few standouts.

Looking around 2 ft.³, the

”MTX15” in small box” is in the lead, with 101.8 dB @ 20Hz

If we scale this up to 16 drivers, we are looking at 34cu ft, and able to hit 125.8 dB.

This is Gjallarhorn and M.A.U.L. territories, give or take a few dB (the advantage goes to the 16 small sealed subs)

Of course 16 subwoofers is going to cost more than 1 large one in a big cabinet, but it’s got other advantages going for it.

Ease of transport, ease of placement,  ease of resale. Perhaps also vertical scalability eg. How many MTX 15” can I buy for one RF TS3 19”?

There’s no free lunch- you need more amps. But what’s the 10+ or 100+ buy price of classD OEM amplifiers modules (400W into 4 ohms). My guess would be under $100 ea.

So is the ideal subwoofer a single large one eg. 18-24” in a very large cabinet eg. 18-36cu ft.

Or multiple smaller ones? Some recent data suggests that multiple small ones help smooth out room modes. These days, some manufacturers are marketing 8” drivers as subwoofers, so surely a 15” driver in a 2.1cu ft cabinet is small enough?! We are bass nuts, it’s probably considered too large for 99% of the general population (or spouses)

But in terms of scalability, it certainly takes the risk/anxiety of the out of the equation of

“Is it low/loud/clean enough?”

 

Well, just build a couple now, and build more later until you’ve quenched your bass desires.

I have owned quad 10” Peerless XLS woofers, and own two TC Sounds LMS-R 15”, but never experience horn loaded subs.

What other factors am I missing? Or should I just go back to letting us dream about our unobtainium 32” subwoofer screwed into the wall of an adjacent bedroom?

[kipman725]

the GAS COMP 12D1 is a 12" with a Qts of 0.2 and presumably high excursion capabilities:

https://www.gascaraudio.com/produkter/comp12d1?lang=en#1462284074811-88c8730f-f38bd1d2-1d988827-f5a0

Also this was brought to my attention:

https://www.ground-zero-audio.com/en/produkt/gznw-38neo-spl/

BL^2/Re = 1043

 

Regarding the MTX:

using 16 of them you would not be able to get enough power from a domestic supply.  I actually do have 16*15" drivers and the next upgrade for my system is to increase the efficiency by moving away from sealed boxes as I can't sensibly increase the amount of amplifier power.  Another issue with running a lot of small boxes is that the cabling gets super annoying to setup and tear down. 

Quote

 

Above 60Hz the compression balloons mightily to a little over 6dB during the 120dB sweep where the power applied has become monumental at around 8Kw into a nominal 8 ohm load. The bridged k10 amplifier was clipping or limiting through parts of the final sweep, so the actual applied voltage may be slightly less than the 256 volts calculated. Normally driver excursion would limit the output sweeps much earlier, but the tiny enclosure volume here limits it greatly 

[Ricci]

On ‎6‎/‎11‎/‎2019 at 6:08 AM, kipman725 said:

the GAS COMP 12D1 is a 12" with a Qts of 0.2 and presumably high excursion capabilities:

https://www.gascaraudio.com/produkter/comp12d1?lang=en#1462284074811-88c8730f-f38bd1d2-1d988827-f5a0

Also this was brought to my attention:

https://www.ground-zero-audio.com/en/produkt/gznw-38neo-spl/

BL^2/Re = 1043

 

Regarding the MTX:

using 16 of them you would not be able to get enough power from a domestic supply.  I actually do have 16*15" drivers and the next upgrade for my system is to increase the efficiency by moving away from sealed boxes as I can't sensibly increase the amount of amplifier power.  Another issue with running a lot of small boxes is that the cabling gets super annoying to setup and tear down. 

Exactly...A ton of small boxes sounds like a great idea until you start to consider the amplifier requirements to get their maximum potential. Also wiring and setup is a PITA. Don't forget cost and weight also. Each of those tiny sealed cabs with the MTX driver is about 75lbs or 34kg...multiplied 16x!

The GAS comp drivers could be good, or not. I couldn't find an owners manual or complete set of specs. If they do not include shorting rings the inductance will be incredibly high with long 8 layer coils and a big steel pole piece. It looks good on paper but if the inductance is out of control it will exhibit high levels of distortion and a very peaked response shape. There could be shorting rings in the motor but there is no mention of them. I don't even consider long throw subs without shorting rings. The same thing goes for the GZ 15. The main issue I always have with car audio drivers such as this is lack of good information. Also that 12" GAS comp driver probably weighs around 65lbs or more judging by the motor. I couldn't find the weight either. That's a lot of weight if you start using multiple drivers.