Bill W.

So dude, just chill out and sleep it off. You don't have to be there until 2:00pm, so if you can't make it there by then, I'm gonna razz you for a year.

Aside from thumb rule formula linking SPL to horn bandwidth, I have not seen much in print. Most of the experience I have had with this issue was in trying to "fix" it in our levitation sources, ultimately, there is no fix, one simply had to reduce the distance (in wavelengths) traveled at high SPL's.
I guess one could say for what we were doing, the answer really was to have a converging sound field that only reached maximum SPL where it was focussed.
It was pretty cool though, at the center of focus, the sources could light a cigarette with acoustic friction (at about 165-170 dB)

Flux rings had been / are pretty much associated with "more expensive" drivers and the advent of accurate magnetic computer models with experience has made many ceramic structures that were the equal of what one could get with the traditional materials (alnico).
On the other hand, as long as the lines of flux are not "pinned" down totally , the magnetizing force in the Voice Coil modulates the set point of the magnetic circuit.
The extent it is modulated also is the amount that the magnetic circuits' non-linearity in its BH curve alters the relationship between the two.

Interestingly, when I went to work at Intersonics in 1979, the sound source they were using at the time was called a St Clair (developed by the department of mines I think).
It was a bar of aluminum, held at the exact center by a thin flange and driven by a shorted turn.. In this driver, the driven coil is wound around the center pole and the bar is set in motion by the "shorted turn" at one end (turned out of the same bar).
I had made several tweeters (a small aluminum dome) with this approach, using the edges of the dome as the S.T. and coil wound on the center pole.
These worked fairly well and I thought there was a possibility of making a compression driver this way.
We applied for a patent on it but after a while there was a budget cutback and the application was abandoned.
Some years later, Tannoy was able to get a patent on it although configured as a "coax" driver.

Anyway, back when the LAB sub got started, a shorted turn was not something Eminance did normally. They were well aware of it though, at the Bass list BBQ I held in 1996 where I first met both Jerry and Nick McKinney, we did sit around and talk about it and the St Clair source (as Nick was building speakers with them). Sometime later, Jerry went to work for Eminance.

A few years ago, when I asked about making the LAB 12, an ST was still not something that they did normally and as I was asking for a pretty strong motor, there was also a question about how long would it take to develop / cost.

Also, when I started that project, I really had no idea that people would be using LAB's in homes so getting the last nth of distortion out wasn't a concern. I just wanted to beat the highly (marketed) thought of bass horns in Pro-sound with something a DIY'r could build.
Since there was no way to brainwash people into thinking anything (like the big companies do) , it had to do this on its demonstrable performance.
At least so far, nothing they have tested out performs it.
I figured this could only happen by following the computer design and not "horn lore" which said one wanted a low mass driver etc.
One wants the right driver for the job not a thumb rule here.

It sounded enough different than what most were used to be an occasional issue with some at first.
Even the fellow on your forum thought it was "un-musical" and wanted it to sound more like a vented box (funny ).
Reducing the distortion further is not going to make it sound MORE like what he is used to.

We use a variation on the LAB 12 in several products also and when we introduced the Bdeap a year ago, immediately about half were going into big home theaters.
Because of the high sensitivity and high power capacity, in the home, this kind of horn is just idling away in the home most of the time. For example in my living room, at the listening position (about 15 feet away) the SPL is about 100dB for a 1 Volt input (2 Bdeaps in a corner). Here (at the listening position), things buzzing and rattling in the room etc are the primary sources of distortion, not the speaker.
On the other hand, many EQ the system to a far lower cutoff than normal and this really drives the system into potential non-linearity.
This is the region I hope may be improved by the ST.

I should have a pair of our drivers with the ST next week and I will take some measurements and keep you informed.
This is exactly like the driver you are talking about too, a LAB12 with shorted turn (in our case the driver had slightly stronger motor).
The Turn (the iron it displaced) was accommodated like you mention by closing the vent dia somewhat so this is a change that needs to be examined carefully.
Well, I have to run, a busy day and the kids are home on break.
Cheers,

Tom

I understand the woofer used in the BDEAP horn is slightly different than the LAB12, so perhaps it doesn't make sense to combine the flux stabilized versions into one project. But they are similar enough the basic woofer structure will be the same, and the efforts to develop them are the same. So in a sense, you and I are cooperating on this one.

I do want to address the comments about 20Hz EQ though. Not to pick a fight, but to clarify. You yourself describe the potential of pushing a horn-loaded LAB12 into non-linearity by EQ'ing the bottom end. That's exactly the point I've made in the past, and what I hope to address with the flux stabilized LAB12.

Since the woofers in a horn act as direct radiators below horn cutoff, it's really the woofer alone that sets performance limits at those low frequencies. Certainly EQ is an option to bring up the bottom octave, where the horn becomes ineffective. Or if 140dB isn't required and 120dB will do, a smaller direct-radiating package is a viable option and EQ isn't necessary. In either case, having a high-quality subwoofer with a shorting ring will really shine in this application.

Horns are great, but if you're looking for operation down to 20Hz, a subwoofer such as this can be used in other smaller enclosure types as well. For maximum power, a horn is the best option. It offers about 15dB greater output above cutoff, which is significant. But the smaller package of using the subwoofer as a direct radiator is a viable alternative, and performance is quite good, particularly at the lowest frequencies.

> This effect is also seen in a sonic boom, here, on the leading
> edges, the air is compressed over 1atm at supersonic flight. Here,
> because of the same temperature/speed relationship, what finally
> reaches the ground as radiated sound is also a saw tooth (on the
> front edge), over all, the waveshape of a sonic boom is like a
> capitol letter N.

> Throat distortion is pretty much a non-issue in bass horns though,
> cone and coil drivers generally cannot produce enough acoustic
> pressure to cause this.

When a direct radiator, I agree that a loudspeaker of this world cannot increase the general air pressure of the entire atmosphere very much (which I'm pretty sure step response shows, showing sound pressure when voltage step function is applied).

In a basshorn though (or any horn), cant you think of the piston being a moving 'pump', forcing air through the throat? A smaller throat would be creating more compression at the peaks or crests on a sine waveform, and so theres more distortion if air pressure is increased to much due to a small throat (caused by high SPL). On the other hand, a larger throat would create less compression at both low and high SPLs because of less restriction, so less changing in the temperature/speed relationship (causing volume changes to be unequal when pressure is changed), so less changing of waveshape. I dont know if a basshorn has the ability to increase air pressure much past 1 atmosphere, I guess it must compress to some degree though. I think I'm getting mixed up with midrange and HF horn throat distortion though... does one need more acoustic pressure to compress the peaks of a very long low frequency wavelength? Why is this? Is it because, for example a 600Hz wave vs 20Hz wave. The peak area/region on the 600Hz wave is a lot smaller than the peak region on a 20Hz wave, which would be very wide. So at the same SPL level, the compression of air atmosphere in the 20Hz wave is more 'spread out' than on the 600Hz wave, so the speed/temp relationship is not changed as much on the low frequency wave, so distortion doesn't occur until you feed the 20Hz wave a lot more power to get a lot more acoustic power to compress the air to the same level? But on the 600Hz wave, compression is now restricted to a smaller area, so it air pressure is compressed by a greater amount when played at the same SPL level as the 20Hz wave.

I would think then, looking at just direct radiators now, if we had a magical direct radiator which could produce HEAPS of acoustic pressure/SPL, and ignoring distortion caused by the driver itself, the mere unequal compression and rarefaction of air pressure will cause distortion of the sine waveshape all by or in itself. Getting back to the horn now... I'm trying to understand how the throat size distorts the waveshape. I know that it does, but I've managed to confuse myself. Is it because, the throat size itself has the ability to change air pressure? It is the only thing I can come up with that seems to make sense. Amplitude of the pressure wave increases at the throat where air pressure is maximum? Then air pressure decreases as you move away from the throat and down the horn? So the a smaller throat increases air pressure over a larger one, causing more throat distortion/modification of the sine waveshape to a saw tooth wave.

But on a basshorn, weather there is a high compression ratio or a low compression ratio, because of the very large wavelengths of low frequencies, you still need a ton of acoustic output for the crest region of the sine waveform to become compressed enough to distort the waveshape?

Since the amplitude/SPL of the fundamental decreases with distance, then compression of air at further distances is less than compression of the air close up. One would then consider that this waveshape distortion is not linear with distance, even if we disregard the fact the horn throat changes air pressure itself. EG: Just consider a sine wave travelling outside, tying back with my previous note/example about the 'magical' direct radiator.

Maybe I've got this all wrong... you've really got me thinking about this, I'm trying to convince myself to believe that throat distortion in a basshorn is not an issue because it is not there!

I've heard of some people breaking their woofer cones from high compression ratio's even if distortion is not an issue, its one reason to keep the compression ratio low. What comp ratio did you use on the labhorn? Have you ever damaged a woofer before because of high comp ratio at high power?

> This distortion is the sum of the VC motor and mechanical system's
> non linearity.
> The "bad" effects of the latter can be minimized by choosing a
> driver with a low Fs which then requires a small Vb to end with
> the right compliance.
> Depending on the sealed volume more means that a more linear
> spring is dominating over the drivers relatively non-linear
> suspension (by comparison) spring.
> Generally, stay away from High Fs driver for bass horns if low
> distortion is a goal.

I remember you telling me before, the idea is to get the reactances to offset each other, leaving the resistive components to interact with each other. What I am saying, is how can you say a larger Vb dictates that the suspension of the driver is not as linear as a driver requiring only a smaller Vb?

So basically, the idea is to get a load as stiff as the throat, but behind the driver instead using a rear chamber - to get even loading on each side, so its more linear on each side, resistive only. Did you mean to say the compliance of the rear chamber Vb should be matched to that of the throat instead and not the driver suspension? As a matter of interest, the rear Vb which cancels or balances out the capacitive reactance from the mass reactance of the throat, may not be the volume which provides the best/most even frequency response, even though it may be the most linear for the driver/lowest distortion combination...

I am pretty sure that in smaller box volumes, the air inside it is stiffer (inductive reactance?) which may be needed to balance out the capacitive reactance from the throat. Just 'how stiff' do you need it though? That is hard to know, unless there are formula's around, which there probably are. I think though that to generalize to say that you must have a small rear volume to make the system linear is a bit of a stretch. It could even be made too small...which would not be good, because then the reactances wouldn't balance. The 'right' rear volume may even be relatively large by comparison...

How come a driver with lower Fs will end up with smaller rear volume? I would think driver Qts would play the biggest part here...

By the way, would you mind checking my horn length in my first post? It is in the 3rd diagram down. I have the horn length measured right down the dead middle of the horn... is this how you would normally measure it? Have you got any comments on horn/path length? http://www.audioroundtable.com/HighEfficiencySpeakers/messages/481.html

BTW: It's almost 2am here in the morning now, don't be too harsh on me if this post sounds like baloney :p

Cheers
Adrian

>> This effect is also seen in a sonic boom, here, on the leading
>> edges, the air is compressed over 1atm at supersonic flight. Here,
>> because of the same temperature/speed relationship, what finally
> >reaches the ground as radiated sound is also a saw tooth (on the
> >front edge), over all, the waveshape of a sonic boom is like a
> >capitol letter N.

>>Throat distortion is pretty much a non-issue in bass horns though,
>>one and coil drivers generally cannot produce enough acoustic
>>pressure to cause this.

>When a direct radiator, I agree that a loudspeaker of this world cannot increase the general air pressure of >the entire atmosphere very much (which I'm pretty sure step response shows, showing sound pressure .when voltage step function is applied).

>In a basshorn though (or any horn), cant you think of the piston being a moving 'pump', forcing air >through the throat?

Yes, unlike a sealed tire pump etc however, on the throat side of the radiator, the "pressure" is not proportional to displacement but rather the velocity. This results in the curious situation where if one looked at the points of minimum and maximum acoustic pressure, one finds that instant in time is where the radiator displacement is zero (but has greatest velocity).

>A smaller throat would be creating more compression at the peaks or crests on a sine >waveform, and so >theres more distortion if air pressure is increased to much due to a small throat (caused >by high SPL). On >the other hand, a larger throat would create less compression at both low and high SPLs >because of less >restriction, so less changing in the temperature/speed relationship (causing volume >changes to be >unequal when pressure is changed), so less changing of waveshape. I dont know if a >basshorn has the ability to increase air pressure much past 1 atmosphere, I guess it must compress to some >degree though. I think I'm getting mixed up with midrange and HF horn throat distortion though...

What your saying is exactly true, a compression ratio increases the pressure in the throat and in any given circumstance that automatically increases the non-linearity of the air.

What makes this "more complicated" is that at the pressures a VC driver can produce in a Bass horn, the non-linearity of the air is often FAR smaller than the typical driver motor and suspension linearity.
Remember, sound at least the way we deal with it is logarithmic.
For example, if one were able to fully modulate one atmosphere, the resulting pressure is 194 dB or so.
This is loud, the sound has a pressure of 14.7 psi peak to peak.
At 174 dB, the physical pressure is 1.47 psi and in a really high powered bass horn like a BT-7 or maybe a LAB Sub, the peak throat SPL should be in the neighborhood of 157 dB or .147 psi peak to peak.
By the time one gets down to just plain "loud bass" one finds that 132 dB is about 4 pounds per square foot of pressure.
I guess the point is that in a really powerfull bass horn, one might be lucky to find a peak throat pressure of 1/100 atm peak to peak. This is not enough to be a real problem and was what convinced me to pursue the more linear unconventional styles of drivers for bass horns.

. does >one need more acoustic pressure to compress the peaks of a very long low frequency wavelength?

Nope, pressure is pressure.

>Why is >this? Is it because, for example a 600Hz wave vs 20Hz wave. The peak area/region on the 600Hz >wave is >a lot smaller than the peak region on a 20Hz wave, which would be very wide. So at the same >SPL level, >the compression of air atmosphere in the 20Hz wave is more 'spread out' than on the 600Hz >wave, so the >speed/temp relationship is not changed as much on the low frequency wave, so distortion >doesn't occur >until you feed the 20Hz wave a lot more power to get a lot more acoustic power to >compress the air to the >same level? But on the 600Hz wave, compression is now restricted to a smaller >area, so it air pressure is >compressed by a greater amount when played at the same SPL level as the 20Hz >wave.

For a simple case horn, one finds (for flat power response) the driver displacement increases by 2 for each octave one goes down, for a direct radiator in a sealed box, the displacement increases by a factor of 4 for each octave (until past Fb where the displacement is constant with decreasing F).
Throat pressure is constant (ideal horn) and output pressure (power) are constant.
A real horn (which is always smaller) usually has more "issues" making some of this hard to see.
Further complicating this is the fact that the actual throat distortion is much more of an issue at the top end of the response , not at all the bottom.
Here is why (or at least what I see / think).
The thumb rule for throat distortion shows the band width being a very strong factor in the result.
The reason is that the wider the bandwidth, the slower the expansion (dictated by the low cutoff) is compared to the much shorter wavelengths of the high cutoff.
This means the HF signal has to travel a greater distance (in wavelengths) at the higher pressures associated with the slower expansion.
With our levitator sources (narrow beam, 21KHz @ 160+ dB), we saw this real time, one could take the mic and starting at the source and moving away say a foot (about 20 wavelengths), one saw the waveshape go from a sine to a sawtooth.

>I would think then, looking at just direct radiators now, if we had a magical direct radiator which could >produce HEAPS of acoustic pressure/SPL, and ignoring distortion caused by the driver itself, the mere >unequal compression and rarefaction of air pressure will cause distortion of the sine waveshape all by or >in itself.

Yep.

>Getting back to the horn now... I'm trying to understand how the throat size distorts the >waveshape. I >know that it does, but I've managed to confuse myself. Is it because, the throat size itself has >the ability >to change air pressure? It is the only thing I can come up with that seems to make sense. >
>Amplitude of >the pressure wave increases at the throat where air pressure is maximum?

Yes

Then air pressure >decreases as you move away from the throat and down the horn? So the a smaller throat increases air >pressure over a larger one, causing more throat distortion/modification of the sine waveshape to a saw >tooth wave.

Yes

But on a basshorn, weather there is a high compression ratio or a low compression ratio, because of the very large wavelengths of low frequencies, you still need a ton of acoustic output for the crest region of the sine waveform to become compressed enough to distort the waveshape?

Yes, another way to look at it is "power density" HF horns cover both a wide bw and are physically very small. For a compression driver to radiate say 1 acoustic Watt, that power (pressure) is concentrated into a very small area.
For a bass horn radiating 1 acoustic Watt, the throat is tens to hundreds of times larger and so the power density is proportionally less. Add in the narrower BW and one finds the air non-linearity is not a big deal in bass horns.

>Maybe I've got this all wrong... you've really got me thinking about this, I'm trying to convince myself >to believe that throat distortion in a basshorn is not an issue because it is not there!

I can't tell you what to think but I would say as evidence of what I have described is that if air non-linearity were a problem, it would have been when the measurements for the BT-7's were taken.
In that case, they were radiating about 180 acoustic Watts each.

>I've heard of some people breaking their woofer cones from high compression ratio's even if distortion is >not an issue, its one reason to keep the compression ratio low. What comp ratio did you use on the >labhorn? Have you ever damaged a woofer before because of high comp ratio at high power?

You bet, this was a real problem in the early days of the Servodrives, we installed a window in the side of a box and we watched (in some what horror) as we swept the oscillator up and down at high power and watched the cones slowly fold up like some paper flower.
We came up with a treatment we dip them in which stopped cone failures and makes them strong enough to stand on.
The BT-7 has a radiator area of 266 sq ins and throat of about 80 sq ins and the Lab sub has about the same throat area but uses 2, 12inch radiators so it has a lower compression ratio.
The Lab 12 has a highly reinforced cone, huge ridged dust cap and a stiffener underneath that so it is not likely to have much "non-piston" motion in use.

>> This distortion is the sum of the VC motor and mechanical system's
>> non linearity.
>>The "bad" effects of the latter can be minimized by choosing a
>> driver with a low Fs which then requires a small Vb to end with
>> the right compliance.
>> Depending on the sealed volume more means that a more linear
>> spring is dominating over the drivers relatively non-linear
>> suspension (by comparison) spring.
>> Generally, stay away from High Fs driver for bass horns if low
>> distortion is a goal.

>I remember you telling me before, the idea is to get the reactances to offset each other, leaving the >resistive components to interact with each other. What I am saying, is how can you say a larger Vb >dictates that the suspension of the driver is not as linear as a driver requiring only a smaller Vb?

It doesn't say that directly but in practice, one finds the air to be a more linear spring than most driver suspensions and spiders. Also, most stiff drivers tend to be of the low Xmax variety as well

>So basically, the idea is to get a load as stiff as the throat, but behind the driver instead using a rear >chamber - to get even loading on each side, so its more linear on each side, resistive only. Did you mean >to say the compliance of the rear chamber Vb should be matched to that of the throat instead and not the >driver suspension? As a matter of interest, the rear Vb which cancels or balances out the capacitive >reactance from the mass reactance of the throat, may not be the volume which provides the best/most even >frequency response, even though it may be the most linear for the driver/lowest distortion combination...

In a perfect world and in the case of a 50% efficient system, one would want to make the acoustic resistance on the radiator to be about equal to the sum of the mechanical and electrical losses.
This condition results in about half the input power being dissipated as heat and the other half as acoustic power.
In practice, with a real horn, one would "tune" to get the best trade off in response /low cutoff and possibly efficiency.

>I am pretty sure that in smaller box volumes, the air inside it is stiffer (inductive reactance?) which may >be needed to balance out the capacitive reactance from the throat. Just 'how stiff' do you need it though? >That is hard to know, unless there are formula's around, which there probably are. I think though that to >generalize to say that you must have a small rear volume to make the system linear is a bit of a stretch. It >could even be made too small...which would not be good, because then the reactance's wouldn't balance. >The 'right' rear volume may even be relatively large by comparison...

The high pass nature of the load and the spring force controlled driver lf response both govern the horn's output. The spring force is the sum of BOTH the driver suspension compliance in parallel with the enclosure compliance.
The air is the more linear of the two so I suggest making its contribution larger and the drivers suspension smaller to still end with the needed total compliance.

>How come a driver with lower Fs will end up with smaller rear volume? I would think driver Qts would >play the biggest part here...

For two drivers of identical area and mms, the one with the lower Fs has a weaker suspension spring and depends more on a smaller box to get the desired Fb.

>By the way, would you mind checking my horn length in my first post? It is in the 3rd diagram down. I >have the horn length measured right down the dead middle of the horn... is this how you would normally >measure it? Have you got any comments on horn/path length? >http://www.audioroundtable.com/HighEfficiencySpeakers/messages/481.html

I don't know if this is a help but I generally figure that the horn has to end with the same internal volume folded or not when I lay one out. In reality, a bend does add a tiny bit of extra inertia.
I had seen your horn in several posts and was thinking of making a suggestion.
I try to end up with a minimum of un-used space, looking at the void in the forward end of you box, I can't help wonder if you reduced the thickness dimension a bit (making the horn take up a little bit more space up and down) if that wouldn't use it up.
Or, possibly increase the front to back dimension a bit and make the horn a bit longer which also makes it take up more space (and of course go lower).
Please do not take my suggestion as being critical, I encourage you to pursue what ever it is you end up with. Horns can be a lot of fun to design and clearly you have the bug.
Cheers,

Tom

Wayne I have a reply started to you too but I have to run, more Saturday chores.

The standard Lab 12, the version we use and the shorted turn samples your talking about that Eminance has built are all the same driver with the exception if the dimensions of the center pole and in one case the addition of a shorted turn and in the other a different VC.
You could say they were different drivers but unless you took them apart down to the core and measured, you couldn't tell the difference as all the radiator, suspension , frame, physical construction and magnetic parts are all the same.
Part of the design (what makes it a LAB 12 and different) here was to make a very ridged cone, one that acts like piston in operation in a horn as well, not just the driver parameters I had specified.
My thought in not calling it by an unrelated name (like mag 12 etc) is that one does not see this generally when one say offers some other minor change, say a dual coil version of a woofer with a proven track record like the LAB 12 has.
Also since the LAB sub project drove the need and then the design / spec's I gave Eminance for the LAB-12, it is perhaps also a part of the objection that one would simply be taking a unique, successful, proven design, make a small change (that was on the wish list but couldn't have been implemented at its conception) and try to infer it as a fundamentally different driver.
Since "MAG 12" is not a new driver design and is identical in every way to the LAB 12 (except for adding the ST), why not just call it the LAB12 ST?

This is perhaps just my view, for example, our version of it has our logo and decals and a somewhat stronger motor and different VC, but I still refer to it as our version of the LAB 12 and not something new..

So far as co-operating, understand any help I have given (doing the LAB sub project etc) or other things is aimed at the DIY'rs and not part of "my job".
It is a somewhat sensitive area too as it is frequently pointed out that my designing things for DIY'rs (like the LAB sub) that may compete with our products at work in not a "good idea".
This stuff is my personal interest though and fortunately I am not constrained by the all rules which usually prevent others in my position from participating, now it is only a lack of time that is the problem.
I see your forum as a good thing for the DIY community so from that standpoint I am trying to help (like commenting on and refering to your forum on LAB etc) and on this I think we do certainly have common ground.

While the Pro-sound guys are happy with the LAB sub and our version of the LAB 12 has worked well in the Bdeap's, I agree too that the driver has other "non-horn" uses, one of the first things I posted about on LAB was that I used the drivers in a vented box for my bass and then in 2002 we came out with the td-1 sub which is a vented box using it which has been well received.
There are a few threads on the LAB from folks who did this too.
I am Working on a hybrid horn system now using it again, it is a great driver and few have failed in the years it has been in use and it is nice the big E can add a shorted turn now.
I am curious to see how much difference it makes on this driver.
I wouldn't expect sales to the Pro-sound market to be very large though, the Lab horn "as is" is the lowest distortion (at a given level) of any of the Pro-sound subs tested, including the other low distortion horn system.

"Driven to non-linearity"
In practice one finds a loudspeaker has measurable distortion at any level one could drive it at, it then becomes a question of how much is too much or can you hear the difference with and without.
Woofers specifically, all produce audible distortion at any level you can hear them at. Dolby Labs found in testing commercial subwoofers some years ago that there were none available to the home market which could even produce an audible 20 Hz at 1 meter and not also have plainly audible distortion components (when taken away).
On the other hand, one can measure levels of 10 % or much more on many speakers at low frequencies at modest listing levels (why distortion is often specified only at 1 Watt or other "low" power).
The idea in the Lab sub was to make it have as high an acoustic power as possible, to run out of excursion and thermal at about the same time.

In the home, this makes for a horn which is literally loafing in the extreme.
For example, in my corner I have two Bdeaps, at the listening position, I have a measured sensitivity of 97.5 dB avg for 1 Watt input. The system can handle two or three times the 2000 Watt amplifier I have driving it If you are using something at -30 or -33 or even -40 dB of full power, driver linearity is going to be very good and be low in distortion at any level your concerned with.
The math, 2 boxes (2 of the 12's in each box) can handle 3200 Watts RMS in band, and the peaks in that rating are +6 dB greater still (requiring 12.8 KW peak).

1 Watt equals 97.5 dB at the couch with a safe +42 dB of peak head room

Now, want to extend the bottom end a little?

Add say +20 dB of EQ which since your dealing with an acoustically small room (at this freq) it is also minimum phase which corrects the response and phase.
Now you may have extended the low cutoff an octave or two at the expense of cutting your peak headroom down to "only" +22 dB left (over 97.5 dB).

Here is a fellow who has gone perhaps off the deep end in his home theater, look at the measured responses.

http://www.avsforum.com/avs-vb/showthread.php?s=4c6ec5466a3c7c6d0bf0174910298bf9&postid=3473618&highlight=bdeap#post3473618

Remember the plots for the single LAB sub in half space (which goes down a tad lower than the Bdeap) are nothing like what one see's in a room, these are not like the "bass horns" most are familiar with and neither is the driver or horn alignment.

As a direct radiator however, the distortion would (for a given SPL) be much greater both from the added excursion (the Lab horn raises the output about 20 dB vs a direct radiator) and from the fact that a proper horn has an acoustic roll off above its operating band (attenuating harmonics).
A horn like the LAB sub or our Bdeap does not just "go away" below cutoff, at an ocatve down from the corner, these horns are still adding about 3 dB over the same excursion as a direct radiator and its low pass acoustic filter (of the horn) also lowers the distortion vs a direct radiator.
Got to run now,

Tom Danley

• EQ'd horns

It is actually an Eminence executive that has voiced his opinion that "LAB12 ST" and "MAG12" might cause market confusion. I don't believe either name violates any trademark, but I haven't asked an attorney to research it either, so I don't guess I really know. In any case, it's not the name I'm particularly interested in.

I'm thinking about calling the woofer simply B12. The main thing I'm concerned about is its specs. Woofers with ferrite magnets are improved significantly when flux stabilization rings are used, so that's the part I'm interested in. Now it's down to the matter of where to put the ring, in the core or in the plate. Hopefully the first prototype will meet specs with the ring in the core, so Eminence can roll them out pretty quickly.