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Help designing Isobaric Sub Box

post #1 of 33
Thread Starter 
A couple years ago, I purchased several subs and two amps. I started building some large enclosure ported subs but my significant other did not approve.

So, now I am hoping to build something smaller.
Subs: 6 x NHT NPT-11-075-2
Amps: 2 x MCM 50-6262

I have included specs for both of these as attachments. In playing around with box designs in WinISD and with suggestions from other users I have decided to try my hand at an isobaric system. Space is a real problem for me.

WinISD says that if I put three pairs of my subs in an isobaric EBS3 configuration I would need an 8.8 cf box.
In theory this would be tuned to 16.5hz with 118db @ 20hz.

Problem is, I have no idea how to design this box. Any help is appreciated.

 

NHT NPT-11-075-2.zip 0.7080078125k . file

 

MCM-50-6262.pdf 126.439453125k . file
post #2 of 33
Thread Starter 
Ok, so I spent some time in Google sketch up. I put together what I think is the start for a design.

What I am confused about is if the sub sets should be isolated from each other as I have them drawn here. Or, should the cavity between the front three and the back three be open?

Also, my current plan is to power the front three with one amp and power the back three with the second amp.

Thanks in advance for any help.
LL
post #3 of 33
I used to be enamored of Isobarik, but now I just think it's a waste of power, since the extra volume required to fit the 2nd driver prevents realizing the supposed half volume.
post #4 of 33
The first problem with your design is you have a port. That totally destroys the entire reason to have subs "stacked" on each other. Remove the port and you'll get the desired result.
post #5 of 33
Quote:
Originally Posted by schan1269 View Post

The first problem with your design is you have a port. That totally destroys the entire reason to have subs "stacked" on each other. Remove the port and you'll get the desired result.

Why's that?
post #6 of 33
Technically Isobaric subs are sealed.

I've never seen an isobaric that wasn't. Besides, doing a "push pull" with a port...who does that?
post #7 of 33
Personally, with the box you've designed...

Remove the port(just remove the entire ported area from the design).

Then mount your woofers on the inside, then the outer, I would get 2 LARGER bass radiators

I have seen push pull bandpass, but that is another animal.
post #8 of 33
The inside drivers need to be isolated from the outside of course and the coupled drivers air space isolated from eachother....just as you have pictured. Your internal volume of 9cuft is a bit much though, as you'll be exceeding xmax above Fb pretty quickly. 6cuft works better with the 11-075's. The 11-083's need the bigger box though. Inner drivers need to be as close to the outers as possible since you're not clam shelling. Driver overall depth - cone depth +1.5(Xlim). If the inner cavities could be round tubes, even better. Limit the airspace in there as much as possible as you're 'coupling' the drivers and the more control of the airspace the better.
post #9 of 33
Quote:
Originally Posted by schan1269 View Post
Technically Isobaric subs are sealed.
Nope, that's just one of the possible alignments.
post #10 of 33
Thread Starter 
mayhem13,

I thought about a clam shell design but I cannot figure out how to make that attractive. I, and more so my wife, do not want to look at the backs of our speakers. Suggestions on design here would be gladly accepted.

Let me run these subs back through WinISD and see. It suggested 8.8 cu ft for those subs.
post #11 of 33
I'll just say, I've never seen anybody do a push-pull subwoofer that was ported.

With a bass radiator, yes...but never ported.

And Isobaric is "how you load them"...not the box design.
post #12 of 33
Quote:
Originally Posted by schan1269 View Post

Technically Isobaric subs are sealed.

Isobarics may be sealed or vented. The problem with the OPs diagram is the location of the ports, which should be venting the rear chamber directly to the air rather than via a plenum as shown, and the port area, which appears OK for one pair of woofers but certainly not three pairs.
Quote:
What I am confused about is if the sub sets should be isolated from each other as I have them drawn here. Or, should the cavity between the front three and the back three be open?

As you have them is fine as it makes the structure stiffer. Extensive bracing of the rear chamber will also be required.
post #13 of 33
Quote:
Originally Posted by schan1269 View Post

I'll just say, I've never seen anybody do a push-pull subwoofer that was ported.

With a bass radiator, yes...but never ported.

And Isobaric is "how you load them"...not the box design.

Correct.....it's 'compound'.....Isobaric is 'clamshell'.

PR, Ported.......same thing.
post #14 of 33
Quote:
Originally Posted by zordac View Post

mayhem13,

I thought about a clam shell design but I cannot figure out how to make that attractive. I, and more so my wife, do not want to look at the backs of our speakers. Suggestions on design here would be gladly accepted.

Let me run these subs back through WinISD and see. It suggested 8.8 cu ft for those subs.

The way you have it is fine. For the area between the drivers, just cut squares of material and route a circle in em and stack em.....done! It'll be HEAVY so maybe ply instead of MDF.
post #15 of 33
Quote:
Originally Posted by schan1269 View Post

I'll just say, I've never seen anybody do a push-pull subwoofer that was ported.

Push pull is not necessarily isobaric and because you've never seen it doesn't mean it doesn't exist or is not possible. Five minutes with on of he sim programs will show you it is.

Quote:
Originally Posted by schan1269 View Post

With a bass radiator, yes...but never ported.

Which only changes the mass of the air in the port for that of the diaphragm

Quote:
Originally Posted by schan1269 View Post

And Isobaric is "how you load them"...not the box design.

Well, duh. I've owned commercially make isobarics (Sara, DMS) as well as made my own, and the only difference between a typical enclosure and a bric is the use of a second driver to reduce the effective Vas and therefore (rear) enclosure size. And twice the driver count and cost for the same output capability as a single driver.
post #16 of 33
Quote:
Originally Posted by zordac View Post

I thought about a clam shell design but I cannot figure out how to make that attractive.

Face to face mount both drivers on the inner baffle in your original drawings, but have the 'sides' (+top/bottom) as they are. Use a fabric grille to enclose that front volume.
post #17 of 33
Quote:
Originally Posted by schan1269 View Post

Technically Isobaric subs are sealed.

I've never seen an isobaric that wasn't. Besides, doing a "push pull" with a port...who does that?

Technically all isobarik does is yield a driver with different T-S parameters.
post #18 of 33
Thread Starter 
Is there a performance benefit to mounting the speakers in clam shell format? I just prefer to be able to see my speakers if I want to pull the grill off.


I have two designs I am thinking about if I could get a couple of you to model these and give me feedback.

Design 1
Volume: 8.8 cu ft
Tuning Freq: 16.5 Hz
Vents/Ports: 1 x 4 in dia x 11.3 in length

Design 2
Volume: 6.5 cu ft
Tuning Freq: 19 Hz
Vents/Ports: 1 x 4 in dia x 11.5 in length

In WinISD, both of these show pretty slopes for me and cone excursion is not a problem until 15 Hz. (if I am doing this correct)

The one thing that bothers me with both of these is that the air velocity coming out of the port seems pretty high. If I add multiple ports obviously this goes down, but we start getting closer to port resonance. If I go to three ports, the length starts becoming an issue.
post #19 of 33
I'll ask a simple question...

With that many drivers...

Why do you even need the port? Acoustic suspension(as in, no port) is always easier to design, and generally gives you deeper bass.
post #20 of 33
Quote:
Originally Posted by zordac View Post

Is there a performance benefit to mounting the speakers in clam shell format? .

It minimizes the size of the plenum chamber connecting the driver pairs. Since the entire reason for an isobaric alignment is to reduce the cabinet size having the plenum any larger than necessary defeats the purpose. Mounting the drivers face to face on the baffle, eliminating the chamber entirely, was the most commonly seen arrangement when these were semi-popular, thirty years ago and more, when huge Vas figures were more the rule than the exception.
Quote:


Technically all isobarik does is yield a driver with different T-S parameters

Only Vas, which is halved.
post #21 of 33
I remember the fad that was the....

Isobarock

So sad the departure of Advent...

(Cymbal crash...)
post #22 of 33
Quote:
Technically all isobarik does is yield a driver with different T-S parameters

Quote:
Originally Posted by Bill Fitzmaurice View Post

Only Vas, which is halved.

Cms is halved
Mms is doubled
Re is halved (parallel connection) or doubled (series connection)
BL is doubled
SPL (reference efficiency) is halved
post #23 of 33
Thread Starter 
So I now realize that doing face2face clam shell allows for the smallest possible box and I will consider that. It seems though that I will not be saving that much space over the front2back design. When I consider the over all space requirements for the speaker, the only thing the clam design does is move the space requirements from inside to outside the box. Maybe I am missing something but isn't the only thing going to change, is whether I have cages or cones pointing out? Won't the overall depth of the sub assembly be roughly the same?

As for overall size, to build a "standard" ported sub with the same characteristics would require between 12 and 18 cubic feet. If I can get this built for under 9 cubic feet total then I will be very happy. And, my marriage will be happier also.
post #24 of 33
Trust me.....no need to clamshell. I build 2 2 driver 11-075's compounds and the worked very well. These were 'play things' mind you but they were very responsive and smooth. The NHT buyouts since they require big volumes are perfect for compound systems.
post #25 of 33
zordac, have you tried modeling non-isobarik to see if you're really gaining anything?

I suspect it makes much less difference than for a sealed system.
post #26 of 33
Thread Starter 
Quote:
Originally Posted by noah katz View Post

zordac, have you tried modeling non-isobarik to see if you're really gaining anything?

I suspect it makes much less difference than for a sealed system.

@noah,

Please correct me if I am wrong, but it looks like the iso design gives me a lot more on the lower end, particularly under 35hz.
post #27 of 33
Just asking; if it does, it does.

Are you using EQ?

That might give the same output w/less power, though with more power compression.

Dunno, depends on the drivers' power handling and available power; Isobarik might be best.
post #28 of 33
Quote:
Originally Posted by zordac View Post

@noah,

Please correct me if I am wrong, but it looks like the iso design gives me a lot more on the lower end, particularly under 35hz.

Compared to the same size box loaded with half as many drivers in a standard alignment yes, that's what you'd expect to see.
post #29 of 33
I've actually been playing around with the isobaric idea for my friends build.
This is going into a cubby hole into his a/v cabinet (wife won't let it go anywhere else). So his max dimensions are 22"W x 16 3/8"H x 25"D.

All I have to give him are a pair of reconed Lab 12 subwoofers. So I've been trying to come up with the best enclosure design possible to eek out every bit of SPL possible for his huge room. Here is what I've come up with. Each of the three enclosure designs houses two Lab 12's and fits within his space requirements. You can judge for yourself whether the Isobaric is a good choice.

Each enclosure is configured with 1200 watts input and 12db 80hz low pass.

3rd pic is X-max and it's 13mm max for the Lab12. That kinda makes the IsoVent look pretty decent in that regard.
The sealed Isobaric is pretty crappy compared to the regular sealed enclosure, when it comes to SPL. But the regular sealed is exceeding xmax BIG TIME.

YELLOW = SEALED
AMBER = ISOBARIC SEALED
GREY = COMPOUND ISO-VENT
LL
LL
LL
LL
post #30 of 33
Port velocity looks awfully high.

You have about 4 cf available; I'd also try two sealed and two vented.
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