# 4th order isobaric compound box



## smgreen20 (Oct 13, 2006)

I'm looking into changing my box from a ported box to a 4th order isobaric compound box. I input the data into WinISD and it came up with a solution of .46 cf for the rear sealed chamber and .825 for the front/ported chamber. On another forum site I was told that I need to halve both of those sizes given. If that's true, then that's pushing it for me to even try to fit the motor assembly in the box. But I know you tune the box and halving that would change the port dimensions, correct?

I'm also wondering, are the volumes given even correct? The title for the box in WinISD states 4th order bandpass. 

So I ask, What do I do?

If I knew how to copy/paste the screen, I would post the WinISD results. I love the looks of the freq curve for this setup, it's exactly what I'm after. Higher low end output w/the SQ of a sealed box.

The subs I am using are two TREO SSi12.22's. 

What I'm after (which is what I got from the WinISD plot) is a flat as possible response from 30Hz to 60Hz. 

Anyone know of a box calculator for making 4th order isobaric compound boxes?

Thanks in advance.


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## smgreen20 (Oct 13, 2006)

This is what I'm trying to design, 3rd box down titled "Isobaric - Bass Reflex (Compound Loading)"


Car Audio Subwoofer Enclosures Fourth Order/Bass Reflex and Bandpass


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## smgreen20 (Oct 13, 2006)

Correction, the 2nd one down IN the box outlined/titled "Single Reflex - Band Pass Enclosures"


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## The Tube Doctor (Nov 24, 2009)

I've got a program called Low Frequency Designer, v3.02.
Made by a company called SpeakEasy. 
It allows user-defined enclosure configurations.
Sealed, ported, BP4, BP6, serial and parallel ported configurations.
Also allows you to choose the driver config. Single driver, dual standard, 
dual isobarik. 
It's great for the quick & dirty approach. Otherwise, I use LEAP, which
allows for a much higher level of enclosure/driver/environment analysis.
Strictly command-line interface (v4.51) and very much GIGO.
If you've got TS for your drivers, I could run them through LFDES, just to give you 
a bit more info to work with, rather than relying on a single analysis/modelling program's 
results.


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## reker13 (Oct 26, 2007)

go back to WinISD again and recheck.  Are you running the pro alpha .50a7 version? I just checked mine and you choose isobaric then click next then 4th order. So, you should be able to stick with WinISD.


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## reker13 (Oct 26, 2007)

go back to WinISD again and recheck. Are you running the pro alpha .50a7 version? I just checked mine and you choose isobaric then click next then 4th order. So, you should be able to stick with WinISD.


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## The Baron Groog (Mar 15, 2010)

Bbpro will model it-what are the driver specs and space requirments.


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## smgreen20 (Oct 13, 2006)

No, I'm using beta. 

I have issues loading driver T/S as they don't have the TREO SSi12.22's in their list. Then when I try closing the program, I get some error message stating that it can't close.


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## minbari (Mar 3, 2011)

not to derail what you are trying to do, but if you are already at about 1.5 cuft total for a 4th order, why not use a single driver and call it a day? (that is really small, what size sub are you using?) an isobaric may require less space, but it is alot less efficient too, ussualy about 3dB less than a single driver.


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## 60ndown (Feb 8, 2007)

great box here

DECWARE / The Death Box - High Output Subwoofer Cabinet


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## smgreen20 (Oct 13, 2006)

Two 12" TREO SSi12.22's. Same as the TREO SSp's. Use the T/S from the SSp12.22.
http://treoonline.com/infosheets/ssp_subwoofers.pdf

Currently I have 2 of them in a ported box, 1.5 cf each. So I'm already "there" as per the thought of just one in a ported box. What I'm after is a "natural" low end response. The curve of the setup I'm after is +/- 0dB from 31Hz to 53Hz and -3dB from 26Hz to 66Hz.


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## The Baron Groog (Mar 15, 2010)

smgreen20 said:


> Two 12" TREO SSi12.22's. Same as the TREO SSp's. Use the T/S from the SSp12.22.
> http://treoonline.com/infosheets/ssp_subwoofers.pdf
> 
> Currently I have 2 of them in a ported box, 1.5 cf each. So I'm already "there" as per the thought of just one in a ported box. What I'm after is a "natural" low end response. The curve of the setup I'm after is +/- 0dB from 31Hz to 53Hz and -3dB from 26Hz to 66Hz.


Whether we can design a box with that response in bbpro or similar is moot as soon as you put it in your car. With that in mind what are your other considerations? Size? Efficiency? etc.


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## Jroo (May 24, 2006)

I thought on a isobaric 4th order, you are only cutting the sealed section in half. Your ported side size (port /lengh tuning) remain roughly the same, sometimes bigger. I had some old credence clearance woofers and had a 4th order iso design done. I by no means am a guru on this stuff, so I tried to match the design to some other resources so I could learn how he came up with it. Nothing every came exactly, but I can say that all the ported sides were very close whether it was the standard bandpass or the iso bandpass. The difference in size was the sealed section which was close to half the size.


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## The Baron Groog (Mar 15, 2010)

I've modeled some enclosures, for flatest response go for a vented isobaric 1.475cf tuned to 22.12Hz. Will dig deeper than the rest, this is the dark blue trace.

Light blue trace is the subs in their optimum vented enclosure, but huge at 5.9cf

Yellow trace is a single tuned BP, total volume is 0.93cf front chamber is 0.384cf and vented, rear chamber is sealed and 0.546cf-also has lowest group delay

A parallel tuned enclosure, purple line, will give the widest BP response, 0.738cf front chamber and 1.745cf rear chamber 2.483cf total volume.


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## smgreen20 (Oct 13, 2006)

Thanks for graphing those for me Baron Groog. The yellow plot is the same as what I was getting with WinISD and that's the one I want, but in a 4th order Isobaric compound box.

What would I need to do to make it a 4th order Isobaric compound? Can you give me the port length for a 3" port and for a 4" port (both round). What I want is both subs facing each other, one wired out of phase as to be in phase with the other sub, one sub in the sealed box and the other in the ported box, is that something you can plot?


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## The Baron Groog (Mar 15, 2010)

That is a 4th order bandpass, with isobaric mounting and that's the good news.

Now the bad news-for a 3" vent you'd need 25.76" length, and get chuffing between 26 and 38Hz. If you go for a 4" vent you can expect to have a length of 46.35" but get no chuffing-in other words this is going to be the mother of all fockers to build.

I don't really play with BP designs-never had the need/want to but will have a mess for you. If I make the box bigger (that actual box wouldn't fit your drivers in it) the response becomes more peaked, but you could EQ out the peak to get the flat response you're after-again, please remember this will be nothing like flat in your car.

What are the gross dimensions the box must fit in?


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## smgreen20 (Oct 13, 2006)

I have 3 cubes total to deal with. A 46" port is nothing, I have just the spot for it. 
I have an audio control dqs I can use to flatten the response out. I under stand cabin gain will change things. 

Again, thanks for your help.


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## smgreen20 (Oct 13, 2006)

Double post, sorry .


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## The Baron Groog (Mar 15, 2010)

IS the 3 cubes including the port or can you go external with it?


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## smgreen20 (Oct 13, 2006)

I can go external with it, but I'm trying to save some space.


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## The Baron Groog (Mar 15, 2010)

Ok, have re-modeled it for you, these are the internal measurements:

Chamber 1 LF:
14" x14" x13.57"

Chamber 2 Vented:
14" x14" x10.09"

3x 3" ID vents, 24.86" long.

Made using 18mm mdf (.709") will give you external dimensions of: 15.42" x15.42" x 27.58"

No port noise, vent speed stays below 30M/S. Xmax, provided the 32mm is correct and is one way, shouldn't be exceeded with 2000wrms.


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## smgreen20 (Oct 13, 2006)

So the port is 9 sq" (3x3 square) in area by 24.86" long? 

And thanks again for your help and time.


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## Patrick Bateman (Sep 11, 2006)

minbari said:


> not to derail what you are trying to do, but if you are already at about 1.5 cuft total for a 4th order, why not use a single driver and call it a day? (that is really small, what size sub are you using?) an isobaric may require less space, but it is alot less efficient too, ussualy about 3dB less than a single driver.


DING DING DING

Isobaric boxes fell out of favor because loudspeakers just don't need large boxes these days.

I love making weird boxes, and I'm always excited about doing something different, but it's really hard to get excited about isobaric.

If you *must* do an isobaric, there are a lot of good reasons to do a SEALED isobaric. Here's why:

In an isobaric sub, you're basically doubling the motor strength, doubling the weight of the cone, but the SIZE of the cone remains constant. This has the effect of halving the required box size, *but efficiency goes down at the same time.*

Have you ever noticed that expensive subwoofers like JL Audio require tiny boxes, but the woofers are humongous? That's because they're very similar to an isobaric sub. Big powerful motor, tiny heavy cone.

Anyways, the reason why isobaric bandpass kinda sucks is because the efficiency of your subwoofer will depend a GREAT DEAL on the size of your vent. If you use a vent that's too small, your box will NOT be efficient. (Note - WinISD CANNOT MODEL THIS. DO NOT USE WinISD.)

Sealed isobaric is a bit compelling, because you take the loudspeaker port out of the equation. It's particularly compelling if you use an array of inexpensive subwoofers, like the ones from MCM. The reason it's compelling is that you double power handling *and you also increase maximum output because the smaller box lowers excursion.*

So an array of cheaped sealed woofers *can* be compelling, particularly if you're using a big amplifier.


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## Patrick Bateman (Sep 11, 2006)

Jroo said:


> I thought on a isobaric 4th order, you are only cutting the sealed section in half. Your ported side size (port /lengh tuning) remain roughly the same, sometimes bigger. I had some old credence clearance woofers and had a 4th order iso design done. I by no means am a guru on this stuff, so I tried to match the design to some other resources so I could learn how he came up with it. Nothing every came exactly, but I can say that all the ported sides were very close whether it was the standard bandpass or the iso bandpass. The difference in size was the sealed section which was close to half the size.


No, it changes the volume of both chambers. The VAS drops by half because the motor strength is double, but *the size of the cone doesn't change*. The reason that big expensive subwoofers have huge motors is because it's virtually the same effect as going isobaric. The old "solo-baric" subs may have started this trend.


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## The Baron Groog (Mar 15, 2010)

smgreen20 said:


> So the port is 9 sq" (3x3 square) in area by 24.86" long?
> 
> And thanks again for your help and time.


No, there are three 3" internal diameter round ports, each 24.86" long.

You could use a 4.716" x 4.716" square port, 24.86" long if that is easier for you.

With regards to what Patrick was saying I've re-modeled the enclosure for the subs conventionally mounted-much flatter response and a wider bandwidth-all in the same sized box. If you can up the box volume to 4.2cf you can get even better results then.

RED trace originally suggested optimum
GREEN trace box as discussed above.
WHITE trace, conventional mounting of drivers same sized box as green trace
PINK 4.2cf BP, subs conventionally mounted
YELLOW 3cf vented box, tuned to 26.84Hz-highest output, flatest response, lowest group delay, smallest phase shift, lowest port noise, easiest to build (actually by a country mile-the port will fit in the box easily unlike the rest). The only area that this enclosure is "down" on any of the BP boxes is in excursion-you'd run out of it at around 17-18Hz.


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