# Isobaric - is it really 1/2 the space?



## Buickmike (Aug 10, 2012)

Can you literally take a enclosure that you like, and get the same results from 1/2 the space if you iso load it? I have heard that all the time and even went with that rule way back in the day with good results, but is it really the same response?

The reason I ask is because I have a couple old school Oz Audio 250L subs that I really like, but the air space to run both is just too much. After playing with different boxes I really like each in a 1.5 ft ported at 35hz. If I go by the 1/2 theory, that means that a .75 ft enclosure will do the same, however when I chart it in WinISD, it comes out to 1.2 ft ported at 30Hz to get the same frequency response. Seems odd, right?


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## XR250rdr (Mar 22, 2011)

Its roughly half the volume you would use 2 for drivers, not half the volume required for 1 driver.

In your case it would be 1.5 cu.ft. for 2 drivers instead of 3 cu.ft. total.


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## Buickmike (Aug 10, 2012)

XR250rdr said:


> Its roughly half the volume you would use 2 for drivers, not half the volume required for 1 driver.
> 
> In your case it would be 1.5 cu.ft. for 2 drivers instead of 3 cu.ft. total.


Hmm, I always thought 2 drivers iso loaded was 1/2 the volume of a single sub. If 2 drivers loaded isobaric would be the same airspace as one, then nobody would bother to take the -3db hit in the first place. Read paragraph 2 here: JL Audio » header » Support » Tutorials » Tutuorial: Isobaric Enclosure Types

"In this case, you might switch from one 150W driver (such as a JL Audio 10W1v2) in a 0.625 cubic foot enclosure to four of the same drivers (2 isogroups) in the same sized box."

If 4 drivers iso loaded take the same airspace as a single driver as JL Audio states, then my perception is true. 

I'm just trying to get at this....is it really as cut and dried as 1/2 the airspace or is there more to it? I'll most likely just get some scrap OSB and mock up a .75 ft iso loaded box and see if it sounds good or not before committing....although porting a .75 ft enclosure at 35hz is going to make for a small port or slot that will come with some noise.


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## Buickmike (Aug 10, 2012)

Here is more info here as the first link I shared could be read 2 ways. This one is better:
JL Audio » header » Support » Tutorials » Tutorial: Isobaric Enclosure Characteristics

"For example, if a speaker is optimized for performance in a 1 cu.ft. sealed enclosure, one iso-group of the same speakers can achieve the same low frequency extension and overall response characteristics in a 0.5 cu.ft. sealed enclosure."

Seems like it is that straight forward. Screw it, I will mock this up with cheap wood and see how it works out. I'm not looking to get these things to go super deep. They are supposed to like 2 ft each ported, and they hit stupid low in that size box, but I don't need that for real music in a hatch back of a car. I worked with sealed and they hit harder than anything I have ever heard, but they lost some SQ. The smaller ported enclosure was the best of both worlds. Very musical and rich, yet could handle double bass extremely accurately and hit very hard....all the while picking up of bass guitar perfectly. If I can iso load these and get the same thing I will be in SQ sube heaven and it will be loud enough for me.


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## lurch (Jan 20, 2014)

I didn't realize iso could work ported. 
the old loud speaker cook book never mentioned this IIRC. 
i'm subbed !


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## hurrication (Dec 19, 2011)

You need to model your port velocity, because when going iso ported the port volume requirement more than doubles. Most of the time, you end up with ports that have more volume than the enclosure itself. 

Isobaric ported enclosures don't save any more space over normal ported because of this. Iso sealed boxes, sure, but not ported.


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## smgreen20 (Oct 13, 2006)

To correct the other reply, it is half space a single driver, not the pair. Phoenix gold has a print up on it using their XS subs. Go to Phoenix Gold Phorum • Index page and go to the literature section. The proof is in the pudding.


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## Bayboy (Dec 29, 2010)

Half the space of one for little gain, or reduction rather in distortion. The issue with that in practical use is output isn't greater, just a tad cleaner and space is still taken up with the basket & motor assembly outside the enclosure. Also, going ported will require a longer vent than usual. Pros & cons.


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## knever3 (Mar 9, 2009)

There are more than one way to load isobaric pairs. Face to face is the most common, but you can also do cone to magnet meaning the second sub is a set distance directly behind the one mounted outside of the box. Car Audio magazine had great results from this setup, it had less distortion but the tradeoff was the inside sub was subject to more abuse then the one on the outside. It was a great article showing the many ways to load subs in a space saving way or to tighten up your bass. It will not however be much louder since you still have the same cone area as one sub. Plus you need to feed both subs with power so you have nearly double the wattage for the cone are of only one sub.


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## XR250rdr (Mar 22, 2011)

Buickmike said:


> Hmm, I always thought 2 drivers iso loaded was 1/2 the volume of a single sub. If 2 drivers loaded isobaric would be the same airspace as one, then nobody would bother to take the -3db hit in the first place. Read paragraph 2 here: JL Audio » header » Support » Tutorials » Tutuorial: Isobaric Enclosure Types
> 
> "In this case, you might switch from one 150W driver (such as a JL Audio 10W1v2) in a 0.625 cubic foot enclosure to four of the same drivers (2 isogroups) in the same sized box."
> 
> ...


Guess I've been wrong :blush:

Sorry for the misinformation.


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## Buickmike (Aug 10, 2012)

Thanks for the replies all. My point of all this was to see if the formula for iso loading was truly a matter of making a box exactly 1/2 the size of what a single driver would take, or if there was more to it. I'm guessing many of the folks that have replied have never tried iso loading subs as it has been almost 20 years since subs were produced that would even make someone consider isobaric. If you have not built one, I suggest finding a pair of old school cheap subs that work well in a large ported enclosure, and iso load them. You will be amazed at how much cleaner and more accurate they are. I had done plenty in my highschool years back in the early 90's, and they all came out amazing. The only thing is that I winged every single one of them. I'd come up with whatever I liked for a single enclosure and chopped it in half. I usually did ported and usually made one face the back of the other because I never liked looking at the basket side of a sub. I modeled them after the old JBL S1 sub tower, that was 2 8" subs iso loaded in a ported enclosure. Iso loading was great when you had a pair of subs that were either cheap and / or needed stupid amounts of airspace. One of my favorites was a pair of Radio Shack paper 8" woofers in a 1.2 ft or so ported enclosure. That setup had a ton of people fooled. 

Long story short I understand that iso loading is more of a novelty, but when working with some sub configurations that would otherwise be overlooked, it can have some advantages.


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## Buickmike (Aug 10, 2012)

So I realized why WinISD was throwing me off. I accidentally selected the wrong sub model (300L instead of 250L) when I ran the iso loaded specs. I ran the specs on the correct sub and indeed it comes out to 1/2 the space.


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## Niick (Jun 3, 2015)

I haven't thought about building isobaric subwoofer systems in a LONG time. This has really got me thinking......hmmm......thanks everyone!


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## Bayboy (Dec 29, 2010)

Buickmike said:


> Thanks for the replies all. My point of all this was to see if the formula for iso loading was truly a matter of making a box exactly 1/2 the size of what a single driver would take, or if there was more to it. I'm guessing many of the folks that have replied have never tried iso loading subs as it has been almost 20 years since subs were produced that would even make someone consider isobaric. If you have not built one, I suggest finding a pair of old school cheap subs that work well in a large ported enclosure, and iso load them. You will be amazed at how much cleaner and more accurate they are. I had done plenty in my highschool years back in the early 90's, and they all came out amazing. The only thing is that I winged every single one of them. I'd come up with whatever I liked for a single enclosure and chopped it in half. I usually did ported and usually made one face the back of the other because I never liked looking at the basket side of a sub. I modeled them after the old JBL S1 sub tower, that was 2 8" subs iso loaded in a ported enclosure. Iso loading was great when you had a pair of subs that were either cheap and / or needed stupid amounts of airspace. One of my favorites was a pair of Radio Shack paper 8" woofers in a 1.2 ft or so ported enclosure. That setup had a ton of people fooled.
> 
> Long story short I understand that iso loading is more of a novelty, but when working with some sub configurations that would otherwise be overlooked, it can have some advantages.




ISO isn't a novelty. It's real, but also outdated as you stated so yourself. In those same 20 years I'm sure you have witnessed the advancement of drivers, specifically subwoofers with higher linearity in extended throw, shorting coils, cone materials, etc. Things that would have been very expensive in those days, but are now going for much less and also can work in less space than the reason for ISO suggests. Novelty? No. Simply a different way to skin a cat with similar results.


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## Buickmike (Aug 10, 2012)

I also want to clarify something on iso loading. It really only makes sense if you are looking at a ported enclosure. Iso loading sealed subs does not have enough space gain to overcome the losses. If you had a sub that takes up 1.0 cu ft sealed, you could iso load 2 into a 0.5 ft enclosure, but you will have to either face one basket outward or create another .3 or .4ft chamber for the 2 subs back to back. Now you are taking up 1 cu ft and haven't really gained anything and now need double the power.

Now let's say you have 2 subs that thrive in 2.5 cu ft ported enclosures each. 2.5 is a bit much and 5.0 is not gonna happen. Now you can iso load these into a 1.25 ft enclosure, which is not too small to port, and now you are getting 75% of the benefit of that 5.0 cu ft enclosure out of 1.25.


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## Buickmike (Aug 10, 2012)

Bayboy said:


> ISO isn't a novelty. It's real, but also outdated as you stated so yourself. In those same 20 years I'm sure you have witnessed the advancement of drivers, specifically subwoofers with higher linearity in extended throw, shorting coils, cone materials, etc. Things that would have been very expensive in those days, but are now going for much less and also can work in less space than the reason for ISO suggests. Novelty? No. Simply a different way to skin a cat with similar results.


Agreed. ISO loading in the car audio world is really only applicable to those of us who are partial to our old school subs and want to use them again without taking up the whole back of the car. I guess the reason I say it is a novelty is because of the application and not the concept.


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## jode1967 (Nov 7, 2012)

you also only get the noise benefit when doing face to face. was pretty cool for the woofers of its day


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## Buickmike (Aug 10, 2012)

jode1967 said:


> you also only get the noise benefit when doing face to face. was pretty cool for the woofers of its day


Yep. What I've always wondered is whether they need to directly face each other or if setting them up like in the pic here:










Fron what I've read as long as the motion of the subs are opposite you get the benefit.


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## thehatedguy (May 4, 2007)

Nice picture of the console from Steve Head's truck.


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## seafish (Aug 1, 2012)

JL has a decent primer on the design advantages/disadvantages of Isobaric subs and the differ met ways to build them...

JL Audio » header » Support » Tutorials » Tutuorial: Isobaric Enclosure Types


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## hurrication (Dec 19, 2011)

You mentioned that you think most of the people here haven't tried iso in real life, but I have. I've tried iso ported, and one thing that was obvious is the importance of the necessary larger port volume as well as the additional length required to keep the same tuning.

When you iso a pair of subs, you are dropping the overall qtc of the enclosure . Characteristics for ported boxes between mid-q and low-q speakers are different as well as the fact that doubling your power for the same enclosure size in itself needs more port area to keep velocity under control. When cutting enclosure size in half with an iso pair, your vent length doubles and it can take up to 4x the power to achieve the same spl as just one sub. 

In the end, a properly designed and implemented iso ported enclosure has marginal space savings compared to a single sub ported enclosure on top of having to quadruple the power to maintain the same output as a single sub. As an example, say you have a 10" 300 watt sub - you might lower the overall enclosure airspace by about half a cubic foot compared to an iso pair, but now you will need to send 1200 watts to the iso pair just to match the output of the single sub.


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## smgreen20 (Oct 13, 2006)

XR250rdr said:


> Guess I've been wrong :blush:
> 
> Sorry for the misinformation.


At least you owned up to it, Props man.


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## smgreen20 (Oct 13, 2006)

Buickmike said:


> Yep. What I've always wondered is whether they need to directly face each other or if setting them up like in the pic here:
> 
> 
> 
> ...


Direction of sub is irrelevant, as long as phase from sub to sub is observed. The air between the subs doesn't matter to a point. It's the air "suspension" between the two that helps. It's been a while since I read the Cookbook.


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## XR250rdr (Mar 22, 2011)

smgreen20 said:


> At least you owned up to it, Props man.


Thanks...its what I get going from memory on something I haven't looked at in a long time. Should have done a quick verification first.


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## hurrication (Dec 19, 2011)

XR250rdr said:


> ..something I haven't looked at in a long time.


How long? .. since SDOT?


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## XR250rdr (Mar 22, 2011)

hurrication said:


> How long? .. since SDOT?


:laugh: Yeah something like that.

When did they finally shut the place down?


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## Patrick Bateman (Sep 11, 2006)

Tapped horns are a pretty interesting use of isobaric loading. You get the advantages of isobaric - small enclosure size - without the drawbacks of a ported isobaric. Because realistically, the entire enclosure is the port! Pictured above is a McCauley 421, made in my hometown of Tacoma. (http://www.mccauley.com/product_overview.cfm?ID=2275)

isobaric is pointless for front loaded horns, because the back chamber is already very tiny.

Having said that, I've only built an isobaric enclosure once. It's hard to get excited about them because it's usually easier to simply use a smaller driver. I guess it would be a good idea if you had a humongous amp, because the power handling doubles.


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## Buickmike (Aug 10, 2012)

XR250rdr said:


> Thanks...its what I get going from memory on something I haven't looked at in a long time. Should have done a quick verification first.


No worries. 

This has been a good thread with lots of great info. 

Not sure when I'll have time, but I will play with these in a scrap wood enclosure one day and will report back how it works out.


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## cubdenno (Nov 10, 2007)

I am trying to figure out how port area has to increase?


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## ChrisB (Jul 3, 2008)

I did a push-pull isobaric bandpass enclosure in the late 90s with some OG Kicker Competition 10s. Unfortunately, the enclosure was built without taking the transfer function into consideration and it sounded like poop in my 1996 thunder chicken. I replaced that abomination with a single Kicker Solobaric 12 and was much happier.


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## hurrication (Dec 19, 2011)

cubdenno said:


> I am trying to figure out how port area has to increase?


Excessive velocity kills port efficiency.

Regardless of driver size, the more power you send a ported enclosure the larger the port area needs to be to keep velocity from being excessive. 

In addition to this, when you half the airspace the port length needed to keep the same tuning doubles.


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## Patrick Bateman (Sep 11, 2006)

^^ exactly.

That's why it's interesting for a tapped horn, and little else.

If you do an isobaric sealed box, the savings in box size will probably be a few hundred square inches, not enough to justify the extra expense really.

If you do an isobaric ported box, the savings in box size is significant, but then the port gets crazy huge. (It's counter intuitive, but the bigger the box is, the smaller a port is, and vice versa.)

But a isobaric tapped horn is kinda interesting, because the entire enclosure is basically a port.

It's also (sort of) interesting for bandpass boxes. In a bandpass box, the port is tuned at least half an octave higher than a ported box, and due to that, the port length is pretty short to begin with. Polk made a famously good isobaric bandpass box back in the 90s, and the owner of Hybrid Audio was running isobaric bandpass for a while.


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## cubdenno (Nov 10, 2007)

hurrication said:


> Excessive velocity kills port efficiency.
> 
> Regardless of driver size, the more power you send a ported enclosure the larger the port area needs to be to keep velocity from being excessive.
> 
> In addition to this, when you half the airspace the port length needed to keep the same tuning doubles.


I understand the port velocity. That is dictated by displacement. 

I don't understand why for a given enclosure volume you would have to increase the port area just b cause you are running isobaric. The displacement remains the same. So if you calculate the port area based on the power you are running and the displacement of the driver at that power, it shouldn't change.


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## Patrick Bateman (Sep 11, 2006)

Because the tuning frequency depends on the volume of air in the box.

The more air in the box, the lower the tuning frequency for a given port length. That's why Logitech uses passive radiators for their computer speakers. When you have a ported box that's ultra small, the port length gets ridiculous.


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## Buickmike (Aug 10, 2012)

From what I have calculated, the port length will be the same for a given size of enclosure regardless whether it is a single driver or iso loaded. The issue is with the fact that there will be a lot more air moving and therefore the velocity will increase. That being said, to my surprise if I port my .75 cu ft enclosure at 34hz with either a 3" or stupid long 4" port, the mach will be .1 max, which is still well under the .16 that LinearTeam says to stay below. The way I will be designing the enclosure will be to tuck in behind the back seat in my WRX hatch, and share very similar dimensions to my current 1.2 cu ft ported enclosure that is in there now......slim and wide. I can do it in a planar load style and the exposed sub will face the back and have a grill over the back of the enclosure, kind of like having it recessed and in a window box if you will so it does not protrude. This will also allow me to run the port into the side of the enclosure where the outward sub sits and let it run across to the outboard side of the box. Hard to explain. At some point I will draw it up and if it will work, I'll post the drawing up here. Then it will be time to put it together with scrap wood and see what it does. That way I'm not out anything but time, and I enjoy experimenting with this stuff so even if it is a total flop I still had fun.


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## Buickmike (Aug 10, 2012)

One more thing I need to get straight in my head.
From what I've read, this will be the drawback of running iso from an efficiency perspective:

Let's say I feed 150 watts to this setup since a single driver can handle 150 RMS in my case. At 150 watts there would be no difference in SPL between the iso loaded .75 cu ft box and the 1.5 cu ft box with a single driver, right? 

Now let's say I feed 300 watts to the iso loaded setup. Now I have broken ahead of my single driver setup at the expense of a bigger amp. Of course running 3.0 cu ft ported would be better, but I am willing to sacrifice 1.2 cu feet MAX! 

If I make the box any deeper, the hatch will not fit my son's hockey gear in the hatch. Doing the calc here, I will need to make the enclosure a little narrower, but can still keep the same depth and height. Even with the space taken to hide the exposed sub and the port, I will be around 1.0 cu ft total. Sure it won't be that loud and will be a bit inefficient, but I guarantee those 2 Oz 10's will sound far better than my single Boston G1 10". 

This will be my last post for awhile since I will probably not try this until over the holidays, but I will report back and let you know if it works out well or is crap. I will run a single 10" in my 1.5 ft ported box first so I can compare.


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## Patrick Bateman (Sep 11, 2006)

Buickmike said:


> One more thing I need to get straight in my head.
> From what I've read, this will be the drawback of running iso from an efficiency perspective:
> 
> Let's say I feed 150 watts to this setup since a single driver can handle 150 RMS in my case. At 150 watts there would be no difference in SPL between the iso loaded .75 cu ft box and the 1.5 cu ft box with a single driver, right?
> ...


Easiest way to wrap your brain around it is to remember Hoffman's Iron Law. Basically the box size and the F3 are going to dictate the SPL.

For instance, let's say you have a 2cf box in your trunk and you think it's loud enough. But you think it's too big, you want more of your trunk back. You can halve the box size, *but you'll need to double the power to get the same SPL.*

OTOH, if you have a 2cf box in your trunk and you don't think it's loud enough, you can double the box size and raise the SPL.

It doesn't matter what the box is. Tapped Horn, sealed box, vented box, front loaded horn, bandpass, dual reflex bandpass, whatever.

They all follow Hoffman's Iron Law.


Now someone will say, "no that's not true. I've doubled the box size and my SPL didn't go up at all." The key here is F3. When you double the box size and the driver stays the same, *the F3 goes down.*

This is the reason that I use these weird-ass computer speakers in my projects. I generally don't WANT a low F3, because a low F3 kills your efficiency.




oh, one thing, haha I forgot : All of this **** is theoretical. In the real world, vents distort and vents compress. That's why people have been having such wonderful results with tapped horns, and that's why the SPL guys use these ridiculously large vents. In the real world, a vented box with an undersized port doesn't behave like the sims predict as the volume knob gets turned up. And using ridiculously large vents can make all the difference in the world, because at low frequencies nearly 100% percent of the SPL is being radiated by the port! The driver literally isn't moving. I see this constantly in the spec sheets; manufacturers frequently spec out boxes with vents that are way too small for the power ratings. I'd speculate this is a marketing thing; the marketing department doesn't want to lose a sale because a competitor's speaker "works" in a smaller box. So they undersize the port to keep the box size small.


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## Rrrrolla (Nov 13, 2008)

Patrick Bateman said:


> Because the tuning frequency depends on the volume of air in the box.
> 
> The more air in the box, the lower the tuning frequency for a given port length. That's why Logitech uses passive radiators for their computer speakers. When you have a ported box that's ultra small, the port length gets ridiculous.


I think what he was asking was why does the AREA of the port have to increase. I can't get my head around it either. I can see why the length must increase, but why th diameter (or area)?


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## Patrick Bateman (Sep 11, 2006)

Rrrrolla said:


> I think what he was asking was why does the AREA of the port have to increase. I can't get my head around it either. I can see why the length must increase, but why th diameter (or area)?


Let's say it's 1995 and you have a four cubic foot vented box tuned to 30hz and it has a port that's 4" in diameter and 10" in length. You're running one hundred watts.

Fast forward to 2015 and you have a one cubic foot vented box tuned to 30hz. Because the box is smaller *you need a longer vent to achieve the same tuning frequency.* If you used a 4" diameter port that was 10" long, it would tune the box to a higher frequency. You have to lengthen the port to get your 30hz tuning frequency.

Even worse, due to Hoffman's Iron Law, you need more power to get the same SPL that you had with that 4'cf box. So now you're using FOUR HUNDRED watts to get the same SPL. So a 4" diameter port isn't going to cut it - you need a dramatically larger port to avoid port compression.

This is why passive radiators and tapped horns are popular these days. A port is like a highway or a river, the amount of traffic or water that can move along it is not dictated by the length of the river, it's dictated by the diameter.


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## oabeieo (Feb 22, 2015)

IIRC , most but not all modern car subwoofers already have very heavily damped cones and increased motor strength , hence they work in smaller boxes, I don't think you can effectively shrink the box vol down much further with such subs, IIRC the whole idea behind isobaric was double the cone and double the motor. Which is exactly what modern car subs have already done with one sub one cone one motor. 

Is it pointless to try? No, but I would consider the sub very closely before I just went and tryed modeling a iso setup for it and expect results that are worth mentioning


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## oabeieo (Feb 22, 2015)

Buickmike said:


> Yep. What I've always wondered is whether they need to directly face each other or if setting them up like in the pic here:
> 
> 
> 
> ...


Eeah those are sexy subs !!!!!


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## hurrication (Dec 19, 2011)

cubdenno said:


> I understand the port velocity. That is dictated by displacement.
> 
> I don't understand why for a given enclosure volume you would have to increase the port area just b cause you are running isobaric. The displacement remains the same. So if you calculate the port area based on the power you are running and the displacement of the driver at that power, it shouldn't change.


Because when you run an iso pair, you have to double the power to keep the same spl.

If you run an iso pair and use the same power as you did with one sub, then the port area wouldn't need to increase. However, it will be half the perceived loudness.

Port area is directly correlated to power level, regardless of the driver size or swept volume. The power level will dictate how fast the air moves through a port of a given size. If you increase power, you have to increase port area. It's just one piece of the puzzle to properly designing a ported enclosure, but sadly gets overlooked most of the time.

You can't look at ported boxes like sealed boxes. Driver displacement means nothing in a ported box. When you're analyzing a ported box, you are simply focusing on the enclosure itself because the enclosure is what is doing the work like Patrick mentioned earlier. The ported enclosure is just a resonator, and all you're doing with the sub is using it to excite the resonator.


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## cubdenno (Nov 10, 2007)

Ok, you said what I said. We are on the same page. 

The sad thing is, isobaric loading will never have the total output capability that simply using the two subs in another orientation. You are still always limited to the max output level of one sub...







hurrication said:


> Because when you run an iso pair, you have to double the power to keep the same spl.
> 
> If you run an iso pair and use the same power as you did with one sub, then the port area wouldn't need to increase. However, it will be half the perceived loudness.
> 
> ...


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## cajunner (Apr 13, 2007)

couple of problems with isobaric construction...

one, you lose your free lunch. 

when you double your driver Sd by adding a second driver, the coupling that results from a standard ported/sealed box and normally adds 3 db to the SPL, is gone when placing the cone face/face or face/basket.

this is 3 db that is irrespective, it is a hit that does not take into account other things, like how 2 coils can dissipate twice the input power, or how a vented box adds 3 db as well.

the other thing is making a suitable install for isobaric using a mounting ring that separates the surrounds from rubbing in clamshell config or a tube that works in basket/face. If you seal it airtight, the basket/face, leaves no air exchange for the motor sitting in that tiny cylinder chamber, it can and will get hot, possibly damaging the motor parts in the front woofer while the back woofer can breathe some, especially if hanging out in a vented config. As the motor in the tube heats up, it expands. Expanding air in a suspended system, means the inside woofer is being held in constant sunken spider, and the front woofer is pushed out. 

If you leave a tiny bit of leakage for the cones to let out air pressure differences due to barometric changes or just heating changes, to then equalize between long bass notes, you run the risk of having one motor being out-of-spec with the other either pushing or pulling on the air mass in the tube or cavity (clamshell) such that the pair of drivers are constantly out of spec, or working in such a way that either pushes or pulls the drivers out of alignment.

And using a porous surround and/or dustcap is not workable, you want the air between the cones to be mostly static or unchanging.

Adding a port to the mix complicates this build, and adding a passive radiator instead of a port simplifies.

changing mass of a PR is easy, you can tune a 6th order, or dual reflex using a passive on one side of the cones, and a port on the other. 

I feel like the graph that shows dual vented boxes making two peaks in response, looking like a pair of horns, is more efficient, you are decreasing bandwidth to create gain.

If you use a port to control the upper response peak, or high side vent on a dual reflex, and you put the passive on the low frequency reflex, you can adjust the tuning of the box in a different way than if you used ports on both reflex vents.

I think that boxes with passive radiators sound differently and more natural when using mid-Q and smaller drivers where the PR can easily control the driver's cone without being too heavily weighted and causing linearity issues within the PR's suspension.


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## Patrick Bateman (Sep 11, 2006)

cubdenno said:


> Ok, you said what I said. We are on the same page.
> 
> The sad thing is, isobaric loading will never have the total output capability that simply using the two subs in another orientation. You are still always limited to the max output level of one sub...











Here's the excursion chart of a box with a ported fifteen, and an isobaric tapped horn with dual tens. *Look what's happening at the tuning frequency.* The cone isn't moving. The output of the box is dictated by the port.









That's why prosound subs look like this. They're all voice coil, just pounds and pounds of copper and cooling mechanisms to keep that voice coil from melting down. *With a carefully selected highpass filter and a really big amplifier, the only thing limiting your SPL is how much power you can sink into the driver.*

Eighteen Sound Speakers - Eighteen Sound 18NLW9600 - Eighteen Sound 18NLW9600 18" woofer handles 1,800 watts AES. Eighteen Sound 18NLW9600 18" subwoofers are available here. 18 Sound speaker components.

That driver is seven hundred dollars and it can handle 3600 watts. Clearly the simplest solution is to throw this thing into a vented box and call it a day. But if you want to get loud on the cheap, isobaric WILL raise your SPL limit for a given box size IF you don't exceed xmax.

And YES there's a lot of "ifs" in that statement  You would need to replace one woofer with four woofers, you would need a bigger amp too. So this isn't a cheap proposition.









I'm a cheapskate, so the application that I find most intriguing is to take cheap woofers and squeeze them into a tiny box. IE, if you go on Craigslist you can find a lot of old sub boxes with old-school woofers for cheap. I have a couple of JL Audio 12W3s here that I bought for $60. They were in a sub box and I don't think the owner knew what they were, and that they were worth about $300 if he simply took five minutes to remove them from the box.

Get yourself a couple of Craigslist woofers for $60, throw them into an isobaric vented box or tapped horn, add a 1000 watts class D amp for $200 and you have a setup that will rival any $500 solution.


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