# Isobaric enclosures have reduced distortion



## The Baron Groog (Mar 15, 2010)

Morning guys,

This thread is an attempt to find out for sure whether running a pair of subs isobarically (cone to cone or mag to mag) will reduce distortion or increase it.

I had always read and been told that isobaric mounting of subs in cone to cone or mag to mag format would result in:

Lower distortion, through cancelation of 2nd order harmonics due to the opposite motion of the drivers
Reduced box volume for the same FB 
Or 
Better extension from the same sized enclosure

This link: Isobaric details one theory behind the claims that isobaric does not reduce distortion-but I can't help but think that 1000s of audio fans and manufacturers have tried it and experienced reduced distortion so the theory must be flawed. Obviously they could all be suffering from the same psychoacoustic delusion or perhaps they're not audiophiles at all and the extra distortion was heard and liked-I think most are aware that increased distortion can be perceived as increased loudness.

Neil_J has agreed to do some testing with his SWR-843Ds-but has no wood working tools and needs a spacer for between the two drivers to be clamshelled, if anyone can help please sort him out.

Otherwise I was wondering whether Erin (Bikinpunk) would be willing to get the klippel out for this one-presuming you can klippel the drivers when mounted isobarically.

Now, let the games commence!


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## Gary S (Dec 11, 2007)

If one of the drivers has a lot of distortion one way it will lower the distortion.

However, whether you will hear a noticeable difference and whether or not it will be pleasing is debatable. You would have to try it and judge for yourself.

It will drop the box size requirements but lower the efficiency - you have a power an extra woofer but get no increase in spl.


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## The Baron Groog (Mar 15, 2010)

Bump! Common guys-no takers?


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## Candisa (Sep 15, 2007)

I won't vote, because I don't think this should be a "race" between the different options, but here is my opinion:
- Putting a subwoofer-motor in the listening-space results in more unwanted, audible motor-noises. How hard you will hear them depends on the quality of the subwoofer, but there are very little subwoofers out there that have an inaudible motor, especially when driven hard.
- Identical subwoofers moving in opposite directions will cancel out *some* of the distortion and enhance other parts of the distortion. Identical subwoofers moving in the same direction won't cancel out any of the distortion.

If you add up these things, you might (I do) conclude a mag2mag tunnel is the best way to do isobaric, but there are drawbacks to this solution:
- It isn't the most compact solution, in fact, since most subwoofers nowadays require rather small cabinets, I doubt it will be even a little more compact to a regular non-isobaric setup with half the drivers, since that tunnel with both woofer-baskets and -motors in it takes up quite a bit of space, which isn't part of the "air-spring".
- Since there's quite a bit of air in that tunnel, especially when using subwoofers that have a pole-vent (so you have to leave 1" of space between the motors for cooling), the isobaric-effect will be decreased, since the air in the tunnel is compressable
- As in all isobaric setups, you have to apply 4 times the power compared to a regular setup with half the drivers, to become the same output level. This might create a lot of heat in the motors (which are mounted in a small sealed tunnel!), increasing the amount of distortion, further decreasing efficiency and possibly damaging the coils...
- As in all isobaric setups: you need twice the amount of drivers to move the same amount of air, so the Vd-to-price ratio decreases by 50% on the subwoofers only (knowing that you also need 4 times as much power...!).

IMHO, isobaric is only worth doing when using very efficient woofers with very high power handling, requiring a box large enough to actually save space when doing this...

Most nowadays subwoofers have a rather high power handling, but aren't thàt efficient and do not require a very large box to create a .6-.8 Qtc...
Result: a very, very low efficiency system that probably isn't any smaller than the comparable regular system.

Another interesting fact: the human ear is very insensitive to distortion in the subbass-range...

My opinion: with modern subwoofers, there is no real (=audible) reason to do an isobaric setup, only a lot of reasons to *not* do it...

Isabelle


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## chad (Jun 30, 2005)

Who cares, it's a subwoofer... In a car (I believe.)


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## Candisa (Sep 15, 2007)

^^Indeed, no audible benefits to expect from this, only a lot of heat, current-draw and money-loss...


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## Hillbilly SQ (Jan 26, 2007)

In a car I wouldn't bother. There's hundreds of car audio branded (and home) subs available that test well and do just fine in a car. I've found that if a sub is "too clean" it leaves you with the sensation of no bottom end. That's why I've never cared for the extra low distortion subs that have been boners over the years. With iso cone to cone you have motor noise. Magnet against cone with cone side out or magnet against magnet the space savings becomes a moot point.


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## ChrisB (Jul 3, 2008)

Isobaric enclosures are the best way to make two subs perform as one. I guess if you have a crappy sub, it's worth it, like back when it was a buzzword in the late 80s to early 90s. These days, I see absolutely no need to go with an isobaric mount in a motorized vehicle, unless one is stuck in reliving their past. Going further, if one is stuck in reliving the past with their "older is better" mentality, there is no help for them. Crap, I should have made another myth thread with that in it!


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## Jonny Hotnuts (Mar 15, 2011)

The cone traveling through the air forward and backwards will have a slight advantage going from positive phase (inwards) then negative phase (cone going outwards) due to the aero properties of the cone itself. The concave side of the cone (bowl) acts like a parachute where the back side of the cone (convex) is more aerodynamic. 

To see if I am full of it, measure the amount of pressure load on a cone facing a fan (in vision the '=' is a fan) and then turn it around so cone is facing the opposite direction (for that matter hold a cone out of your car window driving down the road, first point to wind and then turn it around and see if you can hang on to it). 
IE:

=> or =< 

clearly the fan blowing into the > (or cone) will have a higher load %. 


An iso setup lowers distortion because the cone will have identical pressure loads, no matter what direction the cone is moving.

=<>= 


food for thought.....


~JH


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## chad (Jun 30, 2005)

Ummmm we have a klippel for this.


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## Candisa (Sep 15, 2007)

Jonny: That would be a fair test if you'd want to explain the asymmetric response of a woofer playing in an unbaffled dipole setup, because the woofer itself is more aerodynamic on it's back side...

But as soon as you put the woofer in a baffle, you have to check the aerodynamics of the woofer+baffle and then you'll see a much smaller difference at both sides.
In a box, the difference will be cancelled entirely, because the air in the box has no way to go and the force needed to compress a certain amount of air by x% is the same as the force needed to expand that same amount of air by that same x%.

If you want to avoid asymmetric response of the woofer, there is only 1 variable (except for DC offset in the amp) that one should take care off: make the leakiness of the car symmetrical, so the force needed to leak a certain amount of air out of the car is the same as the force needed to suck that amount of air in.

When standing still, with the blower off, that's not hard to reach, but in a driving car, the only way to do this is by making the car's cabin airtight, which will result in a very bad interior climate and lead to suffocation very rapidly...

An isobaric setup will not solve this issue, not even a little bit, it only solves *some* of the distortion caused by asymmetric response of the woofers themselves, which shouldn't be an issue when using good quality woofers (costing less than twice the amount of lower quality woofers)...

Isabelle


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## The Baron Groog (Mar 15, 2010)

Candisa said:


> I won't vote, because I don't think this should be a "race" between the different options, but here is my opinion:
> - Putting a subwoofer-motor in the listening-space results in more unwanted, audible motor-noises. How hard you will hear them depends on the quality of the subwoofer, but there are very little subwoofers out there that have an inaudible motor, especially when driven hard.
> - Identical subwoofers moving in opposite directions will cancel out *some* of the distortion and enhance other parts of the distortion. Identical subwoofers moving in the same direction won't cancel out any of the distortion.
> 
> ...


Nice to see you back-how are the SAAB and the house going?

Funny I would argue cone to cone would give the best results-provided you have a quiet motor.

Yes, I would agree that the subject is largely moot now that the majority of subs for car use are low sens, high MMS + big motors-so effectively an iso sub without the hassle-however just because it's not hugely important for car specific drivers it may be of benefit for users of PA drivers or other high sens, low MMS and motor power drivers.

Regardless of whether knowing is of any importance I WANT TO KNOW!!! I don't need to know the worlds entire domestic population of golden hamsters is descended from one female hamster caught in 1930-I'ma geek and can't help it!



chad said:


> Ummmm we have a klippel for this.


Yup-this is what I want to see-actual results, conclusive proof, fact not fiction-Bikinpunk where are you


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## Candisa (Sep 15, 2007)

Well, "being back" is quite overstated, this forum is way too busy for me, mostly with very basic subjects that just don't fit the reason why I joined this forum... But I know there are some people "from the old" days that still hang around here, so I "came back" to post our project and give my thoughts about the few subjects that I do find interesting...

The house is improving bit by bit, but that costs quite a bit of money, so it's a work in very slow progress...
We still own the Saab and we actually decided we want to get it reliable. The car-tax system here in Belgium changed very recently, causing buyers of older cars (below 25years, that's another system) to pay a huge amount of money to get it licenced...
They did that to promote young, "ecological" cars... (And still no discount if you run it on LPG, even though this reduces the CO-emission a LOT)
The cost to *keep* it licensed didn't change yet, so for now the running costs don't change for us...

Selling the Saab would be pointless since nobody wants such a car anymore (it costs so much money to get it licensed, that they could as well buy a much younger car and pay the same for buying+licensing), unless as a donor car...
So we decided to keep it and actually start building an install in it. I didn't take pictures of the progress yet, but there are some drawings and a description in the project page I made. You can find it in my signature. Feel free to subscribe.

On topic: the noise coming from the motor of *any* decent subwoofer will be louder than the THD coming from the cone... Cone-2-cone is the easiest, most compact and most "efficient" solution to go isobaric, but it certainly isn't the cleanest sounding one...

Isabelle


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## chad (Jun 30, 2005)

Candisa said:


> The car-tax system here in Belgium changed very recently, causing buyers of older cars (below 25years, that's another system) to pay a huge amount of money to get it licenced...
> They did that to promote young, "ecological" cars... (And still no discount if you run it on LPG, even though this reduces the CO-emission a LOT)
> The cost to *keep* it licensed didn't change yet, so for now the running costs don't change for us...
> 
> Selling the Saab would be pointless since nobody wants such a car anymore (it costs so much money to get it licensed, that they could as well buy a much younger car and pay the same for buying+licensing), unless as a donor car...


Because the disposal of older cars and the pollution involved in producing the newer cars is SOOOO much better for the environment 

I hate theories like this. Good for you for keeping it.


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## Candisa (Sep 15, 2007)

That's what I think about it too... Untill now, if you re-licensed a car that didn't have license for a while, it was free to re-license it again. It is unclear at the moment if this stays that way, but if it does, chances are that my black-smoking old Volvo will be used again in the future.
I was planning to sell it and buy an Audi ur-S6 sometime this year, but since it would cost a whole lot of money to license it with this new system, I might as well keep the Volvo and save for a newer car (probably a slower one than an ur-S6  ), or I should find a typ44 200 Avant (same 230hp 5cyl turbocharged engine, same quattro drivetrain, but >25yr so different tax-system)...


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## TOOSTUBBORN2FAIL (Jan 31, 2011)

chad said:


> Because the disposal of older cars and the pollution involved in producing the newer cars is SOOOO much better for the environment
> 
> I hate theories like this. Good for you for keeping it.


I always thought this was funny. My car is 14 years old, gets 40mpg (like my wifes 2 year old civic), doesnt have as much road noise as my wifes 2 year old civic, and is funner to drive, and being bought used, the enviromental damage from producing the car was already done.

For me, the only reason my wife is in that 2 year old civic, is safety, not the enviroment. Her car is much safer.


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## sqshoestring (Jun 19, 2007)

I thought people did isobaric way back when because they didn't have today's huge motor high power subs....that act just like the old subs do when paired isobaric?

I fail to see why distortions would be less isobaric, but I can see how they could be more as you are running twice the drivers for the same output. Much greater chance of motor/cone noises mostly.


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## The Baron Groog (Mar 15, 2010)

sqshoestring said:


> I thought people did isobaric way back when because they didn't have today's huge motor high power subs....that act just like the old subs do when paired isobaric?


Yes, as stated earlier in the thread modern subs have much heavier cones, higher powerhandling and bigger motors-so the need for isobaric mounting has been significantly reduced-but I still want to know!



sqshoestring said:


> I fail to see why distortions would be less isobaric, but I can see how they could be more as you are running twice the drivers for the same output. Much greater chance of motor/cone noises mostly.


The reason that they're supposed to have less distortion is because the reversed motion of one of the drivers is supposed to cancel nonolineararities-same as when you run any other enclosure with two drivers and one is flipped. Provided the driver you're using doesn't have a noisy motor the theory was that it would reduce distortion overall.


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## Candisa (Sep 15, 2007)

It's time for the magic subwoofer-triangle:
Efficiency - Low-end extension - Compactness
You can pick 2 of these...

Older subs were quite efficient, but needed a big box to have proper low-end extension.
By going isobaric, you trade in 6dB of efficiency to get more compactness.

Most new subs don't need a big box anymore to go deep, because of their powerful motors, but this requires more power = lower efficiency.
You might get a little more efficiency without sacrificing low-end by using a bigger low-tuned ported box instead of a smaller sealed one and this way, you're back in the old days: higher efficiency, good low-end extension, but a huge box...
So then you could go isobaric to make the box more compact and trade in some efficiency...
But why would you? That will bring you back were you started: low efficiency, good low-end extension, and I'm pretty sure the efficiency will be even lower than it ever was, but the low-end extension and compactness won't be better and it'll cost more money.

The only advantage is you can dissipate more power, but if a box is designed well, the mechanical limit shouldn't be higher than the electrical limit, so there is no need for that extra power...

Isabelle


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## Oliver (Jun 25, 2007)

Jonny Hotnuts said:


> The cone traveling through the air forward and backwards will have a slight advantage going from positive phase (inwards) then negative phase (cone going outwards) due to the aero properties of the cone itself. The concave side of the cone (bowl) acts like a parachute where the back side of the cone (convex) is more aerodynamic.
> 
> To see if I am full of it, measure the amount of pressure load on a cone facing a fan (in vision the '=' is a fan) and then turn it around so cone is facing the opposite direction (for that matter hold a cone out of your car window driving down the road, first point to wind and then turn it around and see if you can hang on to it).
> IE:
> ...


Tang Band W6-1936S 6-1/2 Aluminum Cone Subwoofer = "flat coned subwoofer"

What about 2 of these ?



> Product Features
> 
> Full-range capability
> Underhung voice coil
> ...


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## hurrication (Dec 19, 2011)

I have been modeling a lot of 6th order bandpass setups lately and have thought about isobaric drivers as a means to reduce the box size. I have been able to model out some flat-responding low-group-delaying boxes that are 1/4 the overall chamber volume of both sides when the drivers are paired isobaric compared to not. The port lengths seem to want to increase 4x in length, though, for the same cross sectional area. 

Here is one I did with some JL 10w0's. 4 subs in 7 cubic feet before port vs. 2 isobaric pairs in 2 cubic feet before port. What's interesting is that 4 isobaric pairs in 4 cubic feet before port models identical to 2 pairs in 2 cubic feet (duh), so you could theoretically have the same output and response as 4 non-paired subs in a little more than half the space yet needing twice the power. But.. we're talking about 150wRMS per speaker here so it wouldn't shatter your bank account or your alternator!


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## jpeezy (Feb 5, 2012)

it cancels out odd-order non-linearities,it is also extremely tight and accurate,tiny box,yes porting can be tricky,(the ports do get long and the enclosures are usually small),they are not meant for spl. i did a setup in a crx probably (showing my age)almost twenty years ago,it used 8 jl 10w5 in a v-shaped enclosure,ported,in the hatch area he could still use the front and top cargo cover,this was running off a single ppi 2300am running two ohm stereo,that thing would play the low low bass notes so well,i mean like 30-60 hz, not 60 hz and up.real low bass.i think he actually had to replace the seals in the hatch.loud car.but amazingly accurate,especially if you like bass guitar and kick drum,wow .


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## JayBee (Oct 6, 2006)

I ran an isopair, face to face, back in 92-93 for about 2yrs. I was trying to get the smallest enclosure possible in the back of my beretta. It turned out well and would do it again if space was an issue an i had an exact extra driver to burn. However, i doubt i would go out of my way to replicate it.


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## BP1Fanatic (Jan 10, 2010)

I ran 4 MTX RT2154's in a 3 chamber BP4 isobaric box in a 1983 Mercury Grand Marquis. I had an Autotek 7300BTS running 1 ohm bridged. That thing talked to whales. It had 4 ports corner loading the rear of the trunk. Tuning was 60 hz. When I donated the car in 2002 after the head gasket blew, we had a garage sale that same day. The box was the 1st thing that sold. I miss that car!!!


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## chuyler1 (Apr 10, 2006)

Sorry to dig up an old thread, but it's literally the only one discussing Isobaric enclosures. Let me preface this by saying I scored a flea market buy of a pair of JBL 12"s from the early 90's. The owners manual recommended enclosures pretty much outline what has been discussed above. If you can afford more power/subs, but cannot afford more space, Isobaric lets you build an enclosure roughly half the size, or another way to clearly look at it, if you only have 2 cuft to work with, you can lower your -3db cuttoff from 42Hz to 32Hz and it only costs you (in this case) an additional 140 watts and another sub.

That does have me thinking, like hurrication above, could you reach a goal today? By purchasing two cheaper subwoofers could you equal the output (or at least desired frequency response) of one high powered subwoofer without paying more? Could a pair of 12W0s ($200/ea -- rated 300w) sent 500w RMS equal or surpass that of a single 12W3 ($400 -- rated 500w) sent 500w RMS? 

Even if the theory checks out, there's also the issue of mounting them with today's subwoofers not really designed for being clam-shelled together.


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## BP1Fanatic (Jan 10, 2010)

Yes. 

And BTW, I just got a 2011 Crown Vic LX last Friday.


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## BP1Fanatic (Jan 10, 2010)

And, she under the knife right now for travel to Cleveland tomorrow.


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