There's definitely pros to isobaric designs, but I think you've already answered your own question as to why. Plenty of subs on the market that will perform very well in small enclosures (even ported if you don't mind the vent lengths) to solely concentrate on using that style sub enclosure. For cheap sub use that makes it entirely cost effective over other options, then perhaps it's worth considering. The other reason would be for some elitist who is trying to reduce distortion at any cost.

 

I've considered going iso as well but with all the subs out now that work well in small boxes and power being cheap to run them with it's just not something people do anymore. Back in the old days when subs needed huge boxes iso was common. Hell the 10's I'm running I could put the PAIR in .5 net sealed and have a perfectly acceptable response. Or run the pair ported in 1.2 tuned to 35 and have a great ported setup. In the old days a pair of 10's would have needed 2-3 cubes sealed minimum or 4-5 cubes ported from what I understand. Little before my time obviously.

 

most of the reason people dont use it anymore is because speakers dont need the volume they used to.

old school 10" sub might have required 1.5-2 cuft of space, now they require 0.5-1.0 cuft. the VAS are lower, which is what you are decreasing by an isobaric ssetup anyway, that is why you can use half the airspace for two sub that would have for 1.

another reason is sensitivity. you lose 6db with an isobaric setup. yes, linearity and distorsion is decreased, power handling is increased, if you are ok with those trade offs, then by all means do it

 

+1 Best reasons denoted.

 

no that comparison is

1 sealed 12" sub VS 1 sealed isobaric pair of 12" subs.

the isobaric pair are going to be 6db down and the box will 1/2 the size

if you have 2 sealed 12" subs vs 1 isobaric pair of 12" subs

the isobaric pair will be 3db down and the box will 1/4 as big

this assumes you are not adding any power, that graph is done with 300 watts system power.

 

Correct.. I think most people misunderstand what a fair comparison would be. Even though it is a pair of subs, it's output should only be compared to a single sub. Then the advantages will become clearer with the only downfall is the extra power requirement. However, most who go that route highly praise the lesser amounts of distortion even though a more modern "clinical" sub would be as effective. Just like stated earlier, you have to remember the time when it was popular. Subs with shorting rings & high xmax was almost unheard of in car audio.

 

It's a mystery to me why it's not more popular. Here's some reasons that people might try it:

#1 - If you have sufficient power, then the limits on your output are generally displacement and heat, in that order
#2 - If you have sufficient power, two drivers mounted isobarically will generate approximately 3dB more output than one driver
#3 - isobaric mounting 'spreads' the heat out over two voice coils. Therefore, your power limits are higher than with one driver.


As I see it, the argument that isobaric is lower in efficiency is basically moot, because if you have enough power then you're generally limited by the xmax of the driver. And isobaric gives you more output for a given box size.


Personally, I think the easiest way to visualize isobaric is to picture it like this:

Let's say you have unlimited funds, and unlimited power. Four drivers mounted isobarically in a box will generate 6dB more output than one driver in the same box volume.




There *are* two reasons NOT to go isobaric:

1) If you have limited funds, than isobaric might not be the best option. For instance let's say you can hit 120dB with one woofer that costs $100 in a one cubic foot box. To get to 126dB you'll need four woofers in a box that's the same volume, and you'll need more power to get there. (twice as much iirc.) So that could mean that going from 120dB to 126dB could cost you an additional $400 or so. ($400 for four woofers instead of one, and $100 more to double your amplifier power.)

But even this argument is a little iffy in my opinion.

Instead of using four woofers in an isobaric, I would generally recommend using TWO HUGE woofers.

For example, let's say that you can hit 120dB with one 10" woofer that costs $100. Instead of using *four* woofers in an isobaric box that's the same size, I would recommend using *two* woofers that require a box that's twice as big. Does that make sense?

IE, don't use four tens mounted isobarically in a one cubic foot box. Use two fifteens mounted isobarically in a one cubic foot box.




Last but not least, there IS one reason I'd pass on isobaric mounting. And that reason is a very simple one. The sheer size of the drivers can get out of hand.

IE, all these theoretical benefits of isobaric mounting lose a bit of luster when the sheer size of the basket gets up to 20% or even 50% of the box size. Again, this is a reason that isobaric is a lot more attractive with giant drivers. (The last isobaric sub I did was a passive radiator box with dual fifteens.)



You can crunch all these numbers in hornresp of course. It will show that isobaric subwoofers always generate more SPL for a given input level than a conventional sub, as long as you have sufficient power. That last caveat is a big one, but cmon amplifiers are cheap. Walmart has a 4000 watt amp (not "max watts", we're talking real RMS output) for $250

 

Sorry, but this its the first time we disagree. Isobaric is a 6DB loss vs 1 normal sub. That its not a benefit

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Output is less for the same voltage.

Maximum output is higher.

That's the magic of isobaric. If you have sufficient power, two drivers in an isobaric pair will produce approximately 3dB more ouput than one driver.

The easiest way to visualize this is to understand that output is dictated by displacement. It doesn't matter if your driver is 18" in diameter or 1" in diameter, output is dictated by displacement. A 1" driver will need more xmax to produce the same output that an 18" can, but the central factor is one thing:

Displacement.

For instance, if you have two drivers with a 12" cone, and one driver has a sensitivity of 110dB, and the other driver has a sensitivity of 70dB, they both need the exact same displacement to generate the same SPL. Sensitivity has zero bearing on output if you have sufficient power. (This ignores thermal compression, but thermal compression is better for isobaric anyways, so it's 'safe' to ignore it for this comparison.)

So let's say we have two subwoofer enclosures. One enclosure uses a 12" driver with one cubic foot of air behind it. The other enclosure uses *four* of the same driver, with one cubic foot of air behind it. The second enclosure has the exact same alignment as the first enclosure, because four drivers mounted isobarically need the same amount of *one* driver mounted conventionally.


Now what's the difference between the conventional enclosure with one driver, and the enclosure with four drivers mounted isobarically?


That's right : displacement.

The subwoofers mounted isobarically have twice the displacement, because the cone area is doubled.




There *are* some caveats on this:

1) The drivers must be within one quarter wavelength. But this is easy to do with subs. Eighty hertz is fourteen feet long. Get the drivers within 3.5' and you'll be fine.
2) The sheer volume of the baskets becomes a problem. Probably not the best option for sealed boxes, but very nice for ported boxes, back loaded horns, tapped horns, etc
3) For the life of me it's never made any sense how/why second harmonic would be affected. I don't 'get' the arguments that isobaric push-pull lowers it. If you want to lower 2nd harmonic, I personally think that the basket of one driver and the cone of the other would have to be exposed, but you'd need four drivers to do that. (or two drivers in a conventional box. Basically one cone needs to be exposed, and one basket.)
4) Isobaric is completely pointless if you don't have enough power to reach xmax. Well, I guess not *completely* pointless because it *does* reduce thermal compression at any voltage level.

 

But the displacement from the "internal" cone doesn't hit free-space. The displacement of the cone that plays into the listening environment (the visible woofer) still has its same physical limitations WRT displacement (Excursion x Surface Area). So how would the isobaric's internal cone increase SPL (total displacement) in the listening environment when its still limited by the outside driver's design specs (total displacement @ Xmax)?

I always thought Isobaric enclosures were for nothing more than to allow a smaller box as long as you are willing to double-up on drivers and power (which volume reduction - as stated - was more of a benefit in the 90's than today).

 

I believe you are correct if output is excursion limited. Then the displacement is identical and therefore spl. If it is thermally limited it would make sense that isobaric would offer an increase in spl.

 

Here's some sims to demonstrate what I am talking about.
In the example I've pitted four eights against a pair of fifteens mounted isobarically.
I did this because four eights basically have the same displacement as one fifteen. (And two fifteens mounted isobarically basically act like one fifteen with a sensitivity that's been reduced by 3dB and a VAS that's half as high.)

Details here:

http://www.diymobileaudio.com/forum/1836048-post48.html

If you don't want to sift through the whole post, the basic point is that the isobaric box is smaller, cheaper, and has the same maximum output.

 

Noooo...maxium output of the isopair is not more than a single.

 

I crunched the numbers in Hornresp, and Jason's statement is correct when it comes to one driver versus an isobaric pair. According to Hornresp, maximum output is identical.

A few notes on this:

1) Four drivers mounted isobaricly DO offer six decibels more output, with the same amount of air in the box. (Because we have double the cone area.)
2) I'd be willing to bet that 'in the real world' the isobaric pair will generate more output than a single driver, due to reduced power compression. For instance, if you look at the power compression measurements performed by Wayne at Pi Speakers, as you approach the thermal limits of the driver additional power barely increases output at all.

Basically what happens is that the driver gets hotter and hotter, and heat raises the impedance. So as you increase the power, you increase the heat, the heat increases the impedance, and the rise in impedance lowers output.

Oddly enough, this is actually another argument for tapped horns. Due to their multiple impedence 'troughs', I'd expect that the peaky frequency response of tapped horns would 'smooth out' at high power, because heat will basically smooth out the impedance curve. (IE, at one watt your impedance minimum looks like four ohms, but as you approach thermal limits it may go as high as six or even eight ohms.)

 

Not sure what kind of amps and speakers you run - but I wouldn't touch that amp with a 10 ft pole I prefer Crest "CA" series (CA12) and QSC "RMX" series (RMX 1850HD x2 + RMX1450) amps and JBL VGC drivers in my PA System, thanks

My vehicle's sub amp was $800, subs were $300/ea or so. Multiply that by 4, plus extra batteries, a HO Alternator (I'm running a stock alt with the current setup) and I'd be in a world of hurt

You also can't gloss over the additional volume needed for the coupling chamber between drivers, extra baffle and bracing. A 1cu ft enclosure for a single 12" is going to end up a lot bigger than that for 4x 12's (2 iso pairs with 0.5 cu ft for the rear chamber).

Like I said - I'll pass. Volume requrements are so small today - there's just no need for the additional expense when a good 12" is plenty happy in a cube or less...

 

You can do clam shell and not need more space than required. But you still need room for magnet, etc to sit in

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You can do clam shell and not need more space than required. But you still need room for magnet, etc to sit in

Not familiar with a "Clam shell Isobaric" design.

The magnets on my W12GTi's are a good 11"-12" deep - so there's another issue - the drivers would be too far apart (too much volume in the ISO connecting chamber). I'd actually have to swap to lesser drivers to "allow" an isobaric setup - again negating its usefulness at all IMO. I just don't see the need or desire for Isobaric in 2013. In 1989 when a 12" needed 2.5 cu ft and only handled 250WRMS - sure

 

Clam shell = bolt one sub on the other. The cones face each other. One sub in the enclosure, the other sub is not

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Clam shell = bolt one sub on the other. The cones face each other. One sub in the enclosure, the other sub is not

Ah - Of course! I can be a bit thick-skulled at times! But in my case that would eat-up a good deal of my usable cargo area (subs/magnets would protrude a good ~11" into my cargo area) - further negating the proposed benefits of Isobaric in the first place (and the box still has to be at least 11" deep to accommodate the inside woofer). I think it is safe to say I would never have a legitimate application for Isobaric - bar none. I guess others could find a way to create the need

 

Lol, I am not defending it. I have used it on the past. But as others have said, it was when a 10" sub needed 2cuft of space.

There is little use for it now

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I love small boxes. So I got a wild hair a few weeks ago and tried an isobaric alignment with a pair of my 12's.

The box. My 12's like ~ 2cf ported so I settled on 1cf for the box. But porting a 1cf box to 30hz is a PITA, so I went with a PR.

The box was tiny (relatively speaking), but the whole rig really wasn't all that small. The ass of the exposed 12 stuck out, it was crazy top heavy so I had to fab up a bracket to keep it put and honestly... the output wasn't there. It sounded... "muffled." I have a ton of power on tap too so that's definitely not the problem. Maybe it was my design, but after just a few days, I gave up on it.

The other thing is the weight. That little rig weighed in at ~ 50lbs. When I can use half the power, half the weight, but double the size for the same (or more) potential output. I'm not saying I'm giving up on isobaric alignments altogether, but I may need to rethink my strategy.

 

I wonder if this was due to the low distortion?

This is something that I found odd about push-pull subs. They definitely sound different, but I'm not 100% convinced that they sound 'better.' They DO sound quieter, which may be the same as sounding 'muffled' as you describe it.

Basically in subwoofers the harmonic distortion is often more audible than the fundamental. This is due to a couple things. First, our hearing mechanism sucks at low frequency; we 'feel' bass more than we feel it. I used to have a 10hz tapped horn in my bedroom (Night of The Living Bassheads - diyAudio) and I found that the most amusing part of having this beast was that it would shake the room. But it wasn't that I could *hear* the bass, it was that it had so much output at 10hz it would knock you around. Visceral not audible.

The other reason that the distortion is more audible is that it's so high; in some subs you have something like 10% distortion at one watt, so you can only imagine how high it is at 2000 watts!

Geddes has done some studies on this, and basically argues that harmonic distortion doesn't matter until it reaches a point where it's really offensive. (IE, don't worry about harmonic distortion as long as your design isn't pushing your drivers to their mechanical limits all the time.)


Here's some more musings on push-pull subs: Push Pull vs Normal Distortion Compared - diyAudio

At lower power levels, where the Lab12 is quite clean, no difference could be detected. When pushed at a power level where distortion could be noticed, the most distracting artifact was the audible vent noise from the reversed cone of the PP.
The lack of the second harmonic, an octave, made the now predominant third harmonic distortion more apparent in the PP cabinet. This is music related thing, the third harmonic , being a perfect fifth, may sound OK with some compositions, while sounding “wrong” with others.

Although the technical side of me says reduction of distortion of any sort is a good thing as far as a reproduction system is concerned, both my old ears, and a college freshman apprentice thought the push pull cabinet sounded less “musical” when pushed hard."

 

One other little side note on the potential output and power compression...

I did some SPL testing the other day. Granted, this is SPL and SPL only so the numbers are way out there, but I found it interesting.

I was testing a single 12" sub. At 500w clamped power I got 149db. At 1,200w clamped power, I got 152.4. That's slightly more than double the power for slightly more than a +3db gain. Which is right in line with the theoretical. BUT, when I upped the power to 2,600w clamped, I "only" got 153.6db. That's more than double the power for only a +1.2 decibel gain.

Perhaps I will revisit this exact test by dropping in an iso pair. That will be very interesting indeed.

 

Yeah this might be the main advantage of isobaric. Nearly all drivers suffer from TONS of thermal compression. Basically you reach a point where adding power to the woofer won't generate any more output, you just hit a wall.

Here's some data from Wayne Parham. The measurement above is at 100 watts and at 200 watts. We see that Wayne's sub gains about three decibels when you go from 100 to 200 watts. (Which is what the sims would predict.)

Here's the output at 800 watts and at 2000 watts. Even though Wayne nearly tripled the power, output is virtually identical. There's no way that anyone could hear the difference between 800 and 2000 watts here. And this driver is rated for 400 watts each, and there's two of them, so we'd expect that 800 watts shouldn't be "too much." But as Wayne's measurements illustrate, there is really no point in putting more than 800 watts into this box

Basically Wayne's measurements demonstrate that there's simply no point in exceeding 400 watts per driver with this particular woofer. This driver has a 2.5" voice coil, so it's size is relatively consistent with your average car audio driver. (Randy's W12GTIs use a 3" voice coil iirc, and the W15GTI does too.)


Here's Wayne's measurements : ProSpeakers Forum - Re: Hornsub shootout RESULTS - 12? - Wayne Parham, October 15, 2005 at 14:20:02

 

In my last post, I mentioned that the distortion can be more audible than the fundamental. Here's an example of this. In Wayne's horn sub, the distortion at 100hz hits about 90dB. But our sensitivity to 100hz is about 10dB higher than our sensitivity to 50hz, as illustrated by the Fletcher Munson curves. (The second image above.)

So that means that out perception of the distortion is just ten decibels lower than the fundamental! So push pull makes a huge, audible difference. Of course, that leads to the question of whether that difference is a good one or a bad one!

And this is not a small sub by any means:

 

Well you yourself mentioned before that people actually like distortion so.....