View Full Version : Isobaric boxes..

04-23-2009, 11:39 PM
I was just wondering with the benefits that using an isobaric loading(such as push pull) isnt used more often? ive done a search, found some information, but otherwise the only drawbacks i see is its awkward to mount.

So why wouldn't I take my 2 12s ported in a 4 cube box, and build another similar box push pull iso style for increased SQ and improved output?

04-23-2009, 11:41 PM
Isobaric = twice the power, twice the subs for half the output. Just a smaller box. Meh.

04-23-2009, 11:42 PM
well what if im already running 2 12s?

04-23-2009, 11:43 PM
you pretty much end up cutting cone area in half. less output. same power. smaller box.

04-23-2009, 11:44 PM
sobaric - Dual Drivers

Basic Theory

Two drivers are mounted together in an enclosure with a cavity of air between the two drivers. The drivers must operate in phase with each other. The cavity of air between the drivers should be made as small as possible without compromising the operation of either driver. The modeling for this type of enclosure is done just as you would any other speaker enclosure except you take the Vas of the driver and divide by 2. This will in effect make all your speaker enclosures half as big as they would normally be for any particular driver.
Advantages Improved sonic bass response. Bass is claimed as being tighter, faster, more accurate and more pure. Vas of the driver is cut in half. The volume of enclosure required to obtain a specific frequency response can be achieved in only half the volume. This is where isobaric enclosures have their biggest advantage.
Disadvantages Wasted amplifier power to driver the internal sub. Efficiency of the system is down 3 dB as compared to a single driver due to the added cone mass and the reduced Vas. When you compare isobarics to a system which houses two drivers each in their own enclosure, this system would actually be 6 dB less efficient.
Best Applications Where size is a big issue. When you want the box to be very small. Where more accurate bass is more important than lots of bass. If you have a hefty amplifier with plenty of juice to spare and a driver that can handle a good amount of power. Suited for music, home theater and car.

Compound ~ Push/Pull Dual Drivers
Basic Theory

Two drivers share an acoustic volume of air within a single enclosure. The best way to take advantage of this alignment is to mount one driver facing outwards with the other driver inverted and facing inwards. The drivers are then wired so that they are electrically out of phase while remaining mechanically still in phase with one another. Odd ordered harmonics are cancelled out by using this approach according to Vance Dickason. According to M&K who specialize is push/pull subwoofers claim that this approach cancels out even ordered harmonics. So take your pick. Either way, harmonic distortion is reduced in that any anomalies or variations in the two driver's spider, cone or suspension characteristics are canceled out by the other driver's inversely proportional anomalies and variations. The sound is as accurate and pure as it can possibly be with each driver "correcting" the other driver. Of course many times two drivers will share the same acoustic volume of air while maintaining the more traditional look of having both drivers fire forward into the listening environment. Though this does not have the same harmonic cancellation effect, all other characteristics between the two alignments is identical. Box volume must be twice that of a single driver. This can be easily modeled by taking the Vas of a single driver and multiplying it by two. The system has an increased efficiency of 3dB over a single driver. Power handling for the system is twice that of single driver. Frequency response is the same for a single driver in an enclosure excaly half the size.
Advantages Increased output and power handling. Very high SPL capability.
Disadvantages One single huge speaker enclosure that may be both unattractive and hard to move. Response it essentially identical to building two smaller enclosures of exactly half the size but without the versatility of placement of two separate subs. If one of the drivers blows due to too much power, the whole speaker will no longer perform because the blown driver will begin to act like a poorly-tuned passive radiator. There are no real disadvantages to building this kind of enclosure as the speakers will behave just as they would in enclosures by themselves. It's very common to make MMT style speakers and use the two drivers in the same enclosure.
Best Applications

Where one sub just isn't enough. High power high output applications. If you choose to do the push/pull configuration, the sonic advantage may make this sub more suitable for audiophile music and critical listening experiences.

04-23-2009, 11:47 PM
thanks lincoln. already read the entire post, shoulda given reference other than "used the search button" very informative none the less.. so since you use them in the push pull config you actually lose SPL?

04-23-2009, 11:48 PM
so could you not do 4 12's in lets say a 4 cube box and get alot more output?

Rich B
04-23-2009, 11:50 PM
The big plus with isobaric is better control of the woofers, they have a crisper sound because they start and stop their motion better.

I like them myself, but to the boom crowd all they see is that you lose some output by using isobaric.

Of course most component setups up front cant keep up with the amount of volume many people have in subs anyway, and a good isobaric will allow a better balanced system.

Boom or quality, like anything theres always a trade off.

04-23-2009, 11:52 PM
i see, so disadvantages of them truly are less SPL for SQ then..

Rich B
04-24-2009, 12:00 AM
Yeah, about 3db's less output.

If the amp can handle the low impedance, like you asked above you can use 4 subs (two iso'ed pairs) to gain that 3db's back, and it would still only require a box roughly equal to 1 sub.

(2 subs= 1/2 the size for one sub, so x2= a box equal to a single sub. Add about .25cf for each sub in total box size.)

04-24-2009, 12:17 AM
thanks lincoln. already read the entire post, shoulda given reference other than "used the search button" very informative none the less.. so since you use them in the push pull config you actually lose SPL?

no problem, i had the infinity isobaric cubes, dont remeber the model 2 for a total of 4 subs, change them out for some punch dvc's never turned back, the infinity's sounded much better but the dvc's werent bad in a sealed box with much more output