Isobaric sub enclosures
One of my first subwoofer boxes was a simple dual 8″ setup in a 12″x 22″ x 10″ box. That small 1.6 Cu.ft box was more than enough to fill up and rock out my 1991 Celica GT. The subs in the box were facing upwards and reflected beautifully off the glass hatchback door and into the cabin.
The thing is, a lot of first time diy’ers build their first DIY Subwoofer box using a single undivided enclosure with dual subwoofers and a 2 channel amplifier. Compared to having no bass at all, then a single enclosure with dual subs will sound like the greatest addition to the car’s sound system since sliced bread. I know I for sure loved mine, just as much as my friend Julian loved his dual 10s in his CRX hatch, and as much as my friend Dan love his dual 12s in the trunk of his Oldsmobile.
If you think about a typical single sealed box subwoofer install, then the amount of air trapped inside the enclosed and sealed box is fixed. The volume of the box is not fixed though because one of the box walls has a variable excursion part to it which we call a subwoofer. As the subwoofer moves in and out with reference to the box wall when the music moves the speaker cone, the volume of the box increases (when the sub moves out) and decreases (when the sub moves in).
As we know from physics density = mass / volume. So a fixed mass of air is manipulated inside a variable volume box and what this does is create a variable density area behind the speaker cone inside the box. As the sub moves outwards, the desnity and pressure of air inside the box drops. This low pressure behind the speaker (with regular pressure outside of the box) creates a suction on the speaker which tries to dampen it and return it back to its resting position. The opposite is true when the speaker dips into the boxes volume, as the pressure increases inside the box trying to push it back out.
This ‘overdamped’ nature of sealed boxes is what helps them create TIGHT bass with minimal distortion even when the speaker is slightly overpowered as the physics of the box dampen the possible distortions of the speaker motion and accelerate its return to its resting position to prepare it for the next bass hit, even when faced with a series of fast paced rolling bass lines.
Now although sealed boxes are great for clean tight bass, they don’t always hit as hard or as low as we’d like them to, especially when compared to a tuned vented box or a band pass system. One solution to helping a sealed box reach max excursion and create more bass is to neutralize the high and low pressure waves existing inside the sealed box. This gets even worse on a typical ‘newbie’ DIY box with TWO subwoofers in a single enclosure. As both speakers move inwards and outwards in phase, the pressure waves inside the box are exaggerated and neither speaker is allowed to go to maximum excursion.
Isobaric literally means of equal (iso) pressure (baric). And the way we achieve this ‘isobaric’ region is to operate two identically matched subwoofers within a tight enclosure. The two cones of the speakers are arranged so that they can move OPPOSITE each other in a way that the total volume of the box is fixed. Since the amount of air trapped in the box is fixed, and the total volume of the box stays fixed, then the pressure inside the box stays fixed (iso-baric) and the pressure inside the box is always equal to the pressure outside of the box. This way both subwoofers are surrounded on both faces by equal pressure and are no longer over damped by the box physics but rather damped by their own physical damping (electrically by the voice coil and mechanically by the design of the basket and surround material).
There are many different subwoofer arrangements that will allow you to reach an isobaric condition in your sub enclosures, but the simplest are by altering:
1- The direction of subwoofer mounting (flush into the box, or protruding with the basket ouside of the box)
2- The polarity of the wiring of one of the two subwoofers so that it moves in opposite direction to the other subwoofer
So what is the advantage of using an isobaric (or push pull as some may call it) subwoofer network ?
Well one advantage is that as stated before the subwoofer is no longer overly damped, which means more excursion and louder bass.
The second advantage is that now that the subwoofer is neutrally damped, it can actually increase the sound quality of the bass in your system so long as the subwoofer is not overdriven or overpowered.
The third and most interesting advantage of an isboaric arrangement is that you are now driving more power into the same box which combines the effects of both subwoofers resulting in a ’super sub’. This means that if you have a tight box space, that you can build a smaller box to give you the same operating characteristic as you need for a single box.
For example you purchase a 15″ subwoofer that requires a minimum 3.5 cubic feet of enclosure space. However, in the spare tire well in your car you measure out an available 2.0 cubic feet of space which is not enough for this sub. Setting up an isobaric setup with two identical 15″ woofers lower your space requirements by a factor of 2 down to only 1.75 cubic feet. What this also means is that any extra space you get over that 1.75 cubic feet will show up as bass extension as your isobaric subwoofer network will be able to hit harder and reach a LOWER corner frequency, as low as half an octave lower if you use an isobaric network in a 3.5 cubic foot box as recommended.
Here is an example of isobaric loading using two 15 inch subwoofers in a tuned vented box in the trunk of a Golf GTI. This box has been tested and proven to hit as low as 16hz!
So how do we use this information ?
1- If you (like i did) have a regular box equipped with two undivided subwoofers sharing the same space, then reversing the polarity of one of those subs will result in a stronger bass hit, and a lower overall frequency response. Have a box loaded with two 15 inch subwoofers ? Simply reverse the polarity on one of them and gain more power around the 16hz region simulating a big 18″. Have two 8 inch subs but want more of a 12″ sound ? Again do the same.
2- Sometimes you are limited for space, such as on a work truck. Work trucks need pretty much every last inch of space to haul tools and carry people around. So some people resort to installing midbass drivers (such as 6.5 inch “subs”) in their door panels. Mid bass drivers are great for 250 to 600hz mid-bass but they don’t really reach that far down into the bass frequencies on the frequency spectrum, especially if they are NOT installed in a ported enclosure with the right resonant frequency. One smart way to increase the frequency dynamic range of your truck would be to use a 6.5″ spacer ring to isobarically load your door mounted midbass drivers with two identical drivers mounted in plain sight. This will not only double your usage of your tiny in-door enclosure (as explained earlier about cutting your space requirements in half), but it will also allow you to gain more true bass from your midbass drivers and increase the overall feel that you get from such a small and minimally invasive install. (To pull this off correctly the speakers have to be wired out of phase (opposite polarity) so that they move in the same direction with the distance between them constant to maintain the isobaric region between them).
Note: Click here to start building your own isobaric network using undivided isobaric sub enclosures for two 12″ drivers and save $20 off retail.
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