May 2012

The Kaboominator
Big bass in your place.
Article By Jeff Poth

Difficulty Level

  More subs? YES! While I'm not lacking in subwooferage, it is fun to play around with different concepts and design types, especially since I then get to show off on these pages. In this case, we're going to describe a dual isobaric opposed woofer design using the Zalytron 1201PL (at the time of this writing, Zalytron has about 500 in stock, and that'll be the end of them). These were chosen because they're inexpensive, well built, and suitable for sealed enclosure design. Zalytron has them listed in case quantity on their main page. I bought a case of these, eight total.

The 1201PL is a basic poly coned woofer, with a pole vent and a low Fs of around 22 according to my measurements, and a Qts of about 0.5. Cone is straight profile, which tends to mean better cone performance for bass but lower bandwidth. Lower bandwidth is a non-issue for subs, since we only need good performance up to a couple hundred Hz. Since I wrecked one during construction, I was able to dig a little deeper into the design through disassembly.

Xmax is approximately +/- 7mm (Coil height – gap height)/2. This is not high by modern superwoofer standards but is sufficient. Pole is straight and has no shorting rings. The value of shorting rings is minimal in low frequencies, except in terms of heatsinking, so isn't really a concern. This pole has a small, maybe 3-4 mm extension with a beveled edge input to the voice coil vent. The pole extension was an unexpected nice detail that should help reduce distortion by improving symmetry of the BL and Le curves around the cone rest position. The gap was relatively wide, as expected in a low-efficiency subwoofer. The high Le (3.4mH approx) was not surprising in what appears to be a four layer coil.

What Is Isobaric?
Isobaric or "constant pressure" is a method by which there is a volume of air acting as a coupling between two woofers. That volume of air is isobaric, because the two woofers move in and out together, so there's never net pressure on this volume (though in practice, some very small changes in pressure can exist due to loading differences, cone distortion, and phase differences). This method means that the drivers move, coupled by a volume of air, with a normal woofer box connected to the "Inner woofer". This gives you two motors acting on one cone's surface area, which cuts the Vas in half, giving you a woofer that has half the box size requirement for a given alignment (usually sealed), but also half the efficiency.  They're typically wired in parallel for 4 Ohms, versus a nominal 8 Ohms, and are the same sensitivity, so take 3dB more power for the same output. This is the tradeoff of Hoffman's Iron Law, wherein you can optimize for two of three of the following: efficiency, deep bass extension, small box size. The isobaric trades efficiency for small box size.

There are two primary ways to mount isobaric pairs. One has a "normal" looking cone arrangement, wherein there's a small box inside the main box. This arrangement requires the coupling volume to be limited as much as possible. This means that the box between woofers needs to be as small as possible, while the main box will be half the size that it would be for a single woofer, plus the volume of the interior coupling box and interior woofer. There are two versions, A and B, shown. Version A allows for a distortion reduction, since the asymmetrical components of the suspension and motor are moving in equal but opposite directions. The effectiveness of this, however, is limited both by the distance between woofers, and the compliance of the air volume. Version B minimizes the coupling volume so their coupling action is more effective (smaller volume of air represents a stiffer spring), but doesn't allow this "equal but opposite" distortion cancellation. Either will have a high-frequency limit imposed by the distance between diaphragms.

Arrangement (C) is called a "clamshell" mounting, and minimizes both the volume of the coupling air between the woofers, and also allows this "Equal but opposite" distortion mechanism to work to its best effect. The downside is there's some limitation of higher frequency response because you're using the frame side firing into the room, and there's the possibility for pole vent noise at higher excursions. In practice, for subwoofers, neither of these tend to be a concern, unless you require continual high excursion (and thus high pole vent velocity). For those who don't know, a pole vent is an opening in the motor that allows air from under the dustcap to be ventilated, allowing a path for hot air as heated by the voice coil to escape the motor.

More On Distortion
Harmonic distortion is the creation of harmonics in reproduction of a fundamental signal. This is typically referred to as THD and is one of the major sources of distortion in audio. Harmonics are frequencies reproduced as a multiple of the fundamental, so for 20 Hz, the 2^nd harmonic is 40hz. There are different "orders" of harmonic distortion, each being a multiple of the fundamental.

Some harmonics of 20 Hz

These components and their ratio has been the source of much discussion in audio, with lower order (2^nd and 3^rd) generally considered less audible than the higher orders, and even order often being considered less problematic than odd. Harmonic distortion and its profile has been used to explain "tube sound" and other factors. The value and meaning of harmonic distortion is debatable, with some experts saying it's a very minor issue and not to be worried about above some reasonable standard of performance. Others feel that it's a key performance issue. I fall somewhere in between.

In subwoofers harmonic distortion is high. Low frequencies require more cone excursion than higher frequencies for a given SPL, and THD is directly linked to excursion levels in typical speaker designs. The reason for advanced motor types is to limit distortion and increase maximum output by extending and/or flattening the curves used to represent how a speaker changes performance with changing positions of the voice coils. Some examples are BL(x) and Le(x). This is the case in the XBL^2 motors used in the CSS Trio 12s. See this article.

If you want more information on the curves representing loudspeaker performance, I suggest you review Klippel's site as they go extensively into discussion of most everything related to drivers. Here's a cheat sheet from their site at this link.

If you look at these, you notice that often the curves are tilted- this creates even order (2^nd, 4^th and 6^th) distortion components. When you do the "Equal but opposite" arrangement as used in A or C of the diagram, where the cone of one is moving away from the magnet and the other towards the magnet (but arranged and wired to pressurize the same space), the tilt is averaged out to a flatter overall line, giving lower even-order distortion.

Distortion in subwoofers has one very important effect -- it tends to make them localizable. It is generally accepted that with a stiff low-pass filter at 80Hz or lower, it's difficult to hear where a properly set up subwoofer is. The problem is that the distortion components are multiples of the fundamental, and are unaffected by the crossover filters. This is true because they're not present in the electrical input signal, but are instead artifacts of the driver itself. The low pass filter cannot control them, since they're generated based upon the excursion demands of frequencies within the passband. For example: If you have 4^th order distortion at 60 Hz is a 240 Hz component, which is localizable and has nothing to do with how you cross over the driver. Thus, you have your subwoofer calling attention to itself via distortion, which is a distraction from the sonic presentation. This is why I favor giant sub setups- I want my subs to disappear. Ironically, this tends to mean that they need to be physically quite a bit larger and with more cone area, to keep them operating at modest excursions and distortion low.

So even though even-order distortion is sometimes considered to improve sound quality in a system, in subwoofers, any distortion meaning higher frequencies coming from the cone make it more localizable and are undesirable. When the distortion comes earlier in the playback chain, any harmonic distortion of bass frequencies is still affected by the crossover, and thus does not play back through the subwoofer, whereas when coming from the subwoofer itself, it cannot be suppressed by the crossover.

Sooo... Where Do The "Dual" And "Opposed" Come In?
Well, dual means that two pairs are used, and opposed means that they're designed so that mechanical force from one pair works against the other. It's that whole equal but opposite thing again- in this instance, it's the force from the cone motion, with another driver pair applying the same force but in a way which opposes the force from the first pair. This is best accomplished by having the drivers fairly close together but on opposed baffles as in the pic below (Which is a dual isobaric opposed woofer).

W-Frame dipoles and "PPSL" type bass systems also use this layout, and are fairly effective at reducing overall energy in the enclosure. In M-frame dipoles and the Kaboominator, the drivers are mounted close together but at a right angle to each other. This allows some force cancellation but is theoretically not quite as effective as the mirror image opposition of the other opposed types. Ultimately, either is a good way to mount two drivers (or two pairs of clamshell drivers) so as to limit the box coloration imposed.

If Directly Oopposed Is Better, Why Use An Angle?
In dipoles, an M-frame can potentially have a wider bandwidth since the trapped volumes are irregular and thus less prone to high q resonances. In true subs, there's not much reason to use an angle over a mirror image unless you need it that way for the layout. In my case, I am again using clearout M&K cabinets from Deep Surplus, who graciously gave away nearly a hundred pallets of cabinets they bought surplus. This was also the source for the cabinets used in my Trio 12 project. The dual-driver opposed cabinets are well made and adapted themselves well to this project. It is a concept I'd wanted to play with since somewhere around 2005, when I built the red dual isobaric 8"s shown above. Unfortunately, that's still sitting in my brother's storage unit, and never got implemented. Waste of a quad of peerless HDS 8"s, which are very fine woofers. Oh well, my fault.

So we're adapting cabinets again. Being the clever fellow that I am, the 1201 PL was selected as a good match for these cabinets with the bolt pattern being the same 8 hole arrangements and cutout as the cabinets, as well as a single 1201PL being a good match for the sealed volume of the cabinets. The Kaboominator requires the same box volume as a single woofer, with the same efficiency and load when wired series-parallel. 2 8 Ohm woofers clamshelled and wired in parallel require half the volume of a single woofer, with half the impedance (4 Ohms). Since there are two pairs, it requires the same volume and when the two 4 Ohm clamshells are wired in series, so back to 8 Ohms with the same efficiency as a single woofer in the same box. The difference is, it has four times the power handling, dramatically lower distortion, both due to having twice the displacement and due to the isobaric cancellation, and twice the output available. All this with reduced enclosure coloration from the opposed mounting scheme gives this design, or another push-pull design with force cancellation, some major advantages over conventional subwoofer design.

In effect, the combination of isobaric and opposed mounting has converted a quad of modest 12" subwoofers to a single, very high quality 18" subwoofer with a lower-coloration enclosure. Kudos to M&K for being clever enough to use this layout- they used the magnet side out on one woofer (in the manifold) and the cone side out on the other, achieving both the distortion cancellation of a push/pull design (this is the same equal-but-opposite distortion cancellation mechanism of isobaric) and the force cancellation of the opposed woofers, in a single cabinet. The 1201PL demands a larger enclosure than the woofers M&K used, so to make the volume work, dual isobaric was the better match. The push-pull arrangement of the production M&Ks had woofers which required half the volume of the 1201PL to achieve a good sealed alignment, and as such didn't need the enclosure size reduction of the isobaric mounting. The key part is that you are taking output from both the cone side and the magnet side of the woofer assembly, in close proximity, to achieve the distortion cancellation.

Build
The M&K had originally mounted an amp inside the enclosure, and this made an opening that needed to be covered up. I cut a panel and screwed into place, with weatherstripping as gasketing to cover this. Also, there was one panel which was unbraced in their assembly. I decided to add an exterior panel to double the thickness, also acquired from the deepsurplus giveaway as a base plate for these woofer systems. This was given weatherstripping so it didn't rattle and screwed into place on top of the unbraced panel. All pretty straightforward.

There's an access panel for the manifold area, necessary to mount the driver since the manifold won't clear the driver frame. Because the isobaric mounting brings the polepiece vent closer to this panel, I cut out some hole area with a holesaw. You can see some sawdust on the panel, and that it's more of a clover shape than a hole- this is both to make the hole large enough to breathe freely and avoid any resonance. You can also see the black sharpie I used on the magnet to help keep it from showing overmuch through the new vent hole.

I added some of my preferred stuffing, "Ultratouch", wired some leadout wire to the internal woofers, sealed some openings in the enclosure with hot melt glue (there were wiring holes in the manifold panel in the M&K design), and mounted drivers. The cabinets had pre-mounted T-nuts, and were mounted with my preferred stainless steel screws. Because the rubber surrounds partially obstructed the mounting holes, I used the screws to shave away the extra portion (this is a common arrangement) so that the screws would not grab and pull on the surround when I was mounting.

I wired them up with the interior woofers having inverted polarity to the outside, and the drivers in series-parallel, and after figuring out how my testing methodology was giving me funny results, performed an impedance sweep.

Click for larger pic.

Perfect results -- low Fc, Appropriate Qtc, and smooth trace. Some well damped ripple at a couple hundred Hz and again above 1 kHz, but that's to be expected. Not too shabby!

Sound
Not surprisingly, the single Kaboominator, which had about the same midbass efficiency, but dramatically lower output, and low frequency (20 Hz) output, couldn't keep up with the dual passive subwoofer systems in terms of raw output. See my Passive Radiator Subwoofer article.

But there was a goodly amount of output on tap as the Kaboominator is equivalent to a single 18", and the sonic character was very neutral- it disappeared exceptionally well, even better than the Trios, within its limits. This may have been related to the close proximity of the cabinet walls in the tight space of the passive sub. Close proximity to cabinet walls can lead to distortion by creating a difference in airload on the driver from one side to the other. In terms of output, a pair of Kaboominators would still be short of the trios with passives- but with 7mm xmax, we're looking at dual low-distortion 18"s. That's the sort of subwoofer system that might be used in a modest PA stack (read- much higher than home levels).  One major upside is that the woofers for a pair would cost less than the cost of just the passives in the trio passive systems, and the Kaboominators would be just shy of equivalent in overall output (and more than enough for any reasonable person), as well as being more sensitive and thus requiring less amp. As I mentioned and showed before, my own clumsy mistakes lead to a dead woofer, which made my pair of kaboominators impossible.

Fair Warning
One caveat: the 1201PLs needed a little massaging, the suspensions showing some signs of sag (where the cones were slightly displaced towards the magnet structure). This really wasn't a big deal to me, who has worked with many woofers and seen this common problem before. I simply gently pulled them into shape, back to where the ridges of the spider formed a straight line. This was a slow process in which I insert socks into the frame to equally press the cone forward and get it back to normal. Let them sit for a day or two, don't get too aggressive with it, and check them periodically to see if they've gotten where you want them. Gentle is the key word.

So What About That Distributed Bass Thing
You Promised With The Passives Article?!
Well, there were going to be some neat tricks shown with dual Kaboominators. Since I think another case of eight 12"s living in my bedroom getting massaged at night might get me divorced, the Kaboominator will remain lonely for the moment. But never fear- I have something up my sleeve, showing how "normal" sealed subs can leverage some of the distortion cancellation I've referred to in this article. It will also use the 1201PL, so if you didn't want Kaboominators in your room, you could use the ideas of the next subwoofer article instead, and still cash in on the great deal on these entirely decent drivers.