DIY Open Baffle With Widerange
This is the best speaker I've built at the time of this writing (that is mostly January, 2010). The speaker is almost two years old but the woofer... much older. Older than some of you reading this! Let us dive right in: The drivers are the JBL 123A (or, preferred, 123A-3/2213) and Fostex FF85k (preferred FF85KeN).
Out Of The Pool!
This woofer has a "Lansaplas" coated, ribbed, curvilinear cone, a high-ish Qes, and a low Fs. It also has a high quality AlNiCo magnet structure and JBL build quality. The combination of a curvilinear cone with low inductance and a lot of damping material not only gives it a fairly well damped, extended top end, but also a well extended low end- in the right box.
These drivers are available; the L-100 was the top selling speaker in history, in its day. You'll just have to hunt for a nice pair. A known good pair can be usually had for $200, you can find cheaper if you really look. $200 is a perfectly fair price for the level of quality these drivers have, both sonic and build. If you come up with L100s for the woofers, please try not to destroy good examples of the speaker- if the cabs are in good shape, you can offer them for resale, whole, and find another set of woofers. Also, be aware that if the cabs are NOT in good shape, the LE-5 midrange and tweeters are quality drivers too.
Note: These AlNiCo drivers have the potential for being de-magnetized. This leads to a higher Qts and less efficiency. You will need to be able to test these to confirm. I used a Dayton woofer Tester 3 for this task. Proper values should be approximately Fs: 25Hz Le: 0.6mH Qts .5
For those of you who want a significant upgrade, the fully modified FF85KeN from planet_10 hifi is a big improvement, with better clarity, dynamics, and a more complete disappearing act. This is one amazing driver, taking the FF85k a significant notch above it's already extremely impressive sound quality. More information on the modified driver at this link.
Make Me Box Your Ears!
The great part here is the crossover is very simple. A first order filter is used on the 3-inch, 10uF. Use a good quality cap. The 12" does not work with a first order filter via series inductance; its small inductance interferes with both efficacy of a series inductance, as well as creating phase wrapping that cannot be corrected for with electrical solutions. My solution was an acoustic one. A felt (adhesive backed F13, part number 8745K53 at McMaster) pad was implemented on a metal grille from Parts Express that created a low-pass acoustic filter. This allowed excellent phase tracking at the crossover point and an ideal textbook first order filter behavior (read: transient response is superb).
One lump remains. A 200Hz lump, to be specific. This is the result of the baffle reinforcing some frequencies, as well as a reduction in capacitor efficacy at Fs on the Fostex. This is the hardest part of the filter, fortunately, it's not that hard. You WILL want to spend money for a very high quality inductor and capacitors here. I used Solen 10 AWG aircore coils and a composite (several values in parallel) capacitor bank to get what I needed. I recommend avoiding any cored inductors here if possible, and keeping the DCR low. It's a big part, an expensive part, and a part that you don't want to skimp on. 12 AWG would be good, 14 may be sufficient, but absolutely no smaller if you're using aircore. If you're using cored, 14 is okay, but I recommend aircore. Remember, this part is the one that's passing the first few, most energy dense, octaves through it. The capacitors are also quality important, but are likely less of an issue here. Several quality poly capacitors in parallel, 100V or more will do the trick. The resistor can be a generic 10W sandcast type; it is not going to be passing significant current apart from at the notch frequency. Sandcast resistors do have some significant inductance, but this isn't really an issue here. When using these resistors for high frequency circuits take care.
These things do not come without their compromises, however:
There is more to it than above, and the curious reader would be wise to spend some time reading about dipole behavior, most specifically the studies at Linkwitz Lab and Music & Design. A speaker hobbyist who reads and understands the content at those two sites will be fairly well-versed in many of the issues in loudspeaker design.
The critical component here is "Directionality". While this speaker does not qualify as "CD", it is significantly more consistent with its dispersion pattern than many speakers, and this was one of the major design goals, within some other constraints. The nature of an open baffle speaker is that as frequency decreases, the front and rear radiation from the speaker driver is more and more able to cancel. The result is a figure 8 pattern, with the "waist" of the 8 representing the driver on the baffle, and the round portions representing forward and rearward radiation. The frequency specific behavior of this effect will depend largely upon the size and shape of the baffle. Smaller baffles tend to be more consistent with their off-axis losses (good) but lose more bass (bad). It's a balancing act.
In addition to dipole directionality, the driver itself tends to be directional. The larger a driver is, the lower in frequency it will begin to "Beam". In the case of our 12" woofer, we have a beamwidth that has significantly tapered by the 2 kHz crossover point. The FF85k begins to be directional at about 3 kHz, and is fairly consistent in its directional behavior up until about 8 kHz (for a cone driver). It is more directional in this range than might normally be expected for a 3" driver, which is to our benefit in this application. Above 8k it becomes very much a laserbeam, so for very high frequencies, you will want to sit very close to on-axis. This is not a wide sweetspot, omnidirectional speaker, but it has sufficient dispersion for a few good listening positions. You and your sweetie on the sofa should be just fine. But this leaves us with the problem of a significant change in dispersion at the 2 kHz crossover frequency.
There are several design decisions that help us accommodate this problem. One of the major causes of directionality in large drivers is the cancellation from different arrival times (leading to 180 degree out of phase signals) from the different parts of a large driver (when it's large relative to wavelength). This is part of the usage of phase plugs. Our acoustic absorption used to form the low-pass characteristic interferes with this cancellation at some angles, and may well smooth the off-axis response (at the cost of on-axis extension -- a benefit, in this unique case). This is theoretical, I have not measured it, but is very likely. Another benefit is the cone geometry of the 123a/2213. The "Curvilinear" style cone tends to have improved high-frequency dispersion characteristics over straight-walled cones. Finally, a large amount of lossy acoustic felt, as used on the woofer grille, is built up around the edges of the FF85k to create a lossy barrier and induce some directionality at and below the crossover point. This is not sufficient for full idealized matching, but helps. The enhancements of this paragraph help minimize it.
The construction pays attention to maintaining an open flow to the back of the driver. This is a valuable consideration in any speaker build, but moreso with open baffles and small drivers like the ff85k, where it would be extremely easy to obstruct the path of the backwave. It's also important to provide at least some recess for the large woofer. I chose a ¼ inset because it was what tooling allowed. Remember from the intro, that this pair was made with minimal tooling, as my tools proper had not made the move yet. A full flush mount for this driver would be preferable from a performance perspective.
You can see the primary "spine" used to brace the driver magnets (this helps reduce the energy transferred into the baffle by the driver frame) as well as stiffening the baffle itself, and providing the rear foot point which sets baffle tilt so that the baffles don't simply fall over. This particular build also has substantial bracing of the baffle, which is recommended. The method you use is up to you, the "ribs" shown here are fairly effective, and the cutouts are to prevent standing waves from forming between the bracing structures. Yes, I know they're ugly, thank you very much.
In short strokes, you can assemble these, with care, with little more than a jigsaw and a drill (as I did), but a proper toolset, including a circular and/or tablesaw, a router with a circle cutting jig, drill and bits, and other odds and ends are going to be required. The rough aspects of the design are important- the shape of the baffle, the effort towards stiffening it and reducing energy transfer into it (bracing the magnets), the crossover, the tilt of the baffle. Some things, however, you can make your own. How much absorption you want around the edges of the midtweeter for example, and whether you want to make funny looking felt pieces around the sides of the woofer, or are happier with doing a full flush-mount.