March 2012

Ultimate Bass Cabinets
Article By Jeff Poth

Difficulty Level

  Where to begin.... I suppose a disclaimer. Yeah yeah, I'm tired of those too, but they're important for contextualization. Ultimate Bass Cabinets is something of a misnomer, as there are bigger, stronger, higher output cabinets out there. And not just a few but many, both in DIY and in commercial products. There are exotic materials, brute force approaches with 6" thick walls, cabs the size of Volvos (and larger) and horns big enough to park a Volvo inside. Most of these are extremely expensive , time consuming, and difficult to produce. This article focuses on the build process of some very well-done cabinets that work with a wide variety of bass drivers, and effectively address many of the problems that exist in a typical enclosure. These problems include panel resonance, internal reflections, and external diffraction. This cabinet design is carefully balanced between cost, performance, and practicality. Ultimate practical bass cabinets just doesn't have the ring of Ultimate Bass Cabinets.  

Cost
Cost is a killer here. One could easily get $500 to $700 deep into these cabinets, pre-driver. With quality drivers, you can be significantly over $1,000. That's not chump change for most DIYers, and is sufficient to buy many decent completed speakers. Unfortunately, it's what the other requirements call for, and is the best balance of performance and price that could be come up with. Remember, that amount is pre-labor, so don't get sticker shock. If you want quality you have to pay, in raw materials just as completed cabinets.

Time is money, so if you are interested in cost savings look elsewhere. These will take some significant time to do properly. You must enjoy the process for this project to be worthwhile.

Design
In this particular case, the cabinet was designed to avoid interior parallel walls, be extremely stiff, and present a low-diffraction front panel.  There are a number of ways to create a cabinet like this. One can achieve these ends in a box speaker through use of bracing and internal baffling, and careful treatment of the front panel (large diameter bevels or roundovers, or acoustic absorption). Another option is the use of non-wood materials, such as the carbon fiber and resin cabinets employed by Wilson Benesch (amongst others). There are also "bendable" wood materials, some of which are pre-kerfed so that there is a thin skin of material with strips of thicker material attached, which leaves gaps that must be dealt with to avoid panel resonance. Those that aren't pre-kerfed are typically thin layers that must laminated together to achieve a desirable thickness.

In my case, I wanted to create a rounded external shape for both performance and cosmetic reasons, and do it in solid wood material (well, plywood) without the hassles of a laminate construction. This meant the use of mass-produced round wood materials, specifically, a half-cylinder shell of plywood, and millwork joinery that creates a large-diameter round-over. The combination of these components resulted in the requirement of a complex shape of a joined semicircle and trapezoid for the shelf bracing, and a little extra effort in terms of filling, joining, and finishing compared to a "normal" box speaker with the internal baffles, but is otherwise not terribly difficult. The result is that we have the longest diffractive surfaces properly dealt with to avoid response ripple, and only have 1 set of parallel walls in the enclosure- top and bottom. Multiple shelf braces and other structural components contribute to the rigidity, and help create a well-optimized acoustic environment. The shelf braces in particular are designed with large windows to allow free airflow without significantly compromising rigidity, and also provide a method of breaking up any top-bottom standing waves.

More On Weight
Weight is an important consideration when designing cabinets for real-world usage. Appropriate care in designing the enclosure allows significant rigidity without going to the brute-force approach of simply layering materials. Appropriate care here means the use of shelf bracing and ribs as well as extensive effort in coupling braces and panels to each other. This allows panel resonance to be minimized without adding the weight of thick panel walls or damping sheets (typical materials are lead, asphalt, or vinyl). Obviously lower weight with a given amount of enclosure resonance allows the units to be easier to set up and move, and is also desirable from a resonant perspective- something which is light and stiff tends to have resonances that are higher in frequency, where less excitation energy lies, and panels are more easily damped.

Construction
Down to the meat of this article. The design is done for you, the construction will not be. Description will be in broad strokes, with the hard design parts done for you. The rest is up to you.

Materials

1. Plywood Half Cylinders as seen at this link.

2. MDF millwork L-columns: as seen at this link.

3. Baltic Birch or other high-quality plywood (Appleply, full-hardwood ply)

4 Gorilla Glue (or other expanding urethane glue)

5. Shellac based primer

6. Veneer or paint

7. Removable mounting hardware (T-nuts, screw inserts, or other fixed socket materials for screws)

8. Gasketing material for driver mounting

Tools

1. Tablesaw or circular saw

2. Misc. Grit sandpaper (a random orbit sander would be helpful here)

3. Router with roundover and/or bevel bits

4. Others will be needed

First step is to trim the millwork to appropriate size. The cylinders are 31" high, and their form will not be modified in any way. Accordingly, all heights are 31" to match. You'll want to use some sort of chopsaw/mitresaw to do this, a normal tablesaw will be difficult to do effectively due to the thickness of the material. MDF is NASTY stuff, and the dust is extremely fine and will get into your lungs and/or sinuses any way they can. Wear a good mask.

Now that you have this, you're going to test fit the pieces, and get a precise measure of the front panel as it will be once mounted. There's only 1 way to fit these pieces together, so this is self-explanatory. Following this, you'll trim some 31 1/8" x Gapwidth panels. You want a tiny bit of wiggle room left on the width to ensure that when you clamp it up you can get it aligned well.

Now you have a pair of front panels, a quad of front MDF millwork roundover posts, and two shells. Assembled, these would make two misshapen tubes. The next piece is the top and bottom panels and shelf braces. A top and bottom can be applied on top of these tubes, but a well-done inset panel makes the rest of the assembly easier, and you need the oddball shape anyway for shelf bracing. See this PDF file for technical drawing.

Bracing
The bracing consists of shelf braces with holes cut out, ribs joining the shelf braces to each other, and to the overall cabinet.

You can see the ribs, the shelves, and the white paint there for your viewing pleasure (actually it's shellac-based primer, great sealant). To get the braces as described to fit in, as well as for attaching the edge-round, strap clamps are used.

I used recycled denim "Ultratouch" damping material, held in place by the ribs and vertical struts, with a slight airgap between the shell and the damping (spaced away by the ribs). Depending on the woofer and alignment you choose, you'll want to tweak your stuffing to match.

From there, the requirements get much easier- you can do driver cutouts, stuff and vent to taste on the flat front baffle. I allowed the baffle to remain proud of the "squashed cylinder" form, with beveled edges, as this was the most convenient for iterative testing. For some of you, this won't be necessary. I leave it to you.  For me, this meant I needed a panel that would accept the removable baffles for the driver and venting arrangements.

As you can see, there are gaps for both the driver and for a panel allowing me to swap out venting/sealing options ad nausea. If it matters, 37 Hz tuning or thereabouts is my favored point with pro woofers like those in my woofer testing article. Vents have a bevel around their output. The termination helps keep vent noise low.

15” Woofer Survey… A Tale Of Three Woofers

Finishing
I'm probably the wrong guy to ask on this point- as you can see, the finishing is not done. The front panels are both birch, but the first has a sizing applied, giving it a more orange hue than the entirely raw vent panel. The horns are oiled but not lacquered, the tops neither oiled nor lacquered. The main cabinet body is both oiled and lacquered.

Paper backed self-adhesive cherry veneer was used for the horns and the main cabinet bodies. Self-adhesive applied with a scraper is the preferred option here due to the curvature. Below you can see that the veneer follows the curved edge nicely, and you can also see the mounting ring initially used (since eliminated) for driver mounting. Cherry darkens up quite a bit with exposure to sunlight and oil.

Done
I'm quite happy with how they've come out, despite being unfinished cosmetically. This is not a project for a beginner by any stretch, but an experienced worker can get this done and have a slick, curvy, high-performing bass cabinet.