Now that we have taken some preliminary measurements and decided on enclosure sizes, it is time to fabricate some test enclosures. There is no reason these should necessarily be any different from the final enclosures, but at this point, we aren't concerned with aesthetics - AT ALL! The important thing here is that the overall dimensions be the same, and the drivers mounted on the baffle as they will be on the final design. This means flush mounting drivers (if that will be done on the final version) and aligning them properly. We want the port any stuffing installed, as well as rounding over any edges that will have this treatment on the final enclosure. Basically, what we'll have is an "ugly" version of the final design.
This is the part of the enclosure that is constructed entirely of MDF, particle board, or a good quality plywood. I toyed with the idea of encasing the enclosure in solid cherry. I even considered building the entire structure out of cherry, but I decided that would be too risky. Solid woods have a tendency to introduce resonances, whereas MDF and particle board are so randomly assembled, they are essentially accoustically dead due to the structural variation. A void-free plywood is also a decent choice, because it uses multiple layers of woods with different densities, which helps minimize resonances. To add a little to the enclosure aesthetically, solid cherry panels will be added to the sides of the enclosure - again PURELY for cosmetics. Here is a drawing with the basic dimensions of the main/surround enclosure. I like to make a simple rendering of the enclosure to get an initial idea of the proportions of the speaker. It's a little crude, but a better way to decide if I like the looks of the design before I have it all assembled. Easier to change my mind now than after I've spent an hour and wasted 1/2 sheet of material!
The majority of the main structure of the enclosure will be made of 3/4" particle board. I haven't been able to find any hardware or lumber stores that carry MDF, which would be my first choice. Particle board is rough on blades and bits, and doesn't hold an edge well, particularly for the recesses needed to flush mount the drivers. It just plain looks sloppy! A decent quality plywood will cost slightly more, but then end results (atleast aesthetically) make up for the cost. It is much nicer to work with, and if a decent quality plywood is used, it is easy to finish. Here I'm going to just use the plywood on the front baffle to insure that the recesses for the tweeter and woofer don't end up looking like a 3rd grader cut them with dull scissors! I will also be using an internal brace, as well as damping material to maintain a rigid and accoustically dead box. The cherry panels are glued and screwed to the sides, which will also increase the stiffness of the enclosure. I don't think I'll have to worry about any resonances here!
The drivers are mounted along the
vertical center line of the enclosure, and spaced 5-3/4" center-to-center.
This ensures that the drivers are spaced less than one wave length apart
at the crossover frequency. This helps maintain the "point-source"
nature of the system. To minimize some of the diffraction, the side
and top edges of the enclosure are rounded with a 1/2" radius. Because
this enclosure uses the slanted front baffle, it is a little more difficult
to construct than a standard rectangular box. Here are a few pictures
of the prototype enclosure in construction.
The cabinet drawings require the Whip viewer. Download it if you don't already have it. It allows zooming, panning, and better printing capabilities. Right click the image for a menu of options. All sides are constructed of 3/4" thick material. An internal brace may be used. My cabinets use a shelf brace located between the tweeter and woofer, which ties the front, back and sides together. Be sure to leave large holes in the brace to allow proper air movement. Click here for a GIF image of the drawing.
The center channel is slightly easier to construct. Because of the odd shape of the woofer baskets, I won't be flush mounting them. Unless you have a really good template to do the routing, the work that is involved in getting the cutouts correct isn't worth the gains. It is possible to simply route a round recess, but aesthetically it doesn't fair too well, and probably only adds reflective surfaces, thus defeating the whole purpose of flush mounting.
The crucial aspect of this speaker is to vertically offset the tweeter from the woofers. Ideally, this speaker would be used standing upright, but that's not very practical for most applications. When MTM speakers are used on their sides, they often have strange vertical and horizontal polar responses. Offsetting the tweeter helps restore the vertical response as much as possible.
The center channel will also benefit
from some cherry trim, as well as internal bracing and stuffing.
Here is the center channel design and another rendering showing final design,
along with some construction photos.
The center channel also uses 3/4" material for all sides. I used two shelf-type braces to tie the front, back, top and bottom together. They are located to either side of the tweeter. Again, leave large holes in the braces to allow free air movement within the enclosure. Click here for a GIF image of the drawing.