(As seen in Speaker Builder Magazine, Issue 4, 1999)

Also See the Maverick 2C Shielded center channel speaker

Wanna make a more traditional looking design? Look at this plan...

DESCRIPTION: The loudspeaker described in this article is a very special design that uses a very small footprint and an open bottom with an isobaric woofer design.

I recently decided that my first speaker project no longer fit the needs of my current lifestyle (not to mention that the cat decided to use the passive radiators as his scratching post, and he ruined one of the midrange surrounds). I wanted something with a smaller footprint, since I am living in a condo and using a 35” tube TV that takes up a large portion of the room already. I also knew that my emphasis was now on home theater more than stereo sound, so I wanted to be able to match timbre across the room. This is not your usual speaker with six walls and three holes on the front face. It has a semi-open bottom, which is used for the mounting of the woofers. (Photo 1)

It all started about six years ago. I was into car audio and wanted to make the sound in my home stereo better. I got the novel idea to combine the use of a sealed woofer system with a passive radiator. I had heard the arguments for and against sealed and ported speakers systems and wanted to get the best of both worlds in one package. I decided to use 8” Peerless drivers (831709) for the bass, and the rest didn’t really matter quite so much.

CABINET CONSTRUCTION
For the unit to be aesthetically pleasing, I wanted to match the wood colors with some of the other furnishings in the room. I decided to use oak plywood for the cabinet and solid oak ¼ round moldings for the edges. In retrospect, if I were to make these again, I would use solid oak for the front and top panels and use ¾” round over bits for the edges, just prior to sanding and finishing. In doing so, I wouldn’t have to worry about the round over extending down into the base and creating a strange looking joint and having to fill or fix it in another way. I will use this format for this article.

Table 1 gives a parts list for the Maverick 2’s and Figure 1 is a construction drawing. One thing I decided to do was set-up a cutting schedule, before I even had the wood. This enabled me to cut all pieces that were the same width of cuts at the same time and made for quick work with only a few saw set-up changes.

For the construction of the cabinet, I wanted to get some stiffness from the upper portion of the cabinet since the bottom is open. I decided that since the walls on the sides were going to have a wider surface area than the front, it should be made with a double thickness. To do this, I used MDF on the inside of the subwoofer cabinet only. The top of the cabinet has two separate sub enclosures, one for the crossover and one for the midrange and tweeter. (Photo 2)

The front panel has recessed holes for mounting the tweeter and midrange and has a piece of MDF attached to the back of the midrange/tweeter area so there is more material for the screws to grab—see Figure 2. The MDF has been chamfered on the inside to allow for easier airflow for the midrange. The inside wall stiffeners should have dado cuts (see Figure 1) to accept the shelf braces being slid into the dado, so the cut should be ¾” wide and ¼” deep. All joints should be glued and then sealed with silicone to prevent air leakage. (Photo 3)

The last pieces I assembled were the three bottom panels that the woofers mount to and the spacer to separate them, and the back panel. I wanted access to the inside of the cabinet, so I decided to use closed cell foam weather-stripping and screwing the back panel onto the assembled cabinet. Make sure if you do this that the screws are on the outside of the weather-stripping, because you would no longer have an air-tight seal.

Both of the sub-cabinets are sealed with a silicone caulking and filled with a polyester-fill. Each speaker cabinet has about ½ pound of fill for the woofers and the midrange/tweeter sub-cabinet has about a ¼ pound. (Photo 4)

I decided to make the bottom trim pieces out of solid oak and form them to match some of the other furniture I have in the room, but it also has a purpose. The shape allows for the air to flow from the bottom of the woofers, and having the speakers on carpet may filter out a little more noise that the isobaric configuration normally hushes.

I sanded and stained the cabinets then put three coats of varnish on to give it a shine. They blend in well with the other wood colors we have in the living room. (Photo 5)

DRIVERS AND CROSSOVER
I knew that the Panasonic Leaf tweeters I had chosen to use six years earlier were out, because I tried to get more, but found out that they had not been manufactured for over three years. After looking at current designs on the market for several years, I had decided that I liked the sound of the metal dome tweeters over the soft dome versions. So I decided to use the Vifa D25AG-35, a 1” aluminum dome, based on the recommendation of a friend.

I also like the quick sound of cones made from the lightweight materials such as Kevlar and Carbon Fiber for the midrange. Since I wanted to replace the midranges on my current speakers before I even got into the new design, I chose the Audax HM170C0 6.5” midrange so that I could swap it with the old Driver Designs 6.5”. I immediately noticed that there was a large improvement in sound image and quality with the more expensive, lightweight Audax. I decided for my new design that the 93Hz I could get from the Audax drivers in a sealed box would work fine with the woofers, so I chose that instead of the 60Hz or lower I could get in a ported configuration.

I knew that I wanted to use the 8” Peerless drivers that I would take out of the old design, and since I had two for each side, decided that I should use them in an isobaric configuration with a sealed box. I wired the drivers in a parallel scheme, but reversed the phase of one of them so that they both move together, thus reducing the Vas.  This would give me a respectable 39Hz at the lower F-3. I found that at a Qtc of .707, the box size was just over 1 ft3. I decided that I could do quite a bit with this in a small footprint. I was trying to figure out how to use the isobaric design and not have a really strange looking cabinet. I didn’t want a magnet structure hanging out of the cabinet, and I was thinking that it would be great if I could hide the woofer all together. With the woofer hidden, it will be less tempting for my cat to destroy them.

I gave Madisound all the specifications for my design and the drivers and they created a crossover design for me (figure 3). Since I am not good at soldering, I let them make the crossovers as well. Table 2 is the parts list.  When I got the crossovers I was very happy with their appearance. They were mounted on a large circuit board and were very heavy, especially when compared to the crossovers taken out of my original speakers. Going from a first order to a third order crossover requires the addition of a lot more components than I thought it would.  Figures 4-6 show the predicted LEAP measurements that Madisound sent me for this crossover design.

CONCLUSION
I am very happy with the project as a whole. The sound quality and the fit and finish in the room meet my criteria (Photo 6). The first time I hooked them up and really started cranking the volume, my wife, Kathy, said that she really noticed the difference in sound quality from the old design. The bass is much tighter and does not have a tendency to resonate through the house as much as before. The midrange is cleaned up by the use of the 3rd order crossover, so they don’t jump all over the place and distort and bottom-out and the tweeters are nice and smooth to my liking.

I do not have test equipment, so I cannot tell you what the actual frequency response is or give waterfall plots, but I am very happy with them and you may be as well. I am now working on the design of a powered subwoofer and also a shielded center channel with the same drivers as the sides. For the future, when I move into a house, I have several ideas for making dipole surrounds for the back using the same drivers there too. For now, the in-wall Radio Shack speakers will work just fine.

My cat, Maverick has not been tempted to scratch the speakers or jump up onto them like he did on the old design.

Misc Pictures: , Back of cabinet, My old speakers: #1, #2, Maverick the Cat: #1, #2

If you have comments or suggestions, email me