Building DIY Loudspeakers - Trapezoidal Design
Build Phase - A Trapezoidal Speaker Design?
If you haven't noticed, speaker shape has changed in the last 10 or 15 years. They used to be wide with a shallow depth but now they're tall with a narrow width and much deeper. That is because of diffraction of sound on the front surface of speakers; even the thickness of the driver flange can cause diffraction of sound. In order to minimize diffraction I needed to make the surface area around the tweeter and midrange as small as possible, the woofer is not as important because bass is non-directional. B&W Nautilus has optimized this concept by placing the tweeter above the box with no surrounding surface area. I made the speaker front triangular shaped as shown.
I also wanted to phase align the drivers so that the sound leaving each driver leaves together and at the same time. Physical phase alignment is achieved by lining up the voice coils of each driver in a line perpendicular to the floor. Therefore I also angled the "speaker front" backward.
A third benefit achieved from these angles is non-parallel panels. Non-parallel panels help reduce standing waves inside the box. A final benefit, in my opinion, is that I have a pyramid shaped speaker that is aesthetically pleasing. I had to search a few math books to find the volume formula for a frustum of a pyramid which works even though the back panel is straight. The final part of the physical shape will lead you to see why I wanted the box volume to be of medium size. The tweeter and midrange should be at ear level when sitting down. If the box volume was too large and keeping the previous mentioned constraints in mind the tweeter and midrange would be above ear level.
Most speaker manufacturers do not make non-rectangular boxes. I believe this is because of the difficulty in manufacturing odd shapes in large quantities and as I pointed out before most low frequency woofers require a much larger box. It is much more important to keep the tweeter-midrange at ear level than to worry about diffraction, especially if the drivers are pushed to their on axis response. So keeping the shape rectangular allows a larger volume. One design quality I did not implement was rounded edges which reduces diffraction at the edge of the speaker. I read that at least a 4 inch radius is needed to make a sonic difference. Given the complexity of the angles, this parameter was left out of my design. Thiel speakers, for example, use this concept. Although I haven't had the opportunity to listen to Thiel's, their physical design is a very good concept.
Bracing and Dampening the Enclosure Box
Bracing and rigidity was the next build decision to make. I wanted to do shelf bracing (like a shelf inside the box but with open areas). Since the angles made shelf bracing difficult I opted for cross bracing and corner bracing. I have recently read that cross bracing is a better choice to stiffen up the cabinet. The cross bracing was placed above and below the woofer and then braces were placed in the corners to stiffen up the joints. I'm not sure if there is such a thing as over bracing a speaker cabinet except that it s uses up box volume. The cabinet was built from MDF as most speaker manufacturers use. I have seen speaker book designs use plywood but I am unsure why; plywood has voids which gives an inconsistent density. A better choice if MDF couldn't be obtained would be particle board. Finally, the crossover was mounted on the bottom of the cabinet to keep it as far as possible from the driver magnets.
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