Building the Corner Coving
First of all, why cove (round) the interior of a speaker box? The resonance of the box will create peaks in the sound, and this will be between 1 and over 4 DB depending on the shape of the inside of the speaker box. A cube would be the worst shape, and would have over 4 DB peaks, however a perfect sphere would only have just over 1 DB peaks.
By coving the box I am making it closer to the shape of a sphere (although somewhat more elongated), and the coving also makes the walls much more rigid which will keep them from vibrating as much (so it will produce a cleaner sound). This is the overall interior shape of my sub:
The first step was to create the corner coves. I designed the coving in my favorite CAD program: Alibre Design. I reserve all rights to this design.
To began building the corner coving: I glued up a square blank 3 inches thick and about 8.5 inches square (I later glued up 7 more blanks at once for the rest of the inside corners):
Next I made a circle-cutting jig for my band saw. It has a pin that you set the blank on, and rotate it to make a circle (it takes about 30 seconds per blank on this bandsaw) The jig has a runner and a stop, so you can put over-sized stock on it and push it into the blade to start the cut.
I drill a 1/4 inch hole in the blank, fit it over the pin, slide the jig up to the blade and spin the blank.
And the circle is complete:
A pile of blanks:
Here is my table saw corner coving jig. It is clamped securely at the left edge. I have drawn 5 marks about 3/8 inch apart there at the left edge to indicate how much I will slide the jig away from me between cuts (not shown).
So, here the blank is in position. I start the saw with the saw blade all the way down, and slowly bring the blade up until it contacts. Then I raise the blade no more than 1/8 of an inch at a time, and spin the blank. I keep raising the blade and spinning the blank until I have reached the proper height (or depth into the blank).
Here the cutting has just been started. Notice my table saw insert is still in… I took it out later for better sawdust removal.
Almost to depth for the first position. After cutting to the proper depth I will lower the blade, and move the jig away from me about 3/8 of an inch, and cut again to the new depth.
Now the cuts have been completed. You can see the jig I made for setting the depth at each position. I used it only to figure out how many cranks of the blade height handle would be needed for each position… this made it quicker. You can’t really tell, but there is a pretty smooth 8.2 inch radius cut into this blank.
After all the blanks have been rounded out it is time to cut them to the proper angles. I made an equilateral triangle to mark the 3 points.
Then I used a square to draw a line a few inches over for the first cut.
These cuts are really not possible without a full size sliding compound miter saw! Really! With the right tool, and knowing the proper angles it is done in just a few minutes per part.
This first cut gives me a flat face to place against the fence:
The 2nd cut is made with the cut flat face against the fence. The blade is angled to the left 30 degrees, and tilted to the left at 35 degrees.
For the 3rd cut I raised the blade tilt to 90 degrees, and cut down, leaving about 1 inch at the center part of the angled face. This cut gives me a face to place against the fence for the next cut. The angled face is the mating surface for the straight coving.
Then I rotate the blank counter clockwise, tilt the blade to 35 degrees again, and cut again. I am aiming to just leave on the marks I made with the equilateral triangle…
Here the 5th cut is the same as the 3rd cut, and gives me a flat face to place against the fence for the last side (which is cut the same way).
Now I angle the blade 30 degrees to the right and tilt the blade 35 degrees right, and cut down where the point is. This new face will mate to the wall of the box.
I make the same cut on each corner, and it looks like this:
Almost done. From another angle:
The final cuts are optional, and are used to either lighten up the piece, or to provide more clearance behind (I will be filling the cavities behind with sand…). The blade is angled to 30 degrees left, and tilted 45 degrees right. The cut leaves about 1 inch at the narrowest part near the center.
And now the corner cove is finished:
Be careful of the corners, they break off easily!
This is how a cove fits inside a corner of the Subwoofer box. Now ain’t it purdy?
As you can see the mating surface will be square to the straight coving: