by John Wyckoff
Back 30 years ago, when I first started to design speakers, I frequently used open baffles to evaluate drivers. I continued this practice for about 10 years. I stopped when I learned about the aperiodic approach. (Remember that this was long before PCs and the net so information was much harder to come by.) Using boxes of this design allowed me to gather information more quickly, and they sounded better. The aperiodic box has far fewer resonances than most open baffles, and eliminates much of the diffraction of a large flat surface.
The aperiodic (AP) box, like the open baffle, is not a bass champ, but its restriction can be adjusted to produce more bass than most open baffles. APs are also great if you don't want a large room divider. I've even used APs as a highpass, no-cap crossover from mid bass to woofer. Another perk is that an AP reduces the impedance peak at Fs.
Here's an easy cheap way to create a pair of APs: Go to a thrift store, or a garage sale and buy a couple of speakers which have a driver cut outs close to the size of the drivers you want to play with. Then buy a box of those coffee filters that look like giant cup cake holders. Now cut a hole in the back of the box that is smaller than the bottom of the coffee filter. You'll need a couple of pieces of 1/4" steel mesh. Cut the mesh a bit bigger than the hole in the back of the box. Fasten one piece of the mesh to the box, over the hole and caulk it up good with silicone. Once dry place several pieces of the filter paper over the mesh and hold them in place by screwing down the other piece of mesh. Make sure air can only go through the mesh and not around it. You can now adjust the "port" by adding or subtracting layers of filters. Oh yeah, cut the bottoms out of the filters and use only the bottom. Now, have some fun.