Building The Panels
The first step is to make six soundproof panels with the following dimensions (in feet): 4' H, 4.56' W, and 5.56' L. To make each of the six panels soundproof, each panel needs to contain two “leafs,” each made of one sheet of plywood, with an air gap in the middle so that they do not touch each other. First, place one sheet of plywood on the ground, and around each of the four edges screw in a perimeter-fitting piece of 3mm “by the foot” neoprene (a rubber substance oftentimes used in floating studio floors). The neoprene will not only serve to combat sound conductivity, but will also hold the two leaves in place so that the panel is solid. Fill the open space, between the neoprene strips, with rock wool to cancel reflections inside the unit. Finally, use wood glue to affix a top sheet of plywood and then screw into the corners, all the way through the unit, for added sturdiness.
Applying Acoustical Treatment To The Panels
For this project, we’ve decided to make the inside of the panel an absorber, and the outside a diffusor, so that the sounds emanating from the amp are absorbed in the box, but the box itself does not interfere with the room’s tone. To start, cover the entirety of one leaf with studio absorption foam (this will serve as the inside panel), cut to the height and length specifications given in the previous paragraph. On the other leaf, do the same but use 1/2 lb. vinyl, again cut to the aforementioned specifications. This will serve as the outside of the panel, as 1/2 lb. vinyl will diffuse room sound. Repeat for all remaining panels.
Wiring The Panels
Pick two panels to serve as the front and back side of the box. For the front side, you will need to drill three holes, approximately 1" in diameter, one 10" down from the top of the panel and two 10" up from the bottom. Feed in two XLR cables at the bottom holes and a 1/4-inch cable at the top hole, allowing a minimum of two feet of slack. These will serve as permanently affixed mic and guitar/bass cables. For the back panel, again drill one hole, with the same diameter, dead center of the panel, and feed a power strip backwards through the hole. This will also be a permanent fixture. Lastly, fill the holes with silicon for a tight seal.
Constructing The Box
Now affix four small triangular pieces, also made of studio absorption foam, to the inside of each panel, at each 90 degree angle of where the panels will meet inside the box. These will reduce low end (below 100Hz to kill the proximity effect). Assemble five sides of the six-sided box, screwing together at the corners. The final panel, which will serve as the top “cover,” should be left “unscrewed” so as to be removable (after all, you will be taking amplifiers in and out of this box with some regularity). Thanks to the neoprene, the box will still have an adequate seal, so no worries.
Customizing The Box
You may find that you need a bigger or smaller box depending on what make/size of amp you intend to put in the box. The aforementioned measurements are perfect for a large combo, like a Fender Twin Reverb, or a half-stack Marshall. However, if you end up needing the space for, say, an Ampeg 8x10, simply measure the amp and work to scale using the following formula: Height x*1; Width x*1.14; Length x*1.39.
Also, you may decide to apply different materials for your absorption/diffusion needs, and this is fine. Fiberglass for absorption, burlap for diffusion, whatever you may choose — it’s largely a matter of taste. If you don’t want to buy acoustic treatment panels, you can always improvise. In many salvage yards you can find old cubicles that contain acoustic material, and you would be surprised what you can find at an office supply store. And if you want to get really “out there” tones, you can use tempered tin to line the inside of your box for a real otherworldly hum.