How to Make Your Own Audio Diffusers

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A Simple, Cheap Way to Great Room Acoustics

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Brent Butterworth

Room acoustics is one of the most overlooked aspects of home audio -- but it can also be one of the cheapest and easiest parts of home audio to get right. That's thanks largely to the work of Dr. Floyd Toole, whose book Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms lays out a fairly simple and relatively inexpensive recipe for great-sounding listening rooms and home theaters. Toole's suggestions are backed by his decades of audio research at the Canadian National Research Council and Harman International.

The materials you need to follow Dr. Toole's prescriptions are all available from home centers and crafts supply stores, and the devices you need are easy to construct. In this article, I'm going to show you how to build diffusers, one of the two types of acoustical devices you need for good sound. The other one is absorbers, which I'll cover in another article.

Diffusers reflect sound in many different directions. They give your system's sound a much greater sense of spaciousness, even in a small room. They also minimize "flutter echo," or the bouncing of sound between parallel walls.

My inspiration for this article did not, however, emerge from the desire for great sound. Shortly after Toole's book came out, I built some diffusers that met his specifications, but they were bulky and ugly. Returning to Match.com after a recent breakup, I realized that my great-sounding but bizarre-looking listening room might make potential mates think I'm a little nutty or obsessed. Which I am, but why make my flaws so apparent?

Thus I resolved to make some nice-looking diffusers -- the brown half-cylinders you see in the photo above. Pretty cool-looking, huh? The best part is, you can easily make them look like whatever you want. 

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The Plan (Roughly)

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Brent Butterworth

The image above shows a simplified room layout done more or less according to Toole's principles. The blue things are diffusers. The red things are absorbers -- specifically, foam. They're all mounted on the wall, about 18 inches off the floor, and they're all about 4 feet high. None of these measurements is particularly critical, by the way.

The diffusers are made from concrete forming tubes, cardboard tubes with walls typically about 3/8-inch thick. Home Depot sells them in sizes up to 14 inches diameter, in 4-foot lengths. Construction supply stores sell them in sizes up to 2 or 3 feet diameter, in lengths up to about 20 feet, but they'll be happy to cut them to length for you.

To make the diffusers, you split the tubes in half (it's simpler than it sounds), then attach some supports so you can wall-mount them (also simpler than it sounds).

The diameter you choose matters a lot, because the thicker the diffusers are and the further they stand out from the wall, the lower the frequencies they can affect. According to Toole, a geometric diffuser like the ones we're talking about here has to be 1 foot thick in order to be effective through the entire midrange and treble region.

However, 1-foot-thick diffusers are bulky, and the 24-inch-diameter concrete forming tubes required to make foot-thick diffusers are expensive. If you want to make your listening room great, build 1-foot-thick diffusers. If you want it to be very good -- and nice-looking -- and more affordable -- you can use the 14-inch-diameter tubes available at Home Depot. These will give you 7-inch-thick diffusers, still better than many of the too-thin commercially available diffusers sold by pro audio stores. I went one notch better than the Home Depot path, building 8-inch-thick diffusers for my back wall (cut from 16-inch-diameter tubes purchased at a construction supply store) and 7-inch thick diffusers for my side walls.

Positioning of these diffusers isn't ultra-critical, but it's a good idea to put a couple at the point of first reflection on each side wall -- the place where, if you put a mirror flat on the wall, you can see the reflection the speaker nearest that wall when you're sitting in your favorite listening chair. You can also put a couple more further back along the side wall if you like. Definitely put a few along with back wall, which will do a great deal to minimize flutter echo.

Obviously, the size, shape and layout of your room will influence your diffuser count and positioning. Of course, another significant consideration in this decision is your significant other's tolerance for acoustical treatment devices.

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Step 1: Measuring for the Cut

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Brent Butterworth

Once you have your tubes, you'll need to split them in half. The cuts need to be straight and precise in order for your diffusers to sit flush against the wall, and to look like something you bought rather than something you made.

I used a jigsaw (or sabre saw) with the finest-tooth blade (24 teeth per inch) I could buy. The finer the teeth, the smoother the cut. You could easily do this with a hand saw, but your cut probably won't be as smooth or precise.

I DO NOT recommend you attempt using a powered jigsaw unless you have some experience with one. Either get a more skilled friend to do it or study up on proper operation and safety practices, then spend some time practicing cuts on junk wood. Even skilled operators can have accidents; I myself have been to the emergency room due to a power saw accident, and still have the scar on my left thumb to prove it.

If you do make your own cuts, be sure to wear safety glasses and make sure other people and pets are not in a place where they might interfere with your work. You are responsible for evaluating your own skills and following safe practices. I and About.com assume no liability whatsoever under any circumstances for any accidents, damage to persons or property that may occur because you undertook this project.

The first step is to mark your cuts. Here's how I did it. First, I measured the actual diameter of the tube, which if memory serves turned out to be 14-1/4 inches with the tubes I got at Home Depot. Then I took half this distance, or 7-1/8 inches, and marked that height on each tube using a framing square, as you can see in the photo above. But before you make the marks, either chock up under the tube or put something heavy inside the tube so it won't roll. I used an anvil -- you know, like the one Wile E. Coyote used to try to drop onto the Road Runner.

You need to mark the halfway point on the tube on both sides, on each end -- again, making sure the tube doesn't roll.

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Step 2: Making the Cut

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Brent Butterworth

To make a smooth, straight cut, clamp a 1x2 onto the side of the tube as seen above, with the 1x2 aligned with the marks you just made. Don't use cheap 1x2s, because those are usually warped. Use the expensive ones, which are straight and almost always defect-free. It'll be worth the extra few bucks because you'll be cutting these up later to make your mounting brackets.

Now carefully cut the tube by using the 1x2 as a guide for the jigsaw, as you can see above. Of course, because the blade's in the center of the saw, your cut will be offset from your marks. With my saw, the offset was 1-1/2 inches. But this doesn't matter because you'll have a matching offset on the other side.

Go nice and slow, and you'll be rewarded with a straighter and smoother cut.

With one side done, unclamp the 1x2 and move it over to the other side of the tube. Now clamp it along the other marks you made, making sure that you clamp it so you'll get two even halves when you make the cut. If you make the cut on the wrong side, you'll end up with one diffuser that's thicker than the other.

I'm assuming you want to make your diffusers 4 feet high. But if your room design or existing wall decor requires a shorter diffuser, no problem -- you can easily cut them to whatever length you want. To make sure your line is straight, mark the distance on both sides of the half-tube, then stretch a wide strip of something around the tube to serve as a guide for marking your cut line. I used a wide fabric belt. You could also tape a couple of pieces of printer paper end-to-end to make the mark. Then just make a slow, steady and precise cut along the mark with the jigsaw or a hand saw.

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Step 3: Nailing in the Brackets

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Brent Butterworth

For these diffusers, the mounting brackets are just lengths of the same 1x2 you used as a guide for your saw cuts. Cut them to the same distance as the original inside diameter of the tube. (Use a mitre box to assure a straight, square cut.) Now nail them in as you see above. I put two brackets on each diffuser, both so I'd have something to hang them from and so they'd be less likely to warp. I put one bracket 1 foot from each end of each diffuser, but that distance isn't critical.

I used 1-1/2-inch wire brads with flat heads that measure about 1/8 inch in diameter, two brads per side per bracket. Be gentle with the hammer, because the cardboard tubes dent easily. Just get the brad head so it's flush with the tube.

Now mark the center point in one of the brackets and drill a 3/8-inch hole there. You only need to put a hole in one of the brackets. This accommodates my quick-and-dirty mounting method to be discussed shortly; if you want to use picture hangers or whatever to mount your diffusers, you don't need to drill these holes.

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Step 4: Finishing Touches

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Brent Butterworth

Here's where you bring your own creativity to the process: decorating your diffusers.

Of course, if you really dig the Sakrete logo, you don't have to decorate them at all. But that defeats our purpose here, doesn't it? You could paint the diffusers, but keep in mind they're made like giant toilet paper tubes, with a continuous seam wrapping around the tube. You're better off covering the tubes with something. I prefer fabric, but you could also use wallpaper or pretty much whatever you want.

Here's where you can get a lot of spousal buy-in: Let your significant other choose the fabric. I liked the thickness and the low cost of the brown felt I chose, but you can pick whatever you want. Perhaps a whimsical paisley? Or a favorite cartoon character? It's up to you. Just make sure the store has enough of it because you'll be using several yards' worth. 

I do have one suggestion for serious home theater aficionados: If you're using a video projector, you would be well-served to wrap your diffusers in black or dark gray felt. This way, they'll absorb light, and the less light bouncing around your room, the better the contrast you'll get on your screen.

To apply the fabric, use a spray adhesive like Loctite 200. I cut the fabric with about 6 inches to spare on every side, then sprayed the surfaces of the tubes, then applied the fabric, smoothing it out with my hands so there were no wrinkles. I gave the adhesive half an hour to set, then trimmed the fabric to leave about 2-1/2 inches excess all around. Then I sprayed the insides of the tubes on their long sides and folded the fabric in, making a couple of quick cuts with scissors to accommodate the mounting brackets. After letting the adhesive set for another half hour or so, I finished by blasting the insides of the tubes at the ends with a generous amount of adhesive and folding the rest of the fabric in.

I'd go into more detail here but honestly, fabric application is a little outside my areas of expertise. This is stereos.about.com, not upholstery.about.com.

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Step 5: Mounting the Diffusers

diffusers_on_wall.jpg
Brent Butterworth

My mounting system for the diffusers is amateurish but effective: I hung each one from a single drywall screw. The diffusers barely weigh anything, so you don't need to worry about hitting a stud with the screw. Just mark the place where you want to mount it, put the screw in so it sticks out about 1 inch, then hang each diffuser from the hole you drilled in the back bracket.

The downside of this "technique" is that drywall's not very sturdy, so the diffusers can easily be torn off the wall by accidental impacts, kids trying to hang off them, etc. If you need more strength, use molly anchors or toggle bolts or something.

I happen to have a series of long windows along the left rear side of my listening room, with no place to screw in any sort of mount. To use a couple of diffusers alongside these windows, I added three legs each to two of my diffusers so they can stand on their own at the desired height. The legs are just 24-inch lengths of the same high-quality 1x2s mentioned before, attached to the diffusers with two 1/4-inch bolts per leg so that 18 inches of leg sticks out from below the diffuser. You can see them toward the rear of the photo above.

Or you could use some monofilament fishing line to hang them from the ceiling. Or you could make the diffusers 6 feet high and just let them stand on their own. There are all sorts of possibilities here. But whichever way you go, you'll get better sound in the bargain.

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