In the limelight this month is an acoustic treatment that does double duty, both diffusing and absorbing. These diffusing absorbers are very effective pinch hitters when a problem with standing waves and room modes requires multiple solutions. It’s possible to buy excellent products that do this job (and they’re reviewed regularly in this magazine), but in the spirit of offering alternatives for readers with more handiness than spare cash, here’s a very workable alternative
For this DIY project you’ll need a utility knife, a framing square, a metal yardstick and a box of ceiling grid tiles. You’ll also need a big piece of paper to help you with the layout, some yellow carpenter’s glue, and masking, strapping or scribble strip adhesive tape (more on this later). Get a five-dollar framing square and invest six or seven dollars on a finger plane.
My recording room was originally the third bedroom in my house, situated in the basement, 7'4" tall, 9' wide, 18' and short change front-to-back. I did some room mode calculations (please see Joe Albano’s article in the March 2004 issue for an explanation of the math), and learned that the room’s worst issues are at 104.2 Hz and at 208.4 Hz and all of the subsequent multiples. And there are others, of course.
There are enough irregular surfaces in the back of the room (desk, filing cabinets, shelves, closet) to tame the reflections in the back half of the room. Fat lot of good it does at the mix position, though.
Over the years I’ve committed gobs of cash trying to absorb away as much of this modal badness as I could. What did I get for my efforts? For starters I had no bass energy left in the room by the time I was done. Highs were iffy and the stereo image wasn’t always what I thought it should be. Mixes that sounded good in the room were a bit lacking when played back elsewhere. Mixing for other environments required conspicuous holes and some weird overemphasis areas in the sound spectrum and it was very hard to do. It was time to ‘zero-out’ the room and stop assuming that living with large masses of foam was where I wanted to be.
I took out all of the foam, the fuzz and the baffles, leaving only speakers and relevant furniture. I stood at the mix position, clapped my hands together once, and the standing wave echoes that came back sounded a bit like that “piiinnnggg” sound that newly filled basketballs make. I repeated that around the room, listening for that zingy ping both side-to-side as well as front-to-back in the room. The back half of the room had no ping at all. The front half (where I track and mix) was soaked with it.
It had been a long time since I had listened to anything in the room as naked as it was, so I put up some music. Eeeww. No question about it, time to go back to the old drawing board.
I’ve got Auralex Maxx-Wall (reviewed August 2000), and lots of it. I put back six panels directly behind the monitors/mixer desk. I set it up as three panels high and two panels wide and it’s all sitting on the factory-provided stands. This four-inch thick barrier stands away from the front wall by about six inches. Well-respected experts generally agree that that’s a good thing to do, so I stopped there and I listened again. Things got better, but I still had standing waves between the side walls and things got a little weird as I moved towards the back of the room.
I paused to buy (and read!) a book on constructing sound studios. Interestingly, it recommended using diaphragmatic absorbers that offer both reflection and absorption to the left and right of the mix position. I looked closely at what they were and realized that I could make my own! We’ll start by understanding how they’re supposed to work, then apply those ideas to our project.
What’s he talking about?
Let’s break down the description. Diaphragmatic absorbers act something like the front head of a kick drum. As sound energy comes into contact with the panel the diaphragm flexes. By flexing the diaphragms absorb energy (not the same as absorbing sound like foam does!). Thanks to the laws of thermodynamics some of the sound energy hitting the panels will be converted into mechanical energy in order to flex the panel. Any sound energy that’s left over is attenuated and will either pass through the panels or get reflected, mostly dependent on the frequency.
The panels are hollow on the inside. Interestingly, since the interior space is triangular it will trap a lot of the sound that passed through the diaphragm and greatly reduce the chances of it bouncing back out again. Because of the size and shape of the interior space it affects primarily the lower frequencies. So far this is really good news.
The shorter-wavelength mid-frequency sounds will pretty much go through the same ritual, but the higher frequencies will be mostly reflected. And because the reflective surfaces are angled and not parallel to each other we’re greatly reducing standing waves.
The net result is that we’ve taken a sort of broadband approach by taming the lows and redirecting the mids and highs within a single project.
Oh, now I get it. How do I make them?
What you’re making are little Evel Knievel ramps that get placed to the left and right of your mix position, out of ceiling tiles originally meant for suspended grid ceilings. I suppose it’s possible to make these out of the kind of fancy panels that get factory-stamped with geometric indentations. But you’re going to be cutting the panels into shapes and sizes that definitely won’t match too well with the factory made imprinted patterns. I recommend you stick with the plainer looking stuff or these units will likely look pretty goofy after you’re done.
There are generally two types of panels you’ll find at the big-box home improvement stores. One kind has an orange/yellow fiberglass core and the other has a grayish wood pulp core. Stick with the wood pulp product exclusively for this project (the fiberglass stuff isn’t dense enough for our application).
Your finished panels will be 1" thick, made by laminating two half-inch ceiling tiles together (another reason to stick with flat stock). The long leg of the panels is 18" wide, the short leg is 6" wide, and they meet at a 90-degree angle. That makes the hypotenuse (the width across the wall for all the drummers out there) about 19" wide.
These units are simply two-sided, not built with enclosed backs, tops or bottoms. It’s very important that air be able to freely move in and out behind the panels for them to work. If they were completely enclosed then they would become resonant bodies and would possibly rattle and make noi se at certain frequencies. This would defeat the purpose of the design. You can sit the units on the floor or a shelf, but be sure that there is breathing room at the top for sound pressure to escape.
These panels need to handle sidewall reflections from waist-high to nearly the ceiling to be effective while you’re sitting or standing at your mix position. Mine are forty-three inches high, made that size to allow my guitars to sit in the floor racks without touching and still have the panels go to nearly the ceiling.
I used a layout template to create the panels. After I cut out the panels I stood them up on the template drawing to make sure I had done it right. Don’t underestimate the power of this template: it’s the only thing that ensures that this project is done successfully!
The drawing starts out with a single straight line on the page going left to right. This represents the face of your wall. Use your framing square and place one leg on that line at the six-inch mark. Holding the square at that point rotate it around until the long leg intersects your pencil line at the eighteen-inch mark. Trace around the framing square and you’ve just marked out the outside dimension of your panel. Retrace this line with a marker if you have to.
Note that there are no hard-to-lay-out angles to fuss with—so far you’ve only used the numbers six and eighteen. This is specifically why the framing square is such a handy tool for this project.
The individual ceiling tiles are half of an inch thick and we need to show both thicknesses on the drawing to cut all of them to shape and size. Relax and take your time with the drawing. The template guide gives you everything you need to ensure that the later installation will go smoothly.
You can see from the template diagram that you need to create bevels on the inside edges of both legs where they come into contact with the wall. The leading edge of the eighteen-inch wide panel gets a pretty steep bevel and the six-inch leg gets a shallow bevel.
Right about now you’re probably wondering why we’re bothering with these bevels at all. Can’t you just cut and make do with 90-degree corners? You need very solid contact between the units and the wall for them to work as diaphragms. Without this support they just flap in the breeze. Diaphragms only work if there’s a solid perimeter to flex against. The bevel cuts give you a little over four inches in width making contact with the wall, and they provide good support to let them do their job trapping the lower frequencies.
Cutting the pieces
Standard ceiling tiles come in 24" x 48" panels. I needed one box of eight panels and had a handful of narrow strips left over after I was done cutting. This yielded me four finished panels.
The template guide shows that the big outer panel is 18" wide and the short outer panel is 5 1/2" wide. (One butts against the other to make the six-inch short side.) Mark out these dimensions on your first tile and rough cut with your utility knife. Use the metal yardstick as a cutting guide and go slowly, taking multiple shallow cuts. If you’re afraid of scarring up the surface you’re cutting on, only cut most of the way through the panel and then bend the cut open. Finish the cut from the opposite side (a technique that’s a lot like cutting drywall).
Be sure to use a fresh, new blade or this is going to take you all afternoon and create more frustration than it has to (and possibly make you have to buy more ceiling tile to replace badly ripped up pieces).
The blade of my utility knife didn’t extend deep enough into the panel to make the deep bevel cuts on the wide sides in one pass. I made a series of shallow cuts and cut away the material from the inside edge to expose the angle as it progressed.
Cutting the angle from the body of the panel is easy if done in small passes. As you cut deeper into the panel, strip away more material from the inside edge so you have more room for your utility knife. This took a while to complete and I finished up the angle with a small finger plane to clean up the cut and make it flatter.
Stand up the freshly cut pieces to see how they compare to the line drawing. If something is rotten in Denmark and the inside faces look a bit scalloped and scooped out, don’t necessarily worry. You can overcome ragged edges later on. Repeat this process for all of your pieces.
Bringing them together
At this point I need to make two admissions about ceiling tiles in general. First of all, they’re not necessarily built for rock’n’roll. You need to reinforce the edges to keep them from being bumped and grinded and getting ugly. Since these are meant to be mounted on the wall to the left and to the right of your mix position you’re going to presume that there aren’t a lot of hard or sharp things moving around that can mar them up.
The second thing is that the tiles can’t be fastened together in a traditional way. Screws are fun but not really effective. You need more surface area for fastening than is below the screw head because of the lightweight panel density. And nails? What, are you kidding me?
Glue is only part of the answer because all that’s really going to be adhered is the absolute top layer of the compressed wood pulp pile. So you need to devise a strategy to keep the pieces bound together that’s effective, attractive and which will last a good long time.
That said, now that you have four pieces of ceiling tile cut to size and beveled, it’s time to laminate them together. The first part of your construction strategy is glue. Liberally spread glue on the inside face of the outermost tile. Use one of the spare pieces as a corner spacer and lay the inner tile into the glue. You want to mate a dress face against a raw wood pulp face on this lamination. And you want as close to exactly one tile’s thickness left as the corner reveal for an effective ninety-degree corner glue up to happen later.
Repeat the process with the short side. Once they’re laminated to your liking lay them down and put something flat and reasonably heavy on them and let them dry. After about an hour you will be able to mate these two sides together with more glue.
The second part of our fastening strategy is tape. What kind of tape? Pretty much anything but that clear stuff you have on your desk at work will do. Masking tape is good. The twine-reinforced strapping tape is fantastic. Or, since the panels are plain white, how about that white tape you use for the scribble strip on your mixer?
I recommend wrapping the perimeters of each panel, making sure to overlap the edges and bending the tape around the corners. It gives your edges a crisper look and lends a bit of reinforcement to keep any of the paper dress face from getting accidentally peeled back. Wrap across the joints to firmly draw the long leg to the short leg and let the tape run wild on the inside face. The tape gives us large amounts of surface area and nicely reinforces the glue line.
Be careful, though. You’re not going to get a second chance to reposition the tape. Once it comes into contact with the dress face of the tile that’s it. Again, slow and careful is the rule.
If you are left with larger sins than scalloped edges caused by bad cuts you still have some alternatives available to you. Since these tiles are plain white there’s no real reason why you couldn’t dip a brush into some paint and make a visual statement, hiding any ugliness in the process.
Attaching them to the walls
Each one of these units will weigh about six pounds. The commercial manufacturers of products similar to this wrap the entire thing in fabric. They use long strips of Velcro secured to the edges of the panels and attach them to the wall that way. I chose to make shelves for the units to sit upon and drove in finish nails in the upper and lower corners to keep them from falling forward. I can move them pretty easily and don’t have to worry about gracefully removing long strips of adhesive Velcro from the wall at some later time.
Position them so that they cover the wall from between the front edges of the speakers to a little past your head when seated at the mix position. I happened to only need two panels per side for this. Your room may require more panels than that.
After installation I repeated the handclap test from my mix position. The zingy ping was gone. I repeated it about four feet behind the mix position and that still held true. Interesting!
I put up some music and listened. WOAH! There was a stereo image like I had never heard in the room before. I stood at the mix position and marveled at how good things sounded. I was detecting sounds that I’d only ever heard in headphones before. Low notes were crisp and clear, not muddy and fuzzy. The mids were right and the highs were sweet. I can only liken it to having headphones over the entire room.
By chance I moved back to about four feet behind the mix position and found a brand new sweet spot that had never existed before. I can only presume that it’s from the second tier of panels reflecting the sound to a new focal point in the room. Now the other musicians I work with can hear more or less what I’m hearing without having to sit down in my chair when I tweak the sound.
I made and installed these baffles in early June of 2003. They look cool (important) and they make the room sound reliable (substantially more important!).
After a few months I decided to add back some two-inch foam above the mix position. It helps kill some reflections from the mixer surface up to the ceiling and back down to me. (And if you’re curious, the sidewall fuzz you see in the picture is on the inside face of the door.)
New mixes done in the room now translate to other environments better than they ever have. Commercial recordings played back in the room now sound, well, right. As I write this paragraph I’m listening to Loreena McKennitt’s “The Mask and Mirror,” listening from the desk in the back half of the room. I’m now able to hear all of the little quiet finger cymbals and other subtle ear candy buried in the mix—from back here. I was never able to hear that stuff past the mix position before. I can’t guarantee that your mileage is going to exactly match mine, but I’ll wager a shiny new dime that they’ll make you smile and your music sparkle like never before.
Now what about the cost? I paid a whopping $13.52 for a box of eight plain white ceiling tiles, purchased from one of the home improvement warehouse stores. That cost, some glue and scribble strip tape that I already had, and in an afternoon I got a very substantial upgrade to my studio.
There are commercial products similar to this available on the market, and their materials and construction methods are held to much stricter manufacturing tolerances and acoustic standards than we can achieve in a DIY forum. If you’re curious about them, definitely have at it, and keep reading Recording for evaluations of their performance. But as I said before, if you’re handy and short on cash, you’ll be surprised at what these simple panels can do for you.
Matt Seiler (contact him via firstname.lastname@example.org) is a professional carpenter, guitarist, recording engineer, and maker-up of cheap ways to do things oneself that might otherwise cost a bit more, in the Chicago area.