Tune That Room
The art of DIY Accoustic Treatment
This article first appeared Recording Magazine. I reprint it here with permission, and I encourage you to subscribe to that publication, as they are a stand up bunch of folk!
Ask an amateur recordist where they spent the most in their studio, and the answer is likely a piece of gear. Ask any PROFESSIONAL audio engineer what the most important thing in the studio is, and they’ll say acoustics.
Anyone who’s mixed in a boxy room knows how hard it can be to hear what’s going on, and plenty have experienced the sadness of a poorly translated mix that’s boomy, tinny or muddy in other places. Or, you may have noticed the terror of vocals recorded in an empty room – that hollow, boxy sound that should be reserved for bad indie movies and tele-conferences.
The reason for these problems is sound reflecting and interacting, creating nodes (where a given frequency is cancelled by its out-of-phase reflection), peaks (the opposite condition), comb filtering, ring, flutter and plain ‘ol reverb. The size and dimensions of a room determine where and at what frequencies problems occur, but any normal room has 3 pairs of parallel surfaces, which is the worst possible situation for acoustic interaction.
These facts are becoming more apparent to musicians, and there are fewer square boxes with $10,000 DAWs and no treatment. There are still plenty of expensive studios built to spec, but what about bedroom studios, renters and low budget recordists? Can they build a trustworthy space they can use to get great results? Yes, and it’s not that hard.
Before We Begin
First off, let’s distinguish “acoustic treatment” from “sound proofing”. “Sound proofing” means ISOLATION, which is different than treatment. Isolation means making a room that lets no sound in or out. Doing this effectively takes structural work (which renters can’t do), and is a very different physical phenomenon than treatment. Treatment is making a room sound and function better as an acoustic space, by taming or eliminating reflections INSIDE the room that cause problems. We’re talking about treatment, not isolation, and the great thing is we can do all of our acoustic treatment ourselves.
We also won’t talk about measurement. While measuring a room’s acoustic response with testing equipment and by calculating modes and resonances using room dimensions can be useful in some situations, generally the solutions are always the same. So measurement is outside scope of this article.
Low Frequencies First
Often home recordists just tack a lot of acoustic foam or egg shell mattress on every wall (figure 1). This can be ugly, and it doesn’t work very well. In a room like that, you might clap your hands, hear no reflections and think you’re done. The problem is clapping can’t tell you what’s happening with LOW FREQUENCIES. When you tack up acoustic foam, you’re only dealing with high frequencies. To illustrate, consider this chart:
|Sculpted Acoustic Foam||0.11||0.30||0.91||1.05||0.99||1.00|
Absorption coefficients – 2 inch thick material applied directly to a wall.
Absorption coefficient is a number that represents how much sound is absorbed by a given material at a given frequency. In this small chart you can see that at 2 inches thick, all these materials have pretty pitiful absorption below 250HZ. You can also see that Owens Corning 703 and 705 rigid fiberglass (or equivalent) does a better job than typical acoustic foam.
There’s a better approach, and that’s to build from the ground up, frequency wise. Start your design with LOW frequencies. Low frequencies (below about 200hz) behave differently than higher frequencies, so the solutions are different, and it’s low frequency reflections that cause nightmares like boomy rooms, weak low end response, or untranslatable mixes.
There are a couple of common misconceptions to address here. One is that if you listen at a low volume with near field monitors, the room isn’t a factor. Actually, sound is always reflecting and interacting, and those reflections must be dealt with. Another is that low frequencies don’t have time to “develop” in a small room and thus aren’t a problem. In reality, low frequencies are always present and reflecting off walls, even in a small room.
So what to do? For all frequencies, there are two options: absorb or diffuse. It’s mostly impractical to diffuse low frequencies, so we need to absorb them. Since that’s not a matter of throwing up some foam (unless it’s 10 inches thick or spaced 16 inches from the wall), we need other solutions. First, start with the corners. Low frequency build up occurs in corners, so the first and most obvious place to start absorbing is vertical corners. There are several ways to do this, but they all boil down to spanning and/or filling the corner.
An easy, relatively inexpensive and effective way to do this is by spanning a corner with Owens Corning 705-FRK rigid fiberglass (figure 2). These come in 4’ by 2’ foot panels, so in a typical room, it takes two to go from floor to ceiling. 705-FRK (FRK is a paper backing with a foil face) is dense – 6lbs per square foot – and isn’t as good for high frequency absorption as 703 (3lb density), but spanning a corner with it is great for low frequency absorption. A 4 inch thick panel is best here, but 2 inches works too. Optionally, fill the space behind the panel with fluffy fiberglass. Face the backing into the room to reflect high frequencies if you don’t want the room too “dead”, which will improve low frequency absorption a little.
Another method, the “superchunk”, is more costly, but it’s even more effective and easier to make pretty. A superchunk is made using OC 703. Cut the panels into triangles (figure 3) and stack the triangles into the corner. Then cover the entire structure with some kind of acoustically transparent material (muslin, burlap, anything breathable) to make it attractive and keep the fiberglass contained (figure 4).
A warning: Fiberglass, while not toxic, is itchy and breaks off into airborne pieces. Wear sleeves, gloves and a mask when working with it.
There are also commercial “superchunk” products. Auralex makes a “bass trap” which is a corner piece of acoustic foam and RealTraps make a corner trap which is exactly a superchunk module with the addition of a “limp membrane” face, which increases low frequency effectiveness, just like the FRK.
If your budget’s REALLY tight, and you’re willing to compromise on effectiveness, you can fill a corner with a lot of things. I once filled a corner with old clothes and contained it using pegboard (figure 5). This isn’t as effective as a superchunk or 705, but it’s certainly better than nothing. You could also stack fluffy fiberglass bails in corners without even removing the packaging. It’s ugly, but effective if you have room.
Vertical corners aren’t the only corners in a room. There are also the floor/wall corners and wall/ceiling corners. In most rooms, you should absorb as much low end as you can afford to, and you really can’t do too much, so consider doing something there (figure 6).
Corners are a great start, but probably not enough. Especially with LF absorption, effectiveness is about treating a high percentage of the surface area in the room. As mentioned above, it’s effective to put up very thick material or space it out from the wall a good ways (think 4 inches with a 16 inch gap). That takes a lot of space and still isn’t very effective below about 125HZ, but there are other solutions.
In an unfinished space with support beams (figure 7), you can put a couple nails in and rest a piece of 705 on them, creating a ceiling trap. You should probably cover that with fabric (figure 8).
Optionally, fill the space above with fluffy fiberglass. For a no budget alternative, foam and old clothes might work (figure 9).
All of the solutions we’ve discussed so far are called “velocity absorbers”, because speed drives sound into the material, and that’s dampened when waves hit it. Panel Traps work on a different principle and are called “pressure absorbers”. Placed at room boundaries, they take less space and can be effective to lower frequencies (figure 10).
A panel trap is a little more complex to build, but it’s decently cheap and easy. The main difference is that a panel trap is a sealed box with a thin plywood face and fiberglass inside (figure 11). They’re usually sealed to the wall, but can be made to stand alone, as long they’re located at a boundary. I’ve built several, once with futon slats I found in a dumpster! While panel traps take a little more effort to make, they’re an awesome way to increase low frequency absorption with little space, and since they’re reflective at high frequencies, won’t make a room too dead sounding. Plus, you can paint on them and make them pretty.
There’s a great tutorial on how to build panel traps by acoustician Ethan Winer. I highly recommend referring to it: http://ethanwiner.com/basstrap.html
Once you’ve dealt with the lows, it’s time to deal with highs and mids, and the physics here are much easier. High frequencies can be thought of like a ray of light – they bounce off of reflective surfaces at the same angle they come from. Any surface that shouldn’t reflect, you can absorb or diffuse.
Diffusors spread reflections in an even manner. They’re a great way to even things out while preserving liveliness. You can build several types of diffusors or buy them, sometimes for a decent price (figure 12,13). If you’ve got no budget, you can create some semblance of diffusion with book cases and other random items. While an acoustician would say this isn’t true diffusion, in practice it works ok if things are various sizes.
Real diffusion is a great way to even out the sound of a room and keep it live, but it doesn’t work in small spaces. A typical bedroom is too small for effective diffusion, so strategic absorption is often the best bet. For that reason, that’s all we’ll say about diffusion.
High frequency absorption is the easiest to deal with. 2 inch OC 703 absorbs beautifully down to about 400HZ, and there’s a trick you can use to increase absorption even more. Mount panels away from the wall. The ideal distance is the same as the thickness of the material, so mounting 2 inch panels 2 inches away from the wall or ceiling is perfect (figure 14).
Where to mount them depends on the room and your needs, but if you’re treating a mix/control room, you should start with a “reflection free zone” (RFZ) around the main listening area. You can see where to treat with a mirror, or just sit in your chair and look left, right up and down. These are parallel surfaces you should treat. You should maintain left-right symmetry, and you should treat that area of the ceiling, but leave the floor alone. If there’s carpet you’ve got some high frequency absorption already, but that’s not ideal for various reasons, so leave the floor as is if it’s wood or another hard material. You can always throw a rug down for extra deadening.
There are a few options for materials here, and if you have to settle for egg crate foam, do it. If you have to nail a bunch of blankets to the wall, go for it. Ideally, make panels using OC 703, and cover them with an attractive but breathable fabric. Spacing of these panels from the wall can be as simple as four wood blocks from a scrap heap (figure 15).
If you’re treating another kind of room – say a recording room or home theater – then you probably want an even sound everywhere. If you space high frequency absorbers around, alternating with either panel traps or plain wall, you’ll start to create a kind of “pseudo diffusion”. If you have no budget, don’t be afraid to nail blankets to walls, just remember: the thicker the better.
Creating a booth is pretty easy if you have a closet. You can pack the closet with clothes, hang blankets from the walls, or use a more professional approach and install fiberglass panels. In a booth, you want the deadest sound possible, and that’s about all there is to it.
I keep mentioning rigid fiberglass because it’s the best sound absorbing material and it’s cheaper than acoustic foam products, but if you can’t spend money, there are plenty of alternatives. You can even go around your neighborhood and ask if anyone has anything “soft” they need to get rid of! This can work great – but it may not lead to the prettiest space.
Modularity And Aesthetic
We’re talking about DIY acoustic treatment here, rather than permanent professional studio installations. So you may have to move your studio when you move to a new space. Or if your “studio” is a shared space, you might have to try for a pleasing look. Even if it’s not, you may tire of working in a space that looks like the inside of an egg-carton or a construction site. There are plenty of ways to treat a room and make it ugly, and plenty of ways to permanently install treatment and effectively ruin the room once you leave, but it’s just as easy and not necessarily that costly to do things in a way that doesn’t leave a mark, is easy to move, and is nice to look at.
In my current space, everything is built to move. All of my high frequency absorbers are small painting size boxes with pretty frames and colorful fabric faces (figure 16). They hang just like any picture (figure 17).
Ceiling treatments are lightweight and hang from tiny hooks (figure 18).
My “superchunk” is really 4 small modules that I can take out and move (figure 19), and my panel traps are standalone boxes set on the floor or hung with picture wire, with decoupage designs by an artist friend (figure 20).
The room doesn’t necessarily look normal (figure 21), but it’s certainly not crazy making like my first attempts 20 years ago. In fact a lot of people mistake my HF panels for art.
Just Do It
Nothing we’ve talked about here is much different than what would be done with a big budget, except structural work. Other than that, acoustic treatment on a budget can be as good as any expensive installation. You can hire contractors and consultants to design and build, or you can follow some pretty straightforward principles and do it yourself. It’s one place where you really needn’t compromise, and it’s the one place where you really shouldn’t. So have fun and get your room tuned up!