Assuming that one of the goals in treating a mix room is to achieve flat frequency response at the mix position, one must simultaneously grapple with room dimensions; resonant and reflective surfaces; speaker placement and stands; subwoofer placement and settings; crossover frequencies; active and passive acoustic treatments; program EQ; and more. Managing all these variables with any kind of know-how requires precise measurement tools, the skills to use them, and a general strategy that can accurately account for how all of these variables interact. It’s no surprise, then, that very few people really know how to flatten out a room’s frequency response.
To complicate matters, today more and more people (myself included) are mixing in relatively small residential spaces built with no regard for acoustics. To be blunt, most smaller residential spaces are acoustic disaster zones with frequency responses that can look like the Alps. The ability to work from one’s home-based studio has a lot of immediate advantages, but acoustic accuracy is usually not one of them. That’s going to take some doing.
And it gets worse. The smaller your room, the worse your problems will be. Imagine throwing a super-ball as hard as you can at a wall in an 8 x 8 x 8 ft room. You’d probably find yourself ducking for cover as the ball zips from surface to surface. Now compare that to throwing the same ball with the same force in a room three times larger per dimension (24 ft). In this larger room, you’d probably just watch the ball lope around the room. As ironic and annoyingly true as it is, smaller spaces actually require a lot more acoustic treatment to get control of reflections.
Being a New Yorker, my room is predictably small: 15 ft deep by 7 ft wide by 9 ft tall, with the rear wall being a bit out of parallel to the front. I’m actually lucky that my room’s dimensions are not exact multiples of each other and that I have that odd rear wall, but it’s still a small and problematic space. About five years ago, I went to www.realtraps.com, read everything, and then built and installed eight bass traps made from Owens Corning 703 Fiberglas panels. The improvement was night and day, and I felt I could actually do professional mixing in my room.
This year, I installed a new console. This installation shifted the speakers and the mix position, and it introduced new reflections from the console’s surface. These changes provided the incentive to begin a more concerted, multi-faceted approach to flattening my room’s frequency response. I knew my DIY capabilities weren’t going to get me too much further, so I reached out to Ethan Winer at RealTraps as a first step in my approach. (Note that RealTraps only makes and sells passive absorbers and diffusor panels.)
First I emailed Ethan
The first thing I did was replace a pair of my DIY 4 ft x 2 ft x 2” bass traps that hung behind my speakers in the front corners of the room with a pair of standard MiniTraps. MiniTraps measure 4 ft x 2 ft x 3.25’’ and have a proprietary membrane that my DIY traps lack. This membrane not only improves low-end absorption, but it also reflects some mids and highs. The MiniTraps won me about 2 dB of improved low-end flatness at my mix position (this is considerable), and the reflective surface kept the room active in the mids and highs so that we could shift the high frequency absorption to more strategic locations.
Next, I installed the MiniTrap HF (High Frequency) directly above my mix position as a cloud. This panel absorbs floor-ceiling low-end waves, and being absorptive into the mids and highs, also absorbs first reflections from my tweeters. (Note that low-end absorption of the HF model is slightly sacrificed to make them absorptive at higher frequencies. You should choose and place the two MiniTrap models with these features in mind.) To further control first reflections, I hung the RFZ panels on the left and right of my mix position halfway between me and my speakers. In conjunction with the cloud, these panels created a completely different sonic image than I had been used to, but once I listened for a day or two, I adjusted to the new sound and really appreciated what I was hearing. The phantom image (that illusion that there’s something 3D in the middle of the stereo field) and the width of the stereo image were both significantly improved.
The RFZ panels warrant further consideration because they absorb down to 300 Hz, a far lower frequency than the more typical foam offerings can absorb. Doing some measurements with and without the RFZs showed serious improvements in frequency response from 300 Hz up past 7 kHz, with an impressive flattening of peaks and nulls between 500 Hz and 5 kHz. I’m talking about almost completely flattening out a lot of frequencies that were deviating by as much as 7 dB. Because first reflections will reach the mix position slightly later than the direct signal, many of these deviations are the result of phasing (when these phase-related deviations scatter across the mid and high frequencies at predictable intervals, we call this comb-filtering). My measurement software confirmed that phase was involved.
Pausing for a moment, let’s summarize the logic and results of Ethan’s recommendations so far. First, putting thicker and more efficient MiniTraps in the front corners behind the speakers and on the ceiling produced considerable low-end flatness. Second, strategically relocating mid and high–frequency absorption from the rather useless location behind the speakers to the ceiling and side walls created a reflection-free zone around the mix position that drastically flattened the frequency response in the mids and highs. It’s all about getting the right panels into the right spots, and Ethan’s advice was spot-on.
From this point, it was time to address the rear of the room, for which Ethan recommended a pair of MondoTraps (4 ft 9’’ x 2 ft x 4.25’’) and a pair of RealTraps diffusors (4 ft x 2 ft x 6’’). I mounted these panels on optional, free-standing, powered-coated steel stands (a breeze to attach) and placed them behind the sofa in the rear of my room, with the MondoTraps in the corners and the diffusors overlapping them in the center of the back wall.
Listening to music, the two MondoTraps definitely tightened the low end, and while measuring with and without them, I could see that I was gaining roughly another 2 dB of flatness at the mix position at lower frequencies. (MondoTraps are twice as effective as MiniTraps below 100 Hz.) With the Diffusors, which also trap low frequencies starting around 400 Hz, there wasn’t a significant improvement in low end, but the improvement in clarity all around the room was really impressive. The RealTraps Diffusor is a QRD (Quadratic-Residue Diffusor) designed to randomize mid and high–frequency reflections as a means to combating the peaks and nulls created by phasing and comb-filtering.
Ethan’s recommendations for the rear of my room were, again, spot-on acoustically. Unfortunately, the ugly truth about small rooms reared its head, as all these panels were just physically too big. As obsessed with sound as I am as an engineer, I am perhaps equally as concerned with interior decoration as a producer. Here’s the crux of it; I’ve never won over the creative trust of an artist because of an improved low-end response in the mix position, but I have watched many artists decide on the spot whether they can work in a room or not based on the initial vibe they feel. As a good acoustician, Ethan encouraged me to reconsider my seemingly irrational position, but I basically ended up saying that if had to take a super-ball in the face to put an artist at ease, I’d do it. We had a good chuckle over our difference of opinion here.
But I also recalled Ethan talking me through a process by which one can figure out where the best place for a bass trap is. With some coaching from Ethan, I removed the MondoTraps and Diffusors and played a sine wave at my problematic modal frequencies and moved an SPL meter around until I found the spot where those frequencies had the most power. Interestingly, it was way up in the top rear corner of the room. This is an odd little corner near a window that the MondoTraps were going to have a hard time sealing off, so instead I snugged a 2 ft x 2 ft x 4’’ panel I had already built into the corner. This strategically-placed smaller panel bought me close to 1.5 dB of low-end flatness at the mix position, and I was impressed with how well these ghost-hunting techniques worked.
While I was also unwilling to hang the large RealTraps diffusors in the rear of this room, it was coincidentally the same time that Tape Op [#83] published the article on how to build BBC/QRD diffusors, which I did according to plan and hung on the rear wall of my room. The RealTraps diffusors did a better job, but this was to be expected as they are 2’’ deeper than my DIY panels, so effective to far lower frequencies. However, my DIY diffusors and the little square bass trap in the corner struck a good compromise between my need for good acoustics and good vibes.
My situation spells out all too well the challenges of improving the acoustics of small residential rooms, and anyone treating a similar room for serious mixing work will need to learn how to do room measurements and become familiar with the best strategies for placing the right kinds of acoustic treatments available. The RealTraps website is a great place to start reading up, and RealTraps offers passive panels that are among the most efficient available with an incredibly solid build-quality at really reasonable prices. RealTraps brought me to the point where I now need to go beyond passive panels and into the realm of subwoofers, active bass traps, Helmholtz resonators, and more. As difficult and involved as flattening out any room can be, you can make essential steps in the right direction with the help of RealTraps. (MiniTrap $200 direct, RFZ $250, MondoTrap $300, Diffusor $600, with discounts for higher quantities; www.realtraps.com)
–Allen Farmelo, www.farmelo.com
by Andy Hong
In the last three issues, I've reviewed a number of moveable products that I'm using in my tracking room to control acoustics. With only one live room that's suitable for full-volume tracking, I...