Recording - May 2010 - 22

that’s really bad. For irregular rooms where no two dimensions match, you get fewer coincidences, but if one dimension is a multiple of the other or a simple fraction (in our case, 8' is 2/3 or 12'), you’ll still get them. In our example room, we have a deadon coincidence at 141 Hz and a near coincidence with 71 and 75 Hz. Now most small rooms I have measured will start a severe low end rolloff or dropoff with odd peaks at just under 100 Hz, so this example is not surprising. There are two ways of looking at this— the strictly scientific and the real-world practical. The science point of view says that while waves of any length can exist in any room, that frequencies below the lowest axial mode of the room’s largest dimension are not excited by the room’s resonances because there are none in that region. Not to diminish any of the science, I can tell you from many small rooms I have dealt with that, once you get beyond wavelengths and modes larger than the room supports, no matter what the science is, it gets messy trying to control the reflection, diffraction and interference or frequency cancellation. It definitely shows up when you break it down to musical instrument frequencies (musical notes) in wave forms and how they react and don’t react in any given room. Erratic modal spacing is always an issue with small rooms, not to mention the combination of large wavelengths’ direct sound, plus “ghost imaging” of that wavelength of sound coming off the backs and sides of monitors, and reflecting sound of that wavelength coming off walls ceilings and floors. Big waves, big party in a small room! Here is simplified example using this room and a musical note ending in frequency cancellation back at the seating position.
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RECORDING MAY 2010

Let’s take low E Let’s take low E on the bass guitar, 42 Hz. A full wavelength of 42 Hz is approximately 27' long. Lowest supporting room mode dimension necessary to support this, or 1/2 wavelength, is 13.5'. (Our room is 15' long and our lowest mode is 38 Hz.) In Figure 1 you can see that in our 15' length, after allowing our monitors to be positioned 18" from the mix desk wall, that leaves approximately 13.5' to the back wall or approximately a half wavelength of low E on the bass guitar... which means that at approximately the same time the direct sound is leaving the speaker, the reflected sound will be returning to the same point, exactly out of phase! This would be something to consider if you are mixing lots of things in the key of E. Although the math above is not exact, you can see the problems that quickly come about in small rooms. Although it is usually advised not to master your recordings

needed what you would think of as typical bass traps installed in the usual upper corners of the room. We needed to go after the 125 Hz range, with a fairly hefty corner foam trap, which is what the client purchased. Once installed in this small room, they trapped quite a bit of what we were going after, but they also removed the nice response in other octaves that we already had going in a good direction. This shows you how quickly small rooms can change their behavior and become very tricky to deal with. There is hope, though! Plan your room—not just acoustically You may not think that decorating items like bookcases, couches, chairs, storage cabinets, and items like guitars hanging on the side walls, all can be helping you with the way your room will sound. But some of these items can actually be beneficial in some acoustical ways that may have never crossed your mind.

Figure 1. In this illustration, the room setup has the potential for the distance from the monitors to the far wall to cause a null (region of lowered audibility) at 42 Hz, low E on a bass guitar. We can often adjust room sizes and furniture/monitor placements to get the worst of these nulls under control. yourself, I do know that it is something that is being done more and more, so it is even more imperative that you do what you can to help the small room become a mixing-friendly environment. So let’s see what we can do to help smooth things out, particularly with respect to the modal problems at around 70 and 140 Hz that we mentioned before. Acoustical treatment products can get very expensive fast, and unfortunately in small rooms they sometimes can go beyond what you want them to address. For example, one small room I tested out I have measured some small rooms where the results of spectrum analysis changed drastically as soon as some people came in, maybe bringing some equipment in with them. Just the addition of bodies and guitars created entirely different results for that same room. Now I’m not saying that you can’t let anyone come into your room! But keep in mind that a finely tuned setup will be easily affected, the way frequencies react in small places by just introducing different ordinary everyday items such as bodies

Recording - May 2010

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