The EM Poll

Truth or Consequences: Improve Your Studio's Acoustics

Nov 1, 2001 12:00 PM, By Michael Cooper

           

I hate to be the one to break the news, but the control room you've grown to love is a saboteur and a liar. You had hoped that dressing it up with acoustical treatments and fancy reference monitors would make it tell you the truth about your mixes. But no — it continues to mislead you, doling out a confusing mix of weak and boomy frequencies that leave you wondering where the truth really lies.

This is not voluntary prevarication. It's in your control room's nature to lie. Even the best control rooms, in top-dollar studios, do not provide a completely flat frequency response throughout the room. That's because anywhere physical boundaries — walls, floors, and ceilings — exist, sound pressure will build up and break down at select frequencies, skewing the frequency response of the space contained within those boundaries. You want totally flat? Mix outdoors, with your monitors off the ground and pointing at the sky — and good luck staying in the sweet spot above them!

Don't misunderstand me: proper acoustical treatment and accurate monitors can go a long way toward giving you a trustworthy monitoring environment, and both are necessities for every control room. But they are not a cure-all. Most control rooms, particularly small ones, still need additional help to flatten out the frequency response.

For the engineer in search of audio truth, the decisive finishing touch for the control room is electronic equalization. That entails running the mixer's control-room output signals through dual-channel parametric equalizers before sending them to the power amps and monitors in order to boost the room's weak frequencies and cut the boomy ones.

Purists may argue that electronic equalization is not a good solution (I foresee a truckload of angry letters coming my way!), and I agree that it's a compromise. The main shortcoming is that EQ can smooth out the frequency response only for people sitting in the sweet spot; those elsewhere in the room will likely hear an even more skewed response. I would rather have one spot in my control room that I know I can trust, where the frequency response is really accurate, than have every point throughout the room out of whack.

Frequency imbalances in control rooms can be so severe that resolving them with acoustical treatments alone is impractical — you'd have to install so much material (cylindrical bass traps, panel absorbers, and the like) in the room that little space would be left over for gear and people, and even then the problems might not be completely remedied. Room tuning with electronic EQ is therefore often a necessary evil. As long as you understand its limitations, it's also a pretty nifty solution to a vexing problem.

In this article, I will show you how to test and correct your control room's frequency response. I'll also point out pitfalls to watch for along the way. This is not meant to be an exhaustive survey of room-tuning techniques; rather, I'll focus on two approaches I have successfully employed: a simple, coarse, relatively inexpensive method I refer to as “Playing It by Ear” and, for Mac users, an exacting software-driven solution that I call “Heavy Artillery,” which requires more time and expense.

Both methods require you to add a high-quality, dual-channel parametric equalizer to your monitoring setup (see the sidebar “Choosing a Room Equalizer”). In addition to the cost of the equalizer, both methods also necessitate the purchase (or rental) of some additional gear. Of course, you could instead use the money to hire a qualified acoustician to tune your control room for you. But making the investment and learning the techniques yourself offers some advantages; most notable is the ability to tune other rooms or your own should you acquire new speakers, remodel, or move your studio altogether. In addition, doing it yourself will give you personal satisfaction, not to mention an increased understanding of sound and acoustics.

Regardless of the method you choose, I recommend that you read both sections; a lot of information presented in the first section is critical to success in using either method.

WHY ROOMS MISBEHAVE

Before getting into the nitty-gritty, here's a closer look at the origins of the room-response problems you seek to correct. Imagine water flowing under a bridge: where the water hits the pylons, water pressure builds up. Likewise, sound pressure builds up where sound waves encounter hard barriers — walls, floors, ceilings, and other, smaller, surfaces.

FIG. 1: The Gold Line TS-1 Audio Test Set provides a sine-wave oscillator, frequency counter, and level meter in one package.

You perceive the effects of that build-up as changes in loudness levels of particular frequencies at particular points in the room. The amount of change (whether boost or cut) and the frequencies at which it occurs derive largely from the room's dimensions. For example, frequencies with wavelengths twice as long as the length, width, or height of the control room will resonate considerably more than other frequencies. That happens because each reflection of the sound wave off of a room surface combines in phase with the last reflection — a phenomenon called constructive interference — causing a boost in amplitude. Depending on where you are in the room, you might hear a greatly exaggerated level at that frequency (because you're standing where a peak or trough occurs for the original and the reflected waveform) or very little level (because you're standing at a zero crossover point, a point in the room where an original and reflected waveform line up exactly in phase and have an amplitude of zero).

Those resonant waves, also called standing waves or room modes, are responsible for the tonal imbalances that plague virtually every control room. When you tune a control room with EQ, the goal is to compensate for the boosts and notches in frequency response that room modes cause so that the resulting response will be flatter.

Actually, room modes occur in a number of ways: by resonating between two opposing boundaries (whether two walls, floor and ceiling, or whatever, called axial modes); four boundaries (tangential modes); or six boundaries (oblique modes). This article will focus only on axial modes, because they are the strongest of the three types and the only modes whose frequencies can be calculated using simple math.

SIMPLE EQUATION

If you know your control room's dimensions, you can predict the frequencies at which axial room modes will occur by using the following formula:

f = 1,130/2d

In the formula, f is the frequency of the mode measured in hertz, and d is a room dimension measured in feet. One thousand one-hundred thirty, in feet per second, is the approximate speed of sound at normal room temperature and humidity.

For example, a room that is 16.5 feet long will produce an axial mode at roughly 34.2 Hz (1,130/(2 × 16.5) = 34.24). That is the lowest-frequency axial mode that will resonate between the example room's front and rear walls. But axial modes will also occur at whole integer multiples of that fundamental frequency, that is, at 2 × 34.2 Hz = 68.4 Hz, at 3 × 34.2 Hz = 102.6 Hz, at 4 × 34.2 = 136.8 Hz, and so on.

Here's another example, this time for a control room that is 10.5 feet wide. Using the same formula, you can predict that axial modes will occur at 53.8 Hz, 107.6 Hz, 161.4 Hz, and so forth. The room height can also be plugged into the formula to calculate floor-to-ceiling axial modes.

Using the supplied formula, do the math for all three of your control room's dimensions — length, width, and height — and enter the results in a table. (See the table “Calculating Axial Modes” for an example.) When constructing the table, give each room dimension its own column for data entry. Calculate the fundamental or lowest-frequency room mode (f1) and whole integer multiples (f2, f3, f4, and so on) for each dimension and enter the results in rows in the table.

If your room has varying dimensions from wall to wall due to closets or alcoves, enter your results for each dimension in a separate column. Note, though, that you won't be able to use the formula for irregular constructions such as splayed walls or cathedral ceilings. Although rooms with such structural irregularities usually offer acoustical advantages, mathematically calculating their axial modes is not feasible, given the constantly varying dimensions. In the “Heavy Artillery” section, I'll discuss other methods for hunting down room modes in such spaces.

For reasons that go beyond the scope of this article, room modes greater than 300 Hz are not usually that problematic. You therefore need to calculate (and treat) only those that occur in the 20 to 300 Hz range; frequencies above that point should be left alone. In other words, the primary goal is to give the control room a really flat bass response.

When you finish entering the data in the table, look for common frequencies (those falling within 5 Hz of each other) in the columns and put them in parentheses for easy reference. Room modes that pile up at the same approximate frequencies are almost always the ones that cause the deepest notches and spikes in frequency response. You now should have a list of room modes for your room. Some will cry out for treatment; others will present less of a problem.