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.
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.
Before getting started, make sure that your monitor speakers are positioned properly to minimize acoustics-related problems and that you have chosen the position of your mix sweet spot carefully. Those matters are beyond the scope of this article, but they have a direct influence on the amount of equalization you need to flatten your room's bass response. Trying to use EQ to compensate for improper monitor placement is a losing battle. (For information about how to correctly position your monitors, see the sidebar “Now What?” in the June 2001 cover story, “Good References.”)
Also, make sure your speakers are fitted with properly sized protective fuses before you proceed. Loud monitoring levels during testing or overzealous equalization boosts can cause speaker damage if your speakers are not protected. The problem is, you don't always know what's over the edge until you've fallen off the cliff, so both caution and fuses are warranted. If you're not sure what type of fuse to use for your speakers, ask the manufacturer.
Finally, use full-range speakers or a system with a correctly dialed-in subwoofer that is capable of reproducing frequencies as low as at least 40 Hz. If your monitors don't go that low, consult the specifications sheet to determine their 3 dB — down point on the low end and then simply don't attempt to equalize the room below that frequency. Applying massive amounts of EQ to speakers in an attempt to compensate for their inability to reproduce very low frequencies is a recipe for equipment damage (not to mention that it will skew your mixes).
PLAYING IT BY EAR
The simple method of finding and correcting room modes is somewhat inexact, but it can yield fairly good results for those not willing or able to commit to more technical forays. Still, you will need a sweepable sine-wave generator (commonly called an oscillator) for the procedure. The Gold Line TS-1 Audio Test Set ($469; see Fig. 1) is a good choice; it lets you manually sweep a sine wave across the entire audio spectrum with no changes in amplitude.
Refer to Fig. 2 for the gear setup. Patch the output of the oscillator into your mixer's line input and route it at equal intensity (that is, panned dead center) to your L/R stereo bus. Next, patch the mixer's left and right control-room outputs through the parametric equalizer's left and right inputs, respectively. Make sure the equalizer is bypassed or set to flat response initially. Then, patch the L/R equalizer outputs through your power amp's corresponding I/O and, from there, to your left and right monitors (unless you're using powered monitors, in which case you patch the equalizer's outputs directly to the monitors' inputs).
If you wish to equalize two or more sets of monitors, you have two choices. The best solution is to equalize each pair of monitors using a separate dual-channel equalizer. In that case, you'll need a mixer that sports multiple pairs of control-room monitor outputs or you'll have to mult its single pair of control-room outputs at a patch bay before the equalizers.
A less desirable alternative is to perform the test procedures and equalizer setups with your main reference monitors and then use the same equalizer and settings for the additional pair or pairs of speakers. Granted, the equalizer settings for the main pair of monitors won't be ideal for the others, and the selected amounts of boost or cut probably won't be optimal, either; however, the approach will usually provide at least better-than-nothing results and sometimes quite noticeable improvements.
FIG. 2: The amount of gear needed for finding and correcting room modes by ear is modest. In addition to your standard mixdown setup, all you need is a sweepable sine-wave oscillator.
Once everything is properly routed, set the level of the sine-wave tone coming out of your speakers so that it's loud but not loud enough to hurt your ears. If you have a sound-level meter (I use the Radio Shack Digital-Display Sound-Level Meter; $59.99), you can be more exact and can adjust the monitoring level to approximately 80 to 85 dB SPL at the mix position. That is loud enough to excite room modes, yet quiet enough that it won't damage your hearing during short work periods. Even so, you shouldn't perform the following tests for more than 15 continuous minutes without taking a break.
I can't stress too much the importance of conducting the tests while sitting in the mix position. The conclusions you draw, and the corrections you make based on those conclusions, will be beneficial only if they are made from the sweet spot. The first thing, then, is to position the oscillator so that you can sweep its frequency comfortably while sitting in the mix position facing the monitors.
Begin the test with the sine-wave oscillator set to 20 Hz. Slowly sweep the frequency to ever higher frequencies while listening intently for changes in level intensity. When you hear the level dip or get louder, rock the oscillator's frequency control back and forth over that narrow band until you've dialed in the exact frequency at which the boost or cut in the room is the most dramatic. The frequency you dial in will probably correspond to that of one of the axial room modes you tabulated earlier. Refer to your written table to confirm that and to verify that you're on the right track. Make a note of the exact frequency at which you heard the problem in the room.
Repeat that procedure for higher frequencies. Once you reach 300 Hz, stop the test. Look at your table and notes and decide which three or four frequencies are the worst offenders. If your equalizer offers more than four bands per channel, you can treat more modes, but you'll probably discover more room modes than your parametric equalizer has bands to deal with. Because you can't treat them all, you must decide which are most in need of corrective EQ. Usually, the ones below 200 Hz are the most important to treat.
DOWN TO BUSINESS
Now treat the problem modes you pinpointed. First, engage band 1 on both channels of your equalizer (that is, make them active) and set the bandwidth controls to the narrowest settings. Park the oscillator on the lowest frequency you want to treat and then set the L/R channel frequencies for band 1 on the equalizer to that same frequency. Next, either boost or cut to neutralize the room mode. That can be an inexact process because most equalizers don't provide numerical readouts for setting center frequencies for the bands. You therefore initially need to apply overkill amounts of EQ boost or cut, which will be clearly audible, and then sweep back and forth with the band's center-frequency control until you find the setting that best counteracts the effect of the room mode.
If the mode causes a notch at the mix position, turn the monitoring level way down. Boost band 1 of the equalizer at least 10 dB at the outset so that you can clearly hear its effect; you don't want to damage your speakers or hearing in the process. With band 1 boosted at least 10 dB, slowly sweep its frequency control over the range in which the oscillator is fixed. When you hear the room mode increase in level (the effect of the notch diminishes), band 1 is set to that room mode's frequency.
Now, reduce band 1's boost to a reasonable level, about 6 dB, and bring up your monitoring level so you can hear more subtle tweaks. Again, rock the oscillator's frequency control back and forth over the room mode's frequency. Does the sound still dip in level at that frequency (compared with neighboring frequencies)? If so, increase the equalizer's band 1 boost a bit. Conversely, if the 6 dB of boost you provide are too much, you'll hear a boost instead of a dip at the room-mode frequency, and you should reduce band 1's boost. Each time you make an adjustment to the equalizer's boost, sweep the oscillator over the offending tone to judge the results. When the level is consistent with that of surrounding frequencies, the room mode is equalized.
An important point: in most cases, you will not want to boost more than 6 to 8 dB in any particular band to correct deep notches. Excessive amounts of EQ boost will quickly eat up your monitoring system's headroom, causing an increase in distortion. You should adjust your equalizer's input and output levels to prevent the equalizer, power amp, and speakers from clipping. But often it is preferable to only partially correct a deep notch and then learn to live with the remaining dip.
FIG. 3: Analyzing room modes with a DAW-based spectrograph requires routing pink noise through the monitors and capturing it with a measurement mic. The signal is then routed through a mic pre and an A/D converter into the DAW.
Use a similar technique to correct modal spikes in your control room. Set band 1's boost/cut for maximum cut and sweep the equalizer's frequency control over the range in which the spike resides. When you hear the oscillator's tone go down in level, the equalizer's frequency control is set correctly. Next, repeatedly adjust the amount of EQ cut, sweeping the oscillator's frequency after each adjustment until the offending frequency no longer spikes in relation to surrounding frequencies. Feel free to apply more than 8 dB of EQ cut to correct a spike, as it will not compromise system headroom.
If after making EQ adjustments you find that a dip or a spike persists but occurs over a narrower range, try slightly increasing the equalizer's bandwidth control for the affected band. In many cases, however, that will not be an issue, and you'll want to keep the equalizer's bandwidth controls at their narrowest settings.
Symmetrical rooms with symmetrical speaker setups typically require mirror-image equalizer-control settings for both channels. On the other hand, asymmetrical rooms or asymmetrical setups may require slightly different boost/cut settings on different channels for the same band. If that's the case, pan your mixer's control room output completely left to make boost or cut adjustments for the left speaker and completely right to adjust for the right speaker.
You might also be tempted to treat certain frequencies differently from one speaker to the next to compensate, for example, for different room modes getting excited because the two monitors are set up at different distances from nearby walls. Typically, though, that is best avoided; focus instead on correcting modes common to both channels.
Once you tame the lowest offending frequency in your room, repeat the entire process for the other modes using the other available bands on the equalizer. When you finish, all treated bands should sound quite flat at the mix position. Nonetheless, the subjective nature of this test procedure makes it inaccurate as compared with the software-driven approach I will cover next.
The most accurate method for tuning a control room requires four pieces of gear: a pink-noise generator; a measurement microphone; a high-quality, neutral-sounding mic preamp; and equipment that can provide a spectrograph function. A spectrograph shows the frequency response of a signal by plotting its frequency versus its amplitude. A high-quality spectrograph will clearly show notches and spikes in your control room's frequency response.
Fig. 3 shows the proper setup for using the gear. First, patch the output of the pink-noise generator through your mixer to your monitoring system. Make sure the room equalizer is initially bypassed or set to flat response. Set up the measurement microphone precisely at the mix position sweet spot to capture the pink noise emanating from the monitors so that it can be analyzed and displayed by the spectrograph. Although any small-diaphragm, omnidirectional, flat-response condenser mic with extended lows and highs can do a good job of capturing your room's response, a purpose-built measurement mic gives the most accurate results. I use the Earthworks M30 ($500; see Fig. 4), which is an exceptionally accurate and affordable measurement mic.
Patch the output of the test measurement mic through the mic preamp and route the output of the mic pre to the spectrograph's input. I use a digital audio workstation (DAW) — based spectrograph application — a far more affordable solution than an audio test kit — and route the mic preamp's output into the computer with an Apogee Rosetta A/D converter to get an accurate audio capture. If you use a DAW-based spectrograph, set input levels at the A/D for a healthy — 1 dBFS reading inside your DAW.
FIG. 4: The most affordable measurement microphone we know of, the Earthworks M30, delivers flat frequency response within 1 dB from 20 Hz to 20 kHz and time-coherent response from 9 Hz to 30 kHz.
I highly recommend the Macintosh-based Metric Halo SpectraFoo Complete Radical 3 ($800) software program for its spectrograph function (see Fig. 5). Although some other Mac-based applications also offer a spectrograph function, SpectraFoo blows them all away by allowing you to see notches and spikes as narrow as ⅔ Hz. Metric Halo also offers a lite version, dubbed simply SpectraFoo, that costs only $400 and provides a spectrograph function too. Both versions of SpectraFoo can operate in standalone mode or as a plug-in for TDM, Real Time AudioSuite (RTAS), and MAS host applications. They require a PowerPC running Mac OS 7.5.3 or better and 6 MB of RAM.
Now you're ready to embark on your search-and-destroy mission for room modes. Get the pink noise cranking through your monitors at about 85 dB SPL and put on a pair headphones, with no input signal, to protect your ears from the obnoxious noise blasting away in the room. Zoom SpectraFoo down to the 20 to 300 Hz region (remember, that is the only range you're treating) and watch its spectrograph-function display for notches and spikes in the frequency response.
Refer to the table of axial modes you prepared earlier to confirm you're on the right track. Again, pick the three or four worst offending frequencies (the ones with the deepest notches and tallest spikes in the spectrograph display) to treat.
Switch to band 1 on both channels of the equalizer and set the bandwidth controls to their narrowest settings. Go after the lowest-frequency room mode on your hit list first. You know the drill: for taming a notch, crank the equalizer's boost/cut controls by 10 dB or more and sweep the center-frequency control through the band where SpectraFoo tells you that the notch resides. This time, though, let your eyes, rather than your ears, tell you when you have the frequency dialed in. As your equalizer's frequency-control knob approaches and then locks into the offending frequency, you'll see the notch start to decrease in depth in real time and then, hopefully, disappear altogether as the frequency response flattens. Once you have the frequency dialed in, fine-tune the amount of boost/cut for the flattest response. Remember that you don't want to exceed 6 to 8 dB of boost in most cases, or your system headroom will likely suffer.
If the lowest-frequency room mode produces a spike, adjust band 1 for maximum cut. Sweep the equalizer's frequency control until the spike is affected and then adjust the amount of cut for the flattest response. Once you tame the first room mode, activate each remaining equalizer band in turn (going from lowest to highest) and “tune out” the other modes using the same procedure.
After doing all you can using the spectrograph, listen to some CDs in your control room and further fine-tune the equalizer settings by ear. Measurements do not always guide you toward the most musical results, so you might feel that an additional boost or cut is called for in one or two bands. I rely strictly on SpectraFoo for determining the exact frequencies to treat, but I fine-tune the amount of boost orcut by ear.
DON'T TOUCH ME
As mentioned previously, room modes that occur above 300 Hz are not nearly as detrimental to your subjective monitoring experience as those below 300 Hz. That's one reason not to bother hunting down and treating modes that lie above 300 Hz.
But there's another reason. Most equalizers cause significant phase shift, which can make the music you're monitoring sound harsh. Fortunately, the effects of phase shift are not that audible at the extreme ends of the audio spectrum, so when you correct bass-region room modes with EQ, you should not hear any phase-based degradation in the sound quality. It's when you begin mucking around in midrange and higher frequencies, at which the ear is most sensitive to phase anomalies, that you get into trouble. You could, for example, find yourself forever chasing after a warmer mix because of compromised monitoring produced by the phase problems you introduced with EQ.
FIG. 5: Metric Halo SpectraFoo and SpectraFoo Complete feature a spectrograph function that clearly reveals room modes at a glance. The window shown plots frequency on the x-axis and amplitude on the y-axis, here revealing a severe notch at about 130 Hz.
To correct imbalances in your control room not caused by room modes, try using any low- and high-shelving EQ your equalizer offers. For instance, a rigid rear wall (often caused by being fastened too securely to the floor) can create an increase in response below 50 Hz in a control room. (In such a case, I don't recommend loosening any wall-to-floor screws, because the floor might creak.) You could smooth out that response problem by cutting with a little bit of low-frequency shelving. However, some experts recommend using shelving EQ for that purpose only if the equalizer employs first- and second-order filters (as opposed to third- and fourth-order filters).
Likewise, if you have a small control room with heavy carpeting and a lot of absorbent acoustics at the room's front end and problems with imaging or comb filtering prevent you from livening up the room by removing acoustic treatments, a small amount of high-frequency shelving boost can bring some air back into the control-room sound. Just make sure to boost no more than a few decibels and leave frequencies below 10 kHz untouched. That means setting the high-shelving corner frequency considerably higher than 10 kHz. Excessive boosting will make the high end sound noticeably harsh, so be conservative.
Room equalization is an invaluable tool for whipping a stubborn room into shape. You'll be surprised what a difference correcting only three or four room modes will make, both in the accuracy of your mixes and in your enjoyment of music.
Like any quest for perfection, the search for a monitoring environment with perfectly flat frequency response is necessarily doomed — no matter how well the room is designed or how well you implement corrections, “perfectly flat” remains an ideal, not an achievable goal. Just the same, it is a goal worth pursuing, and hopefully, this article has armed you with the information and techniques for doing so.
CHOOSING A ROOM EQUALIZER
Room modes typically cause narrow-band notches and spikes in frequency response, requiring you to treat them with surgical precision. For that reason, only a parametric equalizer can provide satisfactory results in room-tuning applications. A parametric equalizer lets you control the bandwidth or Q of each equalizer band to within a fraction of an octave, enabling you to correct notches and spikes in response without affecting neighboring frequencies much. Graphic and other types of equalizers typically have too broad an influence to zero in on problem frequencies and thus should not be used to tune your room.
Both left and right monitors need to be equalized, so it's also imperative that the equalizer be a dual-channel unit. Furthermore, it should have independent sets of controls for each channel. Although more often than not the EQ should be applied evenly to both sides, an asymmetrical room or a monitoring setup with one speaker closer to a wall than the other may require that you dial in different equalizer settings for the left and right channels.
Ideally, your parametric equalizer should offer at least four bands per channel, with enough frequency-range overlap that at least three bands can simultaneously be tuned to frequencies below 200 Hz. A unit that offers additional high and low shelving or high- and lowpass filters is especially valuable. That's because problems that occur in rooms at high- and low-frequency extremes often require relatively broad-band tweaking rather than narrow-band.
Look for an equalizer that offers continuously variable center-frequency sweeping for each band (so that you can tune in to the exact frequency you need to treat) and narrow Q control (down to at least 1/10- or 1/12-octave bandwidth) for each of its bands. Also, each band should be able to provide 12 dB boost or cut (though 6 dB is often all you need).
In general, go for the highest-quality equalizer you can afford, because usually, the higher the quality, the less group delay (progressive phase shift with rising frequency) the unit will exhibit and the better it will sound. Cutting corners is not advisable, because you'll be making mix decisions based on what you hear pumping through your equalized monitors. You don't need to fork over several thousand dollars for the most pristine EQ available, but do shoot for at least a midpriced unit. Two models that provide excellent flexibility and high-quality results are the TC Electronic TC 2240 ($1,288) and the Klark Teknik DN410 ($1,838). I have also heard good reports about the Symetrix 552E ($749), though I have not tested or used that model myself.