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Truth or Consequences: Improve Your Studio's Acoustics

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

HEAVY ARTILLERY

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.

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.

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.

SWEET SPOT

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.

MANUFACTURER CONTACT INFORMATION

Earthworks
tel. (603) 654-6427
e-mail earthwks@jlc.net
Web www.earthwks.com

Gold Line
tel. (203) 938-2588
e-mail sales@gold-line.com
www.gold-line.com

Klark Teknik
tel. (952) 887-7444
e-mail adam.grath@telex.com
Web www.klarkteknik.com

Metric Halo
tel. (845) 831-8600
e-mail in-foo@mhlabs.com
Web www.mhlabs.com

TC Electronic
tel. (805) 373-1828
e-mail tcus@tcelectronic.com
Web www.tcelectronic.com