Truth or Consequences: Improve Your Studio's Acoustics

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

           

BASE PREPARATIONS

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.