PLACEMENT, SET-UP, INTEGRATION, TWEAKING
The sawdust has settled, there are still a few splinters under your fingernails, and you can always put the tools away later, really. It’s time to make that box jump off the floor. The non-directional nature of bass has already been discussed, and you will recall that specific room placement of the subwoofer is not as critical as other speakers in your system. It is also true, however, that combining subwoofer cabinets into larger arrays can begin to control the dispersion or directivity of even the sub bass frequencies. We also touched on the subject of standing waves inside the speaker enclosure; it shouldn’t surprise us that we will need to address standing waves in the listening space as well. Bass energy is easily reinforced by reflection, and as those reflections recombine, the resulting buildup can cause very irregular response in the room in the form of peaks. Similarly, phase cancellations can occur when the sound waves at a specific frequency cause destructive interference and cancel each other out, and create holes or nulls in the response.
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The common “split stack” arrangement of dividing speakers on either side of an event stage offers easy deployment and reasonable coverage, but will also present opportunities for the frequency response to have significant peaks and dips due to standing waves and phase cancellation. One possible solution to control phase cancellation at bass frequencies is to place the subwoofers together at the front of the stage as a single array. Unfortunately, lining up the cabinets in a long horizontal row will actually narrow the dispersion in that axis and create a “power alley” effect of strong bass in the middle, but not to the left or right. If the split stack configuration must be used, try to stack the subwoofers vertically if possible, or in 2x2 blocks. If the mid/high speakers are suspended overhead and the subs stay on the ground, some attempt should be made to preserve a consistent front-to-back relationship between all speakers in the array. Keeping the subwoofers in alignment along a line drawn between the left/right main speakers and also in the vertical plane will help to keep the speakers in the same time domain (yes, a foot or two can matter here!) and will improve coherence. If the optimum physical placement cannot be achieved, many electronic crossover networks and digital system processors offer onboard time delay compensation to bring the system into correct alignment.
When properly integrated, the actual location of the subwoofer will disappear and the bass will seem to be coming from the mid/high speakers in the system. Test tones and measurement equipment can help you with your system setup, but good results can also be achieved by spending time with a couple of well recorded CDs with which you are especially familiar. An open microphone will help you to identify sub bass frequencies that are peaked enough to cause feedback. Sometimes a microphone located on a resonant stage can be sensitive to the mechanical transfer of sound energy instead of the expected acoustical path. “Shock mount” microphone holders can help to minimize this effect, as can active low cut filters on mixers or equalizers. You may find it useful to have an associate make adjustments to the various system controls, or even move speaker enclosures, while you remain in a consistent listening position. After satisfying yourself that your primary location is OK, move around a bit to make sure that the wider soundstage is acceptable. Now, on with the show!
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