Don’t Feed The Beast
Feedback is not just hard on our ears – it’s also hard on gear, capable of destroying components such as loudspeakers and amplifiers in just a few seconds. As a result, we need to do everything possible to avoid it, and fortunately, there are some basic rules of thumb that, if followed, go a long way in helping to avoid the problem.
Loudspeakers should be placed to the front of performers to eliminate the “loudspeaker to microphone path,” which is a primary cause of feedback. (The output of the loudspeakers is captured by the microphones, which sends it through the system and amplifies it, over and over, and thus the system is “feeding back” via what’s called a “feedback loop,” Figure 2).
Further, all mics on a stage/platform should have a directional pickup pattern, typically called unidirectional or cardioid, so that they pick up sounds in front of the mic while rejecting sounds at the sides and rear.
A mic’s pickup pattern acts like a flashlight beam: any sound present where the pattern points is likely going to be picked up by the mic and amplified. Some mics have a wider cardioid pattern (think of floodlights, which have a wider pattern) while others have a more strict super or hypercardioid pattern (like penlights). Simply, narrower patterns tend to curb feedback because they pick up more direct sound (from the front) while rejecting indirect sound (from the sides and rear).
To further combat feedback, also try to place loudspeakers as far downstage as possible, with the mics pointed in the opposite direction. This puts more distance between the two in addition to aligning the cancellation portion of the mic patterns with the back of the loudspeaker patterns, where there are the fewest mid and high frequencies (Figure 3).
There’s an exception to the previous discussion: while it is indeed usually best to keep loudspeakers and mics out of each other’s paths, there is a cool (and effective) placement technique with columns like the aforementioned EVOX 8 cabinets. Because they don’t generate single point source that in turn creates a beam spot, they’re much less susceptible to feedback even if placed behind the performers (and therefore behind the mics).
I do this regularly with low to moderate volume musicians who are capable of adjusting their own instrument volume while listening to the house mix (Figure 4).
Note the lack of stage monitors, because the columns take care of monitoring. Of course this only works well if musicians can be trusted with their own mixes, so it may not be feasible in your situation. But still, I encourage trying it because it can help a praise team sound spectacular, particularly in smaller rooms/outdoor spaces.
Don’t Forget Monitors
Stage monitors (a.k.a., wedges) are also loudspeakers, so their placement can’t be ignored in relation to microphones or the result could be feedback. Because wedges should be aimed at the ears of performers, the most effective placement is directly in front of mic stands, pointing up.
Many modern wedges now provide two different cabinet angles: nearly straight up for location directly in front of a mic and a shallower angle that provides more coverage toward the back of the stage.
Whatever the case, point wedge horns right at the ears of the target performers while making sure this output is behind mic pickup patterns; this cancels out the sound of the wedges as much as possible.
Purists will note that hypercardioid mics like the first edition Shure Beta 58A offer a very narrow front pickup pattern, but keep in mind that this is achieved by sacrificing some of the rear rejection pattern.
Hypercardioid patterns are designed to work well with either a pair of floor wedges split 15 degrees off the center-rear of the mic or with a single floor wedge slightly offset from the rear of the mic. Pointing the back of a hypercardioid mic directly at a wedge makes feedback more likely than if it’s offset by 30 degrees or so (Figure 5).
Dead Sources Tell No Tales
Switch off extra sound sources when they’re not needed. Leaving an acoustic guitar “hot” while setting it on a stand next to a wedge can lead to a feedback disaster. The whole body of the instrument will tend to vibrate passively like a drum-head, channeling stage noise into the pickup, which then translate to howling feedback. Even if it doesn’t feed back, the extra sound from the pickup mixed into the house system can cause echo and phase-related problems.
Savvy acoustic guitar players use a “kill” switch on each guitar line, muting the instrument whenever it’s out of their hands. Also, always turn down main and monitor outputs on the console/mixer before powering up a system to avoid jarring (and unprofessional) feedback from an unexpected open mic.
Still, we simply can’t control everything at worship services – musicians who refuse to turn down, unforgiving rooms with hard surfaces and massive slap-back echo, old malfunctioning equipment, and so on. It’s a jungle out there!
But by applying the basic loudspeaker and microphone placement techniques discussed here, the result will be maximized system output, improved coverage, and reduced feedback, all of which add up to a cleaner mix. Other upsides include fewer complaints, more compliments, and a better overall worship experience for everyone – and that’s the bottom line.