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Feedback Control

John McJunkin • September 2019Sound Sanctuary • September 11, 2019

There are dozens of things that can go wrong in a live pro audio environment, many of which go completely unnoticed by the audience. Feedback is not one of those. It’s the one thing that can cause the heads of the entire audience to swivel around — in choreographed unison — to see which incompetent idiot violated The Cardinal Rule of Live Sound: NO FEEDBACK!

‡‡         What Causes Feedback

Let’s revisit the actual physics behind feedback. In a perfect world, all of our microphones would be acoustically (and magically) isolated from all of the loudspeakers. In live sound, it does not and cannot happen. There is always a chance that some of the signal from our loudspeakers will find its way to a mic or two, be captured and sent along through our mixer and amps back to the speakers, and so on. Round and round it goes, gaining steam and SPL on the way, and soon we have rumble or howl or screech happening. We can prevent this cacophony by cutting off (or attenuating) the path by which the sound goes round and round. If that’s all we need to know, I could stop writing right here. But I’d be derelict in my duty if I didn’t get into different ways to accomplish said cut-offing (and/or attenuating), so here we go.

‡‡         Choose the Right Mic

In relation to feedback, there’s only one attribute of microphones to consider — the polar pickup pattern. A mic that captures sound from all directions is much more likely to feed back than one that only grabs sound from one direction. Don’t get caught up in the myth that dynamic mics are less susceptible to feedback than condenser mics. The polar pattern is the only thing that matters. Omnidirectional mics can be used in certain applications, but any mic that’s going to be in proximity to any speaker is more susceptible, and if you have to place a mic close to a speaker, directional patterns are your friend. In fact, if circumstances allow, going beyond a typical cardioid to a supercardioid or hypercardioid design can be very helpful, but this requires a little cooperation from the vocalist, who needs to be mindful of how to use it — and must be more on-axis than with a regular cardioid. The notion of training our singers and speakers to help us avoid feedback propels us on to our next topic.

‡‡         Use the Right Mic the Right Way

“Gain before feedback” (or GBF) refers to the maximum amount of gain to which a mic’s preamp can be raised before feedback commences. The more SPL that reaches the microphone, the lower we can set the gain. The obvious solution here is to keep the mic as close as possible to the mouth of the user, and that’s why we need to train our users to do. This can be tough with handheld mics, because people feel free to move about and get animated when using a handheld mic, particularly a wireless one. Caught up in the moment and moved by the Spirit, users can become unfocused on how and where they’re holding the mic.

One solution is to limit — or even eliminate — handheld mics. Musicians who sing in addition to playing instruments use stand-mounted mics, as their hands are otherwise occupied. Pastors and others who speak on a frequent basis should be fitted with a lav or headworn mic. If you still have speakers and/or singers using handheld mics, spend time to ensure they know proper mic technique — close to the mouth and slightly off-axis to help reduce plosives. And avoid cupping the mic, which nullifies the phase-shift network that makes the mic directional, converting an otherwise directional mic into an omnidirectional one, which is more susceptible to feedback.

‡‡         Putting Speakers in Their Place

Placing mics on stands provides a certain amount of control over what direction they’re pointed, and pointing a directional mic away from any nearby speaker is a very good idea. And although it may seem best to point cardioid microphones 180 degrees away from nearby speakers, that may not deliver the best rejection. The polar plot of a cardioid mic reveals that the maximum null is not directly at 180 degrees off axis, but a few degrees away from that, and this can be leveraged with stand-mounted mics. Another (hopefully obvious) way to avoid feedback is to point speakers away from microphones. If possible, all front of house speakers should be located out in front of all microphones; away from the mics and toward the congregation. Another consideration is reflections. Ideally, FOH speakers will direct 100 percent of their energy toward the congregation. However, this is not always the case, and sometimes those speakers direct energy into reflective surfaces (back walls and balcony faces are frequent culprits), which return energy back toward the microphones.

‡‡         The Perils of Monitor Speakers

Monitor speakers create a major feedback risk, so be particularly careful with them. Floor wedges are typically used with stand-mounted mics. Aligning the deepest null of our directional mic with the straight-on axis of the speaker is the right way to go. Monitors pose the greatest threat with improperly employed handheld mics. Caught up in the moment, worship vocalists may lower that mic away from their mouths and directly into a floor wedge, which sends us scrambling to prevent or stop the ensuing feedback. For this reason, and numerous others, in-ear monitors (IEMs) have gained immense popularity in recent years. IEMs can completely avoid directing sound energy into our mics, substantially reducing the potential for feedback.

If monitor speakers are to be deployed, ringing them with EQ is a must. As we know, when speech or music emerges from loudspeakers, not all frequencies are represented equally — some poke out louder than others. The frequencies that poke out the most are the ones most susceptible to initiate feedback. Using a 1/3-octave EQ in the path to the monitor speaker to reduce those troublesome frequencies will reduce the chance of feedback. It’s easy to grab a fader and pull it down a bit, but it’s important to know which frequency is causing the problem. Many digital consoles offer real-time frequency analysis on a display to help determine where to start carving, but there’s no substitute for old-fashioned “name the frequency” exercises. Magical boxes that hunt for frequencies that start to feed back, isolate them and shoot them down are also worthy of consideration.

For miked guitar amps, an elegant solution is to tuck the amp into a doghouse located well away from the stage. This has two benefits: it gets a potentially feedback-causing microphone away from monitor and P.A. speakers, while reducing stage volume and giving the FOH mixer more control over the mix.

‡‡         Turn It Off!

Earlier, I mentioned the notion of “cutting off” the feedback loop — by far the most effective way to solve our problem. One way to do this is using noise gates on microphones. Set the threshold such that a person speaking or singing into the mic will kick the gate open, but a would-be feedback frequency will not. Setting a gate’s release time is trickier than threshold and attack times, but a little work here can go a long way. You can also cut potential feedback using the console input mute button. Muted mics can’t cause feedback. Many times I’ve walked up on a volunteer-manned mixer in a church to find every single mic on the platform was wide open through the pastor’s message. Use mute groups and shut down any mics not currently used.

By minimizing the risks in every possible way, the amount of feedback that we suffer can be reduced to nearly zero. May we never see the congregation’s heads whip around synchronously ever again.

John McJunkin is the chief engineer and staff producer in the studio at Grand Canyon University.

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