How to Diagnose and Eliminate Audio Feedback

by | Audio, Audio Connections, Production

Welcome back everyone, this month’s question comes from Dominic in Syracuse, NY, he asks “As the new tech team member on our tech team, I seem to be struggling with feedback, please help me understand it”. Feedback can easily ruin the mood of a worship service – here are several techniques to help in avoiding the problem.

That low hum you hear is not a distant cargo plane. It’s not your local television helicopter. It’s the beginning of the end…it’s feedback.

Sooner or later every sound system operator experiences feedback. Most people think of it as that dreaded deafening high-pitched squeal resulting from a bad combination of microphone gain, mic location and loudspeaker volume, but feedback often begins as a low-pitched hum. If you recognize it early and correct it quickly, most folks won’t notice. Your reputation as a sound system operator will go untarnished and your sound-savvy colleagues will congratulate you on a good save.

However, let that hum turn into a squeal too many times and your name may stop showing up on the schedule for the sound system ministry. This article is intended to help get you well on the way to banishing feedback.
There are other causes for a low-pitched hum in a sound system, but it’s important to learn to recognize the tell-tale hum of a sound system on the verge of feedback.

First, what is feedback? It’s the result of cycled sound. In short, a microphone amplifies sound coming from a loudspeaker which is then amplified by the sound system and passed out the loudspeaker back into the microphone.

This cycling occurs at an exponential rate. Feedback commonly occurs when stage monitor volumes are set too high and/or the person using the mic is standing too close to a monitor. Now, here’s how to prevent it:

1) Set up the sound system specifically for the room. In any given room, some frequencies will be more prone to feedback. Hire a qualified, experienced, well-respected sound technician to “tune the room.” (The process is actually tuning the system.)

Using a spectrum analyzer, this person should play audio through your sound system and see what frequencies are out of line. Using this information, the tech will set the house-level equalization unit to correct those frequencies. The result is not only a lower chance of feedback, but overall better sound quality.

2) Set up the platform/stage properly. When you arrive an hour before the service (hint-hint) to set up the system, the platform, and test all media, pay particular attention to setting up the platform. Place mics out of range of – and pointing away from – monitors and any other loudspeakers.

Then, talk with the staff, service leaders, worship singers, etc. about proper microphone placement. Lapel mics should be one hand width from each person’s chin when they put their chin to their chest. Handheld mics should be between 3 to 6 inches from the user’s mouth.

Tell handheld users that you control the volume and they don’t need to worry about moving the mic back and forth to do so. The closer the mic is to the source of the sound, the less volume you have to send out and thus feedback is less likely.

3) Know the limits. You can force a microphone to feed back – supply it with enough volume via the mixer (console) and the result is feedback. Why crank the volume so high?

Here’s a common example. Someone not accustomed to using a microphone, and also perhaps uncomfortable with public speaking, is shying away from the mic and speaking softly. So you bring up the volume.

However, mics don’t do a good job of picking sounds that are more than a foot away, so boosting the volume will likely only result in feedback.
When you’re rehearsing for a service, crank the volume up on each microphone as it’s being used. Observe where the feedback starts and then make a mental note not to raise it that far during the service.

4) Turn it off. Plain and simple – if no one is using a particular microphone, bring the fader down and then mute the channel. This isn’t always necessary if the mics are placed in good locations and not apt to pick up secondary sounds.

I place mic channels in groups and control the overall volume to those mics via their specific group. However, if the pastor decides to walk to that portion of the stage wearing his lapel mic, I turn off the mics in that region.

5) Grab some gear. There exists a handy device called a feedback eliminator. Sure, it sounds great but don’t go there just yet. If the system is properly tuned and you’ve followed my previous points, feedback will become a rarity.

So, you might not even need a feedback eliminator. The fact is that there are several components that work towards reducing feedback and also improving overall sound quality.

A compressor places all sound output levels within a set range. In short, nothing sounds too loud and nothing sounds too soft. This is a very important aspect in blending worship music.

A limiter is used as a safety device in live applications to prevent sudden spikes in volume like a dropped microphone or any other unexpected sound blast.

Feedback eliminators work by either limiting offending frequencies as they occur or by shifting the pitch of the sound slightly so the source sound and the output sound don’t match so feedback won’t happen. This isn’t ideal for musical applications because the pitch of acoustic sound coming from an instrument will not match the amplified sound.

If your sound system consists of turning on an amplifier and talking in a microphone, you could benefit from a feedback eliminator. Otherwise, consider the other options presented here. In either case, consult an audio professional so you buy the right gear the first time.

Feedback can easily ruin the mood of a worship service. If you’re having feedback problem, try out some of the techniques as I’ve described and hopefully it will be a thing of the past.

If you have a question for Bill, you can email him at bdipaolo@worshipfacility.com.

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