Yeah, we know what that sounds like. And we’ve felt the suspicious glares from turned heads focused on the unlucky individual behind the mixing console when it happens.
It’s uncomfortable. The moment is as surprising as an electrical shock, and that instant just seems to hang in time.
For the novice sound tech, feedback often evokes a “deer in the headlights” type of response. For others, there is a scramble to turn everything down. And for some, there’s the methodical approach to fixing the problem, as if they knew exactly what happened in the first place.
So how do we stop it?
To determine that, you must understand what feedback is in the first place.
What is Feedback?
Feedback is the result of sound looping between an audio input and an audio output.
In the case of most live sound systems, feedback is caused by the sound present at a microphone being amplified through the loudspeakers, then returning to that same microphone and being amplified again, or re-reinforced. When left unchanged, this loop becomes perpetual and causes that distinct screech or howl through the sound system as the exact same audio signal is layered and reinforced many times over.
A feedback loop starts when the reinforced sound from the loudspeakers becomes louder than the audio source present at the microphone (a singing voice for instance). Again, this is the relative volume of sound measured at the microphone location, nowhere else.
Where does feedback typically happen?
For the average church sound tech, feedback often happens from one of three primary sources:
- The pastor’s lavaliere or headset mic
- Vocal mics or acoustic guitars
- Choir mics (if the choir is miked)
Most feedback issues tend to crop up due to loud stage volume, improper microphone gain/trim settings, or poor microphone-to-loudspeaker placement.
We’ll go explore ways to prevent feedback from happening in another post, but the short version of that post is:
- Place microphones behind the main loudspeakers to prevent common feedback loops
- Use the appropriate microphone pattern and monitor placement
- Set your gain/trim structure so there is a strong enough signal coming into the mixing console
- Plus a few other important tips
(If you just can’t wait for those tips, feel free to download my free feedback report at www.GreatChurchSound.com/freechapter)
3 Quick Tips to Eliminate Feedback
These three tips are always the first place I start whenever I encounter a problem with feedback.
1. Turn it down!
Without being too drastic about it, simply turn down the main output level of the loudspeakers, or if you can find the offending microphone channel fast enough, turn it down. Often you will only need to turn down the level a small amount to stop the ringing. This volume adjustment can sometimes be imperceptible to the listening audience, but can make a big difference in the quality of the overall sound.
Monitor loudspeakers on stage can cause feedback too, especially for vocal mics and acoustic guitars. Sometimes you just need to turn down the individual input level or mix just a little bit in the appropriate monitor channel.
2. Move it!
Simply moving a microphone or loudspeaker out of the pickup or projection pattern of the other can greatly reduce the chance of feedback. Don’t place a microphone in front of a loudspeaker, and conversely, don’t place a loudspeaker behind a microphone.
Another technique is to move the microphone closer to the source. This allows more signal to reach the microphone from the true source than from the reinforced signal of the loudspeakers.
Another common cause of feedback in churches around the world is the improper placement of clip-on lavaliere or “lapel” microphones. While these microphones can be very convenient for clipping onto a variety of clothing or accessories, they are often placed too far away from the presenter’s mouth to be effective for live sound reinforcement. These microphones should be placed high on the chest of the presenter. This gets the microphone closer to the mouth and therefore provides a better signal level. I always recommend placing lapel mics about halfway down the sternum, 6-8” below mouth.
3. EQ it!
If you can’t turn it down or move it, EQ it. It’s amazing what you can pull off with a little finesse on the equalizer. Even a basic High/Mid/Low EQ with sweepable mids on the console can be a huge asset for stopping feedback when you hear it.
Sweepable Mid EQ tips for Feedback
- Set the level knob of the sweepable EQ to about -6dB
- Sweep the frequency knob slowly across the entire frequency spectrum available
- Listen for changes in the tone of the source you are monitoring and note when the feedback frequencies are diminished
- Adjust the frequency level control back to about -3dB if possible to ensure that maximum tone quality is available from the source in the main mix
- Repeat as needed for multiple microphone channels
EQ can be a great tool, but be careful. Try to make incremental and modest adjustments to your channel EQ when using it for feedback. A little bit can go a long way, and it will definitely impact the overall tonal quality of the audio source you are adjusting.
So to recap, those quick tips are:
- Turn it down!
- Move it!
- EQ it!
Stay tuned for the next post for preventing feedback in the first place!
Connect with James and discover more great tips that help you deliver great sound for your church at www.GreatChurchSound.com
Note: this post originally appeared on the LimitlessMinistries.com blog at: http://www.limitlessministries.com/tips-to-prevent-and-eliminate-feedback/