Feedback in a sound system is that annoying howling or squealing tone you hear when the microphones pick up amplified sound from the loudspeakers.

Sound from the loudspeakers enters the microphones, is re-amplified, and goes around in a feedback loop. Almost instantly, the sound builds up until a loud ring occurs - usually at a single frequency.

Fortunately, there are many ways to kill feedback or prevent it in the first place.

Quick Fixes
Before the service, on each mixer/console fader, mark the point where feedback occurs. Don’t exceed that point during the service.

If you still hear feedback during the service, turn down the house master faders or the monitor master until feedback stops.

If you know which mic is causing the feedback, turn down its fader or monitor send, or cut EQ at the frequency that is feeding back.

As Few Mics As Possible
The more mics you have in use, the more likely you are to run into feedback. The gain before feedback decreases 3 dB each time the number of open mics doubles.

Two microphones have 3 dB less gain than one microphone, four microphones have 3 dB less gain than two microphones, and so on.

To reduce the number of open mic uses at the moment, you might prefer to turn them down about 12 dB, rather than off, so you don’t miss cues.

Instead of turning mics down manually, you might want to do it automatically with an automatic (gated) mic mixer. It reduces the gain of mics that are picking up little or no sound at the moment. This cuts down the risk of feedback by keeping the number of open mics at a minimum.

With electric guitar and bass, try using direct boxes(also called DI boxer) instead of mics. Direct boxes pick up no feedback. You can plug the direct box into the musician’s effects output, or plug directly into the bass guitar.

Place Loudspeakers & Microphones Apart
Distance weakens the sound traveling from loudspeakers to microphones, so it diminishes the feedback loop. So try to mount the house loudspeakers up higher so they don’t fire directly into microphones.

Loudspeakers that are more directional, better focusing sound where you want it to go - on the audience - can also help.

Unidirectional Microphones
“Uni mics” reduce feedback and leakage, better rejecting sound firing from the sides and/or rear of the mic, such as floor monitors. Some examples of unidirectional patterns are cardioid, supercardioid, and hypercardioid.

Most uni mics also boost the bass when you mike close. This is called the proximity effect. At low frequencies, it provides free gain (extra volume without feedback). If you want to roll off this excess bass with your mixer EQ, you also reduce any low-frequency feedback picked up by the mic.

Loudspeakers Behind Mics?
With any unidirectional mic, there is a certain angle behind the mic where sound pickup is least. This angle where the mic is relatively “dead” is called the null angle, and it’s advised to locate monitors in the null of the mics.

For example, a cardioid mic is least sensitive in the rear, so therefore, it’s best to place the floor monitor directly behind the mic. A supercardioid has two nulls of least pickup at 125 degrees to either side off-axis. Place the monitors at these locations for less feedback potential.

Close Microphone
Close miking increases the sound level at the microphone and makes the sound system louder. Place each mic within a few inches of its sound source (except for the choir). Keep mics as close as possible, but no closer than necessary to get enough volume before feedback occurs.

If a person speaking is not loud enough through the PA without feedback, do this: Move the mic closer to the person speaking, or ask them to talk closer to the mic. Demonstrate to them how they can’t be heard if they’re too far from the mic.

If you’re miking a group of singers with a single microphone, and you get feedback from the floor monitors, give each performer a close-up mic instead.

Although close miking boosts the sound level, it tends to color the tone quality, giving an unnatural sound. Here’s why: Most musical instruments are designed to sound best at a distance (say, one or more feet away).

So, a flat-response mic placed there tends to pick up a natural or well-balanced timbre. But when you mic close, you emphasize the part of the instrument that the mic is near. The tone quality that is picked up very close may not reflect the tone quality of the entire instrument.

For example, the sound hole of an acoustic guitar resonates strongly around 80 to 100 Hz. A mic placed close to the sound hole hears and emphasizes this low-frequency resonance, producing a bassy, boomy timbre that does not exist at a greater miking distance. The close-miced sound is harsh, too.

To make the guitar sound more natural when miked close to the sound hole, you need to roll off the excess bass on your mixer, or use a mic with a bass rolloff in its frequency response. It can also help to lower (“dip out) some 3 KHz signal to reduce harshness.

A sax miked in the bell sounds like a kazoo. To mellow it out, cut around 3 kHz and boost around 300 Hz. Better yet, locate the mic above the bell so you pick up the tone holes.

Noise Canceling Mics
A noise-canceling or differential mic for vocals is designed to cancel sounds at a distance, such as instruments on stage or monitor speakers. Such a mic provides outstanding gain before feedback and almost total isolation.

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