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Sound Advice

Controlling Feedback Onstage Using Phase To Your Advantage: Part 1 by Peter Janis

Anyone who has played an acoustic instrument onstage knows that feedback can be a serious problem. What few realize is, there are solutions beyond radically altering the EQ. The following looks at the various problems and solutions at hand. For simplicity, we will discuss an acoustic guitar – but the same principles apply equally to a violin, mandolin, banjo, and contra-bass.

Before we get too far ahead of ourselves, we need to first identify the problems. There are basically two types of feedback that occur onstage: high-frequency whistles and1low-frequency resonance. Both are caused by the sound emanating from the loudspeaker being so loud that it overtakes the instrument by feeding itself back through the pickup, causing the sound to feed back into the PA system forming an audio loop. This endless cycle is called a feedback loop, or feedback for short.

High-frequency feedback is often caused by sound from a wedge monitor going directly into the instrument’s microphone. This can also occur with piezo-type transducers. The usual fixes for high-frequency feedback are to turn down the volume, reposition the microphone, or employ some form of equalization to eliminate the problem frequency.

Low frequency feedback occurs when bass energy from the speaker system causes the instrument to vibrate. This is also known as resonant feedback. The sound system causes the soundboard (top of the guitar) to vibrate in sympathy with a particularly loud bass frequency. The vibration is picked up by the instrument pickup and recycles itself as feedback. Some musicians will seal the sound hole using a rubber plug. This can reduce feedback, but also degrades the sound quality of the instrument.

The Down Side To Using EQ To Solve Feedback Problems:
The most common approach to eliminating feedback is to use some sort of notch filter to find the offending frequency and remove it with a narrow band EQ. The problem with this approach is it winds up as a catch 22. For instance, if you reduce midrange to eliminate feedback, you are actually removing the “meat” or most important part of the sound out of the monitors. To make up for the loss in the midrange, folks invariably increase the stage volume and guess what … more feedback.

To make matters worse, some will introduce a form of automatic feedback filtering system to “magically” solve the problem. These devices introduce a series of very narrow filters that rapidly move around to squash feedback as soon as it occurs. The resulting sound is best described as comb-filtered, an effect that studios spend thousands trying to eliminate!

Does this mean that using an equalizer is bad? Of course not. The number one rule with EQ is and will always be less is best. In some situations, the only option you may have will be to introduce some radical EQ curves into your monitors, but when you do so, keep in mind that you are moving further and further away from the natural sound of the instrument instead of actually solving the problem.

Peter Janis is the President of Radial Engineering, the Port Coquitlam, BC-based manufacturer of music and audio equipment. Visit www.radialeng.com for more information.

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