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Moving The Problem Out Of The Way: Using Phase To Manage Feedback
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The other day while testing one of our new products, I noticed significant resonance in the low end.

But the last time we tested the same device, it was not there. What changed? Well, it turned out that the polarity reverse switch had been turned on. As simple as this may sound, depressing the switch completely cleaned up the low-end resonance and mud.

What was actually occurring was the room resonance at one of the given frequencies happened to be amplifying itself right at the listening position. When two low-frequency waves are in phase, they will combine to reinforce each other. But when they are out of phase, they will cancel each other out. This is known as a room mode.

Studios deal with this problem all the time by either shifting the listening position or by adding bass traps. The same problem often occurs on stage. Let’s say we’re amplifying an acoustic guitar. The guitar is being sent through the stage monitors and through the artist’s guitar amplifier.

Low-frequency resonant feedback is often a major problem when we try to elevate the audio level, causing the top of the instrument to vibrate and howl. When this occurs, the common cure is to try to locate the problem frequency and take it out using some form of radical EQ.

A room mode in a studio. (click to enlarge)

 

But applying severe EQ curves to the instrument not only eliminates the feedback, but also dramatically changes the tone of the instrument.

If we examine the problem a little closer and apply a little “scientific good sense,” we can often reduce the feedback without mucking with the natural sound of the instrument.

Amplify Each Other
The root of the problem is often the interaction between the stage monitor and the musician’s amplifier. It has to do with frequency, wavelength and distance.

Just as in the studio, when two frequencies combine in phase, they will amplify each other. And just like in the studio, it is relative to where you’re standing.

On stage, the same phenomenon occurs when the amplifier and monitors combine. They can create a phase “boost” at a given frequency, depending on where the two sound sources combine.

Instrument amplifier and stage monitor, relative phase. (click to enlarge)

 

By simply moving the amp, monitor or artist relative to each other, the problem will likely disappear.

Another simple solution is to reverse the polarity (relative phase) on the direct box so that the signal going to the monitors will be inverted. This can produce the same positive effect.

And if you want to take things further, try shifting the phase using a phase adjustment tool.

In all cases, you’re not messing with the EQ, but physically or electronically moving the problem out of the way.


Source: Live Sound International

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