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To eliminate feedback is it good to reduce the gain and raise the fader? (Part 2)

An RP reader has feedback problems. But will clever manipulation of the gain control and fader provide the cure? In Part 2 of this two-part article, we look at the relationship between these two controls.

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Part 1 of this article explored some important concepts of feedback (howlround), so preferably that should be read first.

Now the question is whether carefully balancing the gain and fader (of the same channel) will improve the situation regarding howlround.

Quick question... What does the gain control do?

Answer: It boosts the level of the signal.

Another question... What does the fader do?

Answer: It lowers the level of the signal.

Clearly both of these controls have an effect on the loop gain of the system and can therefore affect feedback.

But if you raise the gain by 6 dB, the output from the loudspeakers goes up by 6 dB (assuming no compression). If you then lower the fader by 6 dB, then the output goes down to what it was before.

In fact, however many decibels you change the gain, if you move the fader by an equal number of decibels but opposite in direction, then the output level and loop gain will stay the same.

So the short answer is that you won't improve anything however much you play about with the relative gain and level, assuming that you always use one to exactly compensate the other.

But there is a 'but'...

The exception is if you have your gain too high on the point of clipping. (If you do this you will almost certainly have your faders set very low to compensate.)

Distortion induced by clipping adds an uncertainty into the feedback equation by changing the balance of frequencies.

The result is not going to be good. In general, distortion adds energy to the higher frequencies. Howlround is always unpleasant but high frequency howlround can be ear splitting.

So set the gain correctly using the normal methods and concentrate on the factors that really can reduce howlround...

Firstly if you can get the microphone closer to the sound source, you can get a greater proportion of the sound source you want to pick up, in comparison to the sound coming from the speakers.

Secondly, as much as possible place the speakers so they don't fire sound directly at the microphone. And from the opposite point of view, position the microphone as carefully as you can so that it doesn't point at the loudspeakers.

There are more weapons in the anti-howlround arsenal, but these two are the biggest guns.

By David Mellor Monday January 3, 2011
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