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Q: How can someone avoid feedback and distortion in live concert shows/recordings?

An Audio Masterclass student asks how a sound engineer can avoid two of the major problems of live sound and recording.

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Question from an Audio Masterclass student: “How can someone avoid feedback and distortion in live concert shows/recordings?”

Firstly feedback. Feedback should only happen in live sound. If it happens in a recording studio then the monitor loudspeakers are not properly separated from the recording area.

In live sound it happens when too much sound from the loudspeakers returns back to the microphones.

Any microphone will pick up some sound from the loudspeakers, so that it is amplified again. This colors the sound, but does not necessarily cause feedback. At a certain point however the sound will build up each time it goes round this circuit and cause a dreadful howl.

There are four possible solutions. The first is to move the microphone closer to the sound source. This makes the sound source appear louder and less gain is required. The second is to set out the stage so that the loudspeakers do not radiate sound towards the microphone. The microphones should be behind the loudspeakers wherever possible.

The other solutions are not recommended, but they do work. You can turn down the level coming out of the loudspeakers, but of course it will now be quieter than you would like. Or you can increase the level of the sound source, but this will change the character of the sound.

The usual cause of distortion in live sound and recording is setting the gain control too high. The signal from the microphone is a few tens of millivolts and needs to be boosted to around a volt by the gain control.

But if the gain control is set too high, then the signal will be clipped causing distortion. Lowering the fader will not help.

The usual procedure to set gain on a mixing console is to press the PFL (pre-fade listen) button on the channel you are setting. Tell your singer or player to start, then adjust the gain to get a healthy reading on the console's meters, but no red lights.

By David Mellor Thursday November 30, 2006
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