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Article Preview :: Preventing Acoustic Feedback On Stage
Technique : Live Sound
Whether you’re a performer or an engineer, these simple feedback-reducing tips should ensure your gigs remain free from squeaks and howl-rounds!
Anyone who’s ever played or engineered a live music performance being amplified through a PA will be familiar with the sound of feedback — that nasty squealing or howling sound that gets louder and louder until either someone turns something down or something breaks! This unpleasant phenomenon is not only annoying for members of the audience, it can also be extremely distracting for the musicians and, if left unchecked, could easily ruin a performance.
In general, it’s the job of the engineer to minimise the risk of technical problems, leaving the musicians to concentrate on what they do best, which is playing music! That said, however, a little knowledge on the part of the performers about what causes feedback, and the steps that can be taken to minimise it, will help the engineer enormously — not to mention the performers themselves.
Theory Lesson
Wherever possible, the front-of-house speakers should be placed in front of the stage, so as not to direct any sound back into the microphones.
The theory goes that if too much of the sound from the PA speakers leaks back into a microphone, it will circulate around the system, growing louder all the time and quickly building up into a continuous whine or whistle. With sufficiently high gain, where the overall loop gain is greater than unity (ie. the sound going into the mic from the PA is loud enough to loop back into the PA), an oscillation will occur, just as in the oscillator circuit of a synthesizer. This phenomenon is often called a ‘howl-round’. Where the gain is high, but less than unity, the system can sometimes appear to ‘ring’, when, for example, someone speaks into one of the microphones. The more of the sound from the speakers that is able to find its way back into the microphone(s), either directly or via reflections from walls and ceilings, the more likely it is that feedback will become an issue.
It’s very important to note that the onset of feedback is linked to system gain, not to the absolute volume level at which you’re running your microphones. So a very loud singer working close to a mic may create no feedback problems, because the mic gain can be kept fairly low, while a singer with a quieter voice, or one who may not be quite as close to the mic, will be more likely to give rise to feedback. The quieter singer needs more mic-amp gain to bring up their level, and as the gain goes up, so does the risk of feedback.
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Thu 20 Jun 2013
Sound On Sound : Est. 1985