Gain Before Feedback
We all know it as one of our worst nightmares, everything is sounding great, everyone is happy and then out of nowhere comes this giant, show ruining massive noise. In the Sound Engineers Bible, the Book of Show Revelations has feedback instead of a dragon. So, why does it do it? Why do we get feedback.
What is feedback?
Whilst we could talk all day about Ebay, feedback in this industry in its most commonly described form is ALWAYS negative. Feedback occurs when sound from an input device hears it's own output from an output device. In its most common form, this is a mic picking up sound from a speaker, amplifying it and then picking it up again, and again, and again. An endless increasing loop. More info can be found on the Feedback page.
Gain before Feedback
Gain before feedback is effectively the maximum amount of gain we can apply to a microphone or other input device before feedback occurs. This depends on how loud the speakers are, how close and at what position relative to distance and axis the microphone or microphones are to the speaker or speakers and the frequency response of the microphone and speaker and any colouration and frequency response and characteristics of the performance space.
We can maximise gain before feedback by position speakers and microphones in better places, and by altering the response of one or both using graphic or parametric eq's. Frequencies that feedback can be removed using an EQ. Though this should be a last resort after proper mic and speaker placement as it can colour the sound un-necessarily.
The more microphones you have, the less overall system gain before feedback you have. The more speakers you have in susceptible areas the less overall system gain before feedback you have. However the more speakers you have in non susceptible areas (ie delay speakers) the quieter the main speakers can be and thus the more overall system gain you can achieve before feedback.
Compressors can reduce your gain before feedback and increase feedbacks likeliness. A compressor is a gain reduction device so that when it compresses it reduces the gain, often the output gain needs to be increased to keep the level acceptable. When the device STOPS compressing the gain level automatically rises. This can sometimes pass the gain before feedback level and cause the engineers second worst enemy (the worst enemy being the lighting engineer).