hey guys, I am a sound volunteer for my church and I learned concepts of sound system design. The issue we have most is "Feedback". Our church is an L-shaped room with audience seating to the left of stage and in front of stage. The speakers which are 12meters away from the mic is still feeding back into the system. How can we overcome this problem. We are using shure sm58 for speech, and vocals and our main speakers are QSC K12.2 series speakers. Also we have distribiuted speaker system.
You have to start out with a basic understanding of feedback and what causes it. Feedback requires TWO things..... a pickup/mic and a speaker that is amplifying that pickup/mic. Feedback occurs when the audio being picked up by the microphone is outputted through the speaker AND is picked back up in the microphone again. This starts the cycle of the speaker amplifying the mic, which is pick up again by the mic and amplified even louder in the speaker, which is picked up by the mic and made even louder in the speaker, etc, etc, etc until it hits the limits of the system. As a general rule, the longer it takes for feedback to "build up" the farther away the speaker is from the source/mic. If the feedback gets very loud very quickly, the speaker causing the feedback is close to the mic (probably a monitor). If the feedback builds up slowly over time, the speaker causing the loop is likely located farther away from the mic (more likely to be a house speaker). It's generally easy to determine which mic is causing the feedback, but it can be more challenging to determine which speaker(s) is looping back into the mic and causing the feedback. Is it a musician's monitor on stage, or is it one or more of the facility's main speakers? etc, etc, etc. You have to know this before you can start to "fix" the problem.
You should also have a basic understanding of "gain structure" and how to properly set the gain of an input/source. If your input's gain is set wrong/too high, it can cause feedback in unexpected ways because while you might be lowering the gain in the output of your audience speakers farther down the signal chain, it might be outputting too loud in the monitors and therefore causing feedback more quickly in those speakers. Simply setting the gain level correctly might stop the feedback in the monitors and still allow you to have the audience hear the same volume as before.
You should understand what a microphone's response pattern is and where a microphone is more sensitive and less sensitive to audio. The easiest first step is to place your microphones where the less sensitive area are pointed towards the speaker causing the feedback and the more sensitive areas are pointed at the sources you wish to amplify. That one step might fix you problem.
Along with that, understanding what pattern your speakers send out their audio (all speakers are not the same with this regard) and what objects that audio might be reflecting off of back towards the mics will also help. Again, changing the physical placement of the speakers and/or microphone based on this knowledge can go a long way in preventing feedback.
Finally, you should lean how to "ring out" a speaker using EQ to lower the frequencies that are ringing (ie causing feedback). The guides out there generally talk about during this with a musician's monitor (because that is the most common speaker that causes feedback), but the principal applies to any system/speaker. There are plenty of Youtube and other videos that demonstrate and teach this technique.
That is a lot to digest, but understanding these concepts will really help you prevent feedback as much as possible.
