“Live” sound is the amplification of a series of microphones and other sources into loudspeakers that are in the same physical space. Most other forms of sound are in controlled environments, such as studios, automobiles, home “stereos”, DJ systems, etc. In none of these are multiple microphones used in the same space as speakers. In the same space, the output of the speakers is picked-up by the microphones and re-amplified, and re-amplified, etc. This level of re-generation reaches a point where it is heard as howls, squeals, high and/or low tones. This is feedback and it is always present in “live” sound. Aside from being annoying and disruptive, it can be dangerous to people’s hearing and can ultimately burnout amplifiers/speakers.
The primary method for keeping this “feedback” condition under control is to use the appropriate speaker(s), with a predictable pattern, and directional microphone(s) in the proper position, with pickup direction away from speakers. Locating the microphones on a plane behind the speakers takes advantage of the canceling effect of both. There are electronic devices that are very effective in reducing the incidence of feedback, but the best result can be achieved by using an experienced audio engineer to design a system for your venue, using all of the techniques available. Training your personnel will also help reduce feedback and help provide clear, defined sound, by having them turn off all unused microphones and optimizing those in use.
Audio operators new to the audio field tend to be anxious to please and may sometimes force the system into areas that will produce feedback. Understanding the limitations of the sound system, and all systems have limitations, can help control feedback, with the help of digital feedback controllers. Close-miking is also a technique to help control the sound, but must be reduced in level to match the more-distant pickup of other mikes. Solo vocalists and/or preachers using handheld mikes are easy to set at a high level, but hanging choir mikes (distant) will be almost impossible to equal to the former mikes. Setting the level of the most difficult microphone groups as the highest volume, in the mix, will then permit the higher level mikes to be lowered to match the difficult microphones, so that the overall mix will come out even. This is also very important in the recording mix, as well as the main sound mix.