The screeching ring from a microphone is the familiar and annoying sound feedback makes — which can be especially disruptive during a church service. Sound feedback is among the most basic and ubiquitous of sound system problems, but it's also one of the most easily remedied once its causes are understood.
Feedback is an autological term that more or less explains itself. When the energy from a loudspeaker is literally fed back into an acoustical input device — usually a microphone — it creates a sonic loop that produces a howling sound as it moves from microphone to loudspeaker over and over again, constantly reamplified. Feedback's tone can vary from a low and continuous moan to a high-pitched squeal depending on the frequencies going into the loop. Whether high or low, all feedback is the same principle.
The best way to deal with sound feedback is to avoid it in the first place. The simplest solution is to position the microphone so it's not facing any of the loudspeakers in the PA system. Placing microphones onstage behind the main loudspeakers minimizes the chance for a feedback loop to occur. When loudspeakers are behind the receiving end of a microphone, the chances of encountering feedback increase.
Microphones designed to stay on their stands, such as those used by backup vocalists or choirs, can be positioned with loudspeaker locations in mind. Handheld microphones increase a performer's range onstage, but they also increase the risk of interacting with sound system loudspeakers. Simply informing high-energy vocalists of that possibility can help ward off feedback moments.
Logistically, it's easier to move a microphone than an installed loudspeaker. Depending upon how a church is staged, though, the potential for feedback may be built into a stage design — such as one using a thrust that puts a pastor or performer out in front of the loudspeakers. Furthermore, the increasing use of handheld wireless microphones onstage gives vocal and pastor microphones more opportunities to interact with loudspeakers, including monitor loudspeakers.
One solution is to select a microphone based on a particular microphone's pickup pattern. A mic with an omni pickup pattern has a wide field of capture, which allows more off-axis sonic energy to reach its diaphragm and create sound system problems. Microphones with cardioid patterns — for instance, floor wedge monitors in front of microphones — are designed to reject sound from behind the microphone, while those with cardioid or hypercardioid patterns are better at rejecting sound from the sides.
The most basic technique for dealing with feedback is to quickly turn down the overall volume of the sound system, though that should be a last resort. Instead, identify which sound source is causing the feedback. It's almost always a vocal microphone, but acoustic guitars are also occasional culprits. Once the sound system's master volume is eased back up, one can determine which microphone is the problem and mute all the microphones on the mixing console, then bring each one back up individually. The solution may be as simple as repositioning the microphone or vocalist or lowering the volume on the monitor loudspeaker nearest to them.
Lowering a particular microphone's volume level can help avoid feedback, but it also means an important voice may not be loud enough. At that point, it's time to move on to more sophisticated feedback remedies.
Another solution is to use equalization to "notch out" specific troublesome frequencies. "EQ" can cut or boost specific frequencies. Since most feedback-related sound system problems occur with vocal microphones, attenuating frequencies above and below the typical voice frequency range of the human voice, with peaks ranging from between 1250 Hz and 1600 Hz (1.2 to 1.6 kHz). Thus, cutting frequencies below about 500 Hz and above 8 kHz leaves room for vocal expression while reducing the potential for feedback. This attenuation can be done on a church's front-of-house mix console using the EQ controls on the channels used for vocals.
There are also technology solutions for dealing with sound system problems like feedback. Bose T4S ToneMatch mixer, for example, includes the ToneMatch Audio Engine, a proprietary technology that optimizes system equalization to preserve the natural sound of specific instruments or microphones. Presets are calibrated to match the characteristics of various types of microphones and instruments, optimizing their natural tonality while also minimizing feedback potential.
You can also compensate for a lower volume level on a microphone by asking the pastor or vocalist to move closer to the mic. Each halving of the distance between the lips and the microphone allows approximately another 6 dB of gain before feedback. This relationship in physics is known as the Inverse Square Law. This means that the difference between working a microphone at 2 inches and 1/4 inch is 18 dB — more than twice as loud on the logarithmic decibel scale.