Why Do Microphones Sometimes Make A Screeching Sound? Sound System Feedback, Explained

At this point, it is an overused joke. Someone in a movie steps up to a microphone to say something serious or awkward, and when they start talking, a screeching sound can be heard. For as rote as it may be in media, microphone feedback is a very real thing that anyone has the possibility of encountering. You could be a well-seasoned sound engineer working with a band with decades of experience on a concert, and feedback could still occur. Although the actual science and math behind feedback is rather complicated, the true explanation of how it occurs is rather simple.

Basically, everything that makes a noise operates at a certain frequency. Microphones and sound systems have their own frequencies as well, and no two pieces of equipment are going to vibrate at the same frequency, leaving you vulnerable to irregularities that could cause feedback if they interact with each other. In the simplest way possible, feedback occurs when you make a noise into a microphone. That noise is then projected out of the sound system, and the microphone captures that projected noise, causing an endless cycle of frequency clashing. Feedback problems are more likely if your microphone is very close to a speaker, if the person using the microphone is not very close to it, or if the sound of the microphone itself is too loud. Other factors, such as the size and shape of the room you are in or using multiple microphones together, also contribute. Thankfully, there are ways to minimize microphone feedback.

The simplest way to avoid feedback when using a microphone with a sound system is to simply have the microphone positioned behind the speakers. Speakers are designed to project sound in a very specific direction, and while there will always be some reflecting and bouncing depending on the space you're in, there shouldn't be enough sound that travels backwards to your microphone in order to cause the feedback loop in the first place. If you are a musician who relies on speakers to hear your fellow bandmates, you really should invest in some in-ear monitors. 

The kind of microphone you are using can also play a part in why you are having feedback issues. If you have an omnidirectional microphone, it is more likely to pick up sounds from all around you, rather than just at the front of the microphone. A better bet would be a cardioid microphone, which has a far more directionally-focused area of where sound can be captured. If you are an act who uses multiple microphones in your performance, switching off the other microphones when they aren't in use can also reduce the chance that feedback will occur.

If you have control of the space you're in, be it a live venue or a studio, treat the room properly. Install cushioning or cover up places that are notorious for easily reflecting sound. If you don't have that kind of control, hire a professional or get an automated processor to ring out your room, effectively tuning your room to perfectly EQ your equipment to avoid feedback.