Microphone Mythbusters Vol #3: Microphone Feedback

By Gino Sigismondi – Shure Corporation

Gino Sigismondi addresses this question, “What microphone can I use that doesn’t cause feedback?”

photo showing a variety of microphones - Does all feedback?Here’s another frequently asked question here at Shure Applications Engineering: “What microphone can I use that doesn’t cause feedback?” The answer to the question is, of course, that no such microphone exists. Feedback results from a combination of many factors, including loudspeaker placement, microphone placement, the frequency response of both devices, and room acoustics. Or, as we like to say, “Microphones don’t feed back, sound systems do!”

What is feedback?

Feedback is characterized by a sustained, ringing tone, which can vary from a low rumble to a piercing screech. Echoes and reverberation caused by room acoustics, as well as ground buzz and other extraneous noises, are not the same thing as feedback, and cannot be cured in the same manner.

What causes feedback?

Feedback occurs whenever the sound entering a microphone is reproduced by a loudspeaker, picked up by the microphone, and re-amplified again and again. The familiar howl of feedback is an oscillation that is triggered by sound entering the microphone. The easiest way to create feedback is to point a microphone directly into a loudspeaker. Placing the microphone too close to the loudspeaker, too far from the sound source, or simply turning the microphone up too loud exacerbates feedback problems. Other contributing factors are too many open microphones, poor room acoustics, and uneven frequency response in either the microphones or loudspeakers.

What can I do about feedback?

The single easiest way to reduce feedback is to move the microphone closer to the desired sound source. Additionally, using a directional microphone (cardioid, supercardioid, etc.) will typically increase the amount of gain before feedback. Reducing the number of open microphones with an automatic mixer will also improve the situation. Try to keep microphones and loudspeakers as far away from each other as possible. Lastly, acoustically treat the room to eliminate hard, reflective surfaces such as glass, marble, and wood.

When all of the above solutions have been exhausted, the next step is to look towards equalizers and automatic feedback reducers. A common technique used by sound engineers is “ringing out” a sound system by using an equalizer to reduce the level of the frequencies that feedback first. While the graphic EQ is often employed for this purpose, a parametric equalizer, which allows you to dial in the exact offending frequency with very narrow bandwidth, will do a much more precise job. Graphic equalizers only offer a limited number of frequency choices, with a much wider bandwidth. The end result is that attempting to reduce feedback with a graphic EQ can greatly affect the overall frequency response of the sound system.

Automatic feedback reducers are essentially just automatic equalizers; they find and cut the frequencies that are feeding back to the appropriate level, and with very narrow bandwidth. The same precautions listed above apply to feedback reducers as well as equalizers. Automatic feedback reducers can be very helpful in wireless microphone applications. Remember that microphone placement is crucial to eliminating feedback, and the temptation to wander away from the ideal microphone position when using a wireless is great. If the performer gets too close to a loudspeaker, feedback will result; a good feedback reducer will be able to catch and eliminate the feedback faster than a human operator.

Proper implementation of the above techniques will go a long way towards eliminating feedback in your sound system. Do not rely solely on equalizers or feedback reducers, and remember that feedback results from more than just the microphone!