By Chris Huff
Not all feedback is eliminated in the same way. Feedback typically occurs when a microphone and a loudspeaker are too close together thanks to a singer dropping the mic to their side. In this case, let's call it user error. But have you ever had multiple people use the same microphone and suddenly you hear the ringing of feedback? Have you ever created feedback by altering the EQ of a channel? Let's find out why.
What is feedback?
Audio feedback is the sound created when a sound loops between an audio input and an audio output. A simple example is a microphone and a monitor. The monitor is broadcasting sound the microphone then picks up. The monitor then is amplifying that sound and broadcasting it back out where the microphone picks it up again. Eventually, when the volume going into the microphone is the same as the volume coming out of the monitor, feedback begins.
The first frequency that feeds back is the one that requires the least amount of energy to excite resonance. Resonance is a vibration of large amplitude caused by a relatively small stimulus of the same or nearly the same period as the natural vibration period of the system.
What are the common reasons for audio feedback?
- Microphone located too close to a monitor
- Gain structure set too high so as frequencies primed for feedback
What can be done to stop audio feedback in these cases?
- Move the microphone
- Move the monitor
- Use a more directional microphone to meet your mic'ing need.
- Turn down the monitor volume
- Turn down microphone gain (likely the person needs to hold the mic to their lips if singing)
- Watch for reflective surfaces that might be bouncing the monitor sound to a microphone not directly in line with the monitor. Then, make changes then using one of the above.
- Simple but common...turn off microphones when not in use. A stage arrangement can change for an event and create the right conditions for an open mic to cause feedback.
- Equalize the microphone channel signal, lowering the frequencies which are causing the feedback.
How does the equalization-for-feedback process work?
In the first part of the article, I mentioned the frequency that required the least amount of energy to excite resonance. Let's lasso that one to the ground!
Frequencies by their sound:
- Hoots and howls: Likely caused by a feedback frequency in the 250 to 500 Hz range.
- Singing: The range is in-line with 1kHz.
- Whistles and screeches: most likely above 2 kHz.
Determine the likely frequency range and then apply a cut to that range by 3dB.
What about creating feedback when EQ'ing a channel?
It's that very EQ process where we can cause feedback ourselves. For example, one time I had choir mic's all set and EQ'ed to my liking. During a specific song, I decided to try boosting the mid-range EQ a bit more (that 1kHz range). That's when the feedback started. I quickly cut that mid-range frequency back before anyone (except my sound guy, Jeff) noticed.
The keys to feedback control
Eliminate the conditions in which it can appear. Teach singers to hold the mic right up to their lips...and never drop down next to a monitor, establish proper gain structure, and turn off unused mic's.
When it does appear, know that you have an immediate alternative to turning down volumes...you might just be able to EQ it out.
Question(s): What have you done to control feedback in your environment?