06/05/2024
The reason why the monitors switch off when the mic is turned on
GUIDE TO PROFESSIONAL AUDIO ENGINEERING PRACTICES
Audio System Feedback: How To Avoid It.
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 requiring the least amount of energy to excite resonance.
What are the common reasons for audio feedback?
Microphone located too close to a monitor.
Gain structure set too high.
What can be done to stop audio feedback in these cases?
Move the microphone.
Move the monitor.
Use a microphone with a directional polar pattern such as a cardioid.
Turn down the monitor volume.
Turn down microphone channel’s gain.
Watch for reflective surfaces that bounce the monitor sound to a microphone.
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. “Ring out” the microphone channel signal, lowering the frequencies which are causing the feedback, which leads to…
How does the “Ringing Out” process work?
Once again, the first frequency that feeds back is the one requiring the least amount of energy to excite resonance.
Let’s eliminate that one first!
A common technique used by audio engineers is “ringing out” a sound system by using a graphic equalizer to reduce the level of the frequencies that feedback.
Here's how you do it.... Slowly bring up the system level until you begin to hear feedback.
Now go to the equalizer and pull down the offending frequency roughly 3dB.
If the feedback is a “hoot” or “howl”, try cutting in the 250 to 500 Hz range. A “singing” tone may be around 1 kHz. “Whistles” and “screeches” tend to be above 2 kHz.
Very rarely does feedback occur below 80 Hz or above 8 kHz. It takes practice to develop an ear for equalizing a sound system, so be patient.
After locating the first feedback frequency, begin turning up the system again until the next frequency begins ringing.
Repeat the above steps until the desired level is reached, but do not over-equalize. Keep in mind that equalizers can only provide a maximum level increase of 3 to 9 dB.
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 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 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 feedback does appear, know that you have an immediate alternative to turning down volumes....you might just be able to EQ it out.
